Entrainment system and method

Abstract

Methods and systems are disclosed for signal processing, in particular of an entrainment signal. A signal processing module is described which comprises a first signal input for receiving an input signal x in, a signal output for providing an output signal x out to an output device, and a second signal input for receiving a recorded signal y. The recorded signal y represents a recording of the output generated by the output device. The signal processing module further comprises signal processing circuitry configured to generate the output signal by adding the input signal and a difference α (x ref - y) between a reference signal and the recorded signal: x out = x in + α (x ref - y). The reference signal can be the input signal, the output signal, or an external reference signal. The input signal can be an entrainment signal.

Description

[0001] Entrainment system and method

[0002] Technical field

[0003] The disclosure relates to signal production, and in particular, though not exclusively, to methods and systems for generating an acoustic entrainment signal.

[0004] The environment in which a sound (or other signal) is played, affects the acoustic properties (e.g., amplitude spectrum and phase spectrum) of the sound. The effects depend on, e.g., air temperature, humidity, distance to any boundaries (such as walls), used materials, and the presence of objects (including human beings) in the environment.

[0005] For certain applications, it may be desirable that the sound that is sensed by an observer in the environment, deviates as little as possible from an intended sound. One such application is the therapeutic use of sound, in particular acoustic entrainment signals, as described for instance in WO 2022 / 039598 A1. However, as the aforementioned environmental effects may reduce the efficacy of the entrainment signals, a system is needed that optimises the generated signals, taking into account the environment.

[0006] Hence, from the above, it follows that there is a need in the art for a system that compensates for the effect of the environment on the generated signal.

[0007] It is an aim of embodiments in this disclosure to provide a system and method for processing or generating a signal such as an entrainment signal that avoids, or at least reduces the drawbacks of the prior art.

[0008] In a first aspect, embodiments in this disclosure relate to a signal processing module. The signal processing module comprises a first signal input for receiving an input signal xin; a signal output for providing an output signal xoutto an output device; a second signal input for receiving a recorded signal y, the recorded signal representing a recording of the output generated by the output device; and signal processing circuitry configured to generate the output signal by adding a difference a(xref - y) between a reference signal and the recorded signal to the input signal. The reference signal is one of: the output signal, the input signal, or an external signal. The external signal may be a recording of the output generated by the output device in response to receiving the output signal and being synchronised with the input signal.

[0009] Thus, the signal processing module adjusts a signal based on a recording of the generated signal. For example, if the recorded signal comprises a lower amplitude for certain frequencies than the reference signal, the signal processing module may increase the amplitude of those frequencies such that the amplitude spectrum is more similar to, or even equals that of the reference signal.

[0010] Typically, the signal comprises a plurality or range of frequencies with different amplitudes. The reference signal may represent a target signal.

[0011] The input signal can be an entrainment signal. It has been found that the properties of an entrainment signal are affected by the subject (typically a person) being entrained. By compensating for this subject-dependent effect on the entrainment signal, the effect of the entrainment can be enhanced. In the context of sound applications, entrainment may be defined as: “The process whereby two interacting oscillating systems assume the same periodicity. Entrainment is a phenomenon that can be observed in physical and biological systems." This definition follows the definition in P. Oomen et al., ‘The Effects of Spatial Sound on Human Wellbeing’, The Works Institute (2021), available on-line at https: / / osf.io / udsjy.

[0012] In other technological fields, the term entrainment is typically used to describe entrainment of signals. For example, in the context of (acoustic) noise cancelling, an antinoise signal is entrained by the adaptive filter in stages to become more similar to an acoustic noise signal (also referred to, in that context, as a reference signal), which as a result can be more effectively cancelled. In the present disclosure, the acoustic signal produced by the loudspeaker (and generally modified by the presence of the human subject) is entrained to the reference signal (the input signal or a signal derived thereof), so that the acoustic signal becomes more similar to the reference signal. Both are examples of entrainment in physical systems.

[0013] However, the entrainment in the term “entrainment signal” refers not primarily to the entrainment of the signals themselves, but to entrainment of the physiology of a human subject as a result of exposure to the entrainment signal. Consequently, in the context of this disclosure, the term “entrainment” refers to entrainment between a physical and a biological system, unless otherwise specified (implicitly or explicitly). Thus, the term “entrainment signal” generally refers to a generated signal improving entrainment of the biological system, and not to, e.g., entrainment of the acoustic signal to the reference signal.

[0014] It is noted that the circuits described herein differ from, e.g., active noise-cancelling circuits in the choice and function of the reference signal. In noise-cancelling circuits, the reference signal is, essentially, a noise signal that should be compensated for in the output signal; in the embodiments described herein, by contrast, the reference signal is the signal that is, in a sense, the aim to be achieved by the output signal. The reference signal is therefore one of: the output signal, the input signal, or an external signal derived from and synchronised with the input signal or the output signal.

[0015] In an embodiment, the output signal represents an audio signal. In such an embodiment, the output device may comprise one or more loudspeakers, and the recorded signal may be recorded by a microphone.

[0016] In other embodiments, the output signal may represent an optical signal, a microwave signal, or a different electromagnetic signal. In such cases, corresponding output and input devices may be used, such as lights and cameras, microwave source and detectors, et cetera.

[0017] In an embodiment, the reference signal is one of: the output signal, the input signal, or an external signal. The external signal can be a pre-recorded signal obtained using the same system in a similar environment. In particular, the reference signal can be pre-recorded in the same physical environment absent a person. That way, the presence of the person in the environment can be adapted for. If an external signal is used, means may be provided to synchronise the reference signal and the input signal.

[0018] In an embodiment, the input signal, the (modified) output signal, the reference signal, and the recorded signal are analogue signals. The signal processing should happen essentially in real-time. In general, digital signal processing (in particular analogue to digital conversion and digital to analogue conversion) is too slow for the present application.

[0019] In an embodiment, the circuitry comprises an inverter for inverting the recorded signal, a first adder for creating a difference signal by adding the inverted recorded signal and the reference signal, and a second adder for adding the difference signal and the input signal.

[0020] In an alternative embodiment, the circuitry comprises a first inverter for inverting the reference signal, a first adder for creating a difference signal by adding the recorded signal and the inverted reference signal, a second inverter for inverting the difference signal, and a second adder for adding the inverted difference signal and the input signal.

[0021] In some embodiments, the circuitry further comprises an attenuator. If the circuitry comprises a recursive part, the repeated addition of an attenuation difference signal with an attenuation factor of 0.5 leads to a total factor of 1. The attenuator may also be used to control the balance between input signal and difference signal.

[0022] In a further aspect, this disclosure relates to a system comprising one or more signal processing modules as described herein, one or more output devices, and one or more groups of one or more recording devices. Each output device is connected to a signal output of a respective signal processing module, and each group of one or more recording devices is connected to the second signal input of the respective signal processing module. The group of one or more recording devices connected to the same signal processing module as a given output device is positioned in close proximity to the given output device.

[0023] The number of output devices is typically the same as the number of groups of recording devices; that is, for each output device, there is a group of one or more recording devices.

[0024] As used herein, close proximity may refer to a distance of 10 cm or less, 5 cm or less, 2 cm or less, or even 1 cm or less between an outer edge of the recording device and an outer edge of the output device.

[0025] Such a system may be used as an entrainment system.

[0026] In an embodiment, the system is configured as a headphones comprising a set of earcups, wherein each earcup comprises one output device and one recording device on the inside of the earcup.

[0027] In an alternative embodiment, the system is configured as a set of earbuds wherein each earcup comprises one output device and one recording device configured to be inserted into a wearer’s ear canal.

[0028] In such an embodiment, the output device is typically a transducer and the input device is typically a microphone. The configuration as a pair of headphones I earbuds allows the microphone to be placed very close to the transducer. Such a system may further comprise, e.g., known noise cancelling features.

[0029] In an embodiment, each output device and each group of one or more recording devices that are connected to the same signal processing module are configured as an environmental sound loudspeaker.

[0030] Environmental sound loudspeakers are described in WO 2023 / 287291 A1, which is hereby incorporated by reference in its entirety.

[0031] In an embodiment, the one or more output devices comprise a plurality of transducers configured to be worn on or to be in contact with a predetermined set of body parts. The predetermined set of body parts may correspond at least approximately to one or more of: crown, left shoulder, right shoulder, left hand, right hand, sternum, chest bone, sacrum, left knee, right knee, left foot, and right foot. The predetermined set of body parts may also correspond at least approximately to the chakra points. The predetermined set of body parts may be determined based on points of contact with a device comprising the transducers, e.g. a wearable device or a piece of furniture, such as a meditation chair.

[0032] In an embodiment, the system further comprises a signal generator for generating an input signal, preferably the input signal being an entrainment signal. The signal generator may comprise a storage device for storing the input signal and playback hardware to generate an analogue signal based on the stored input signal. In an aspect, this disclosure relates to a method for processing a signal, such as an entrainment signal. The method comprises receiving or generating an input signal, preferably the input signal representing an entrainment signal; generating, in an environment, an output signal, based on the input signal; recording a recorded signal, the recorded signal representing the output signal having interacted with the environment; and modifying the output signal by adding a difference between a reference signal and the recorded signal to the input signal.

[0033] In an embodiment, the output signal is an acoustic signal generated by one or more loudspeakers, and the recorded signal is recorded by a microphone.

[0034] In an embodiment, a subject is present in the environment, preferably the subject being an organism, more preferably a human, and wherein interaction with the environment comprises interaction with the subject.

[0035] In an embodiment, the method is a non-therapeutic method. For example, the method may be used to create a maximally faithful rendition of an, in principle, arbitrary sound or other input signal.

[0036] In other embodiments, the method is a therapeutic method. Therapeutic use of entrainment signals is described in, e.g., WO 2022 / 039598 A1, which is hereby incorporated by reference in its entirety. Similarly, the system described above may be used as entrainment systems in a therapeutic or non-therapeutic setting.

[0037] In an embodiment, the method further comprises obtaining the reference signal. Obtaining the reference signal may comprise receiving the input signal; generating, in a reference environment, a reference output signal, while the reference environment does not comprise the subject; and recording the reference signal, the recorded signal representing the reference output signal having interacted with the reference environment without the subject.

[0038] The reference environment in which the reference signal is recorded can be the same environment as that in which the (modified) output signal is played back, but without a subject present.

[0039] Entrainment is the process of synchronization whereby two interacting oscillating systems assume the same periodicity. The phenomenon can be observed, for example, in physical and biological systems, such as the human body responding to stimuli in the environment. Entrainment methods have a direct or indirect use in many therapeutic practices, ranging from vibrational therapy, promotion of bone and soft tissue regeneration, destruction of kidney stones, tumour ablation and brainwave synchronization for reducing stress, anxiety and depression.

[0040] These uses generally consider unidirectional entrainment of a subject acted upon by a stimulus that is predictably controllable, i.e., the stimulus itself is not accounted for to be affected by the (biological) system it is acting upon. However, the inventors have discovered that the stimulus is, generally, affected by the response of the subject it is acting upon and may be significantly modified by this response in certain cases. The subject-specific response function may be caused by various environmental conditions including first and foremost the presence (and properties) of the subject itself within the environment.

[0041] This subject-specific response function can be measured as frequency-dependent amplitude and phase differences compared to the original stimulus. This, in turn, may be understood as a lack of effective entrainment at those specific frequency components, as the signal implies that frequency oscillations at these components are not (fully) synchronized. Thus, a stimulus used for entrainment is affected by the response of the subject, and the degree to which the stimulus is modified indicates the degree of effectiveness or resistance of the subject to the entrainment method. It is hypothesised that the better the entrainment, the smaller the effect on the entrainment signal.

[0042] Many different (kinds of) signals can be used as entrainment signals. An aural entrainment signal typically has at least one main component that has one or more (usually all) of the following characteristics: i) a steady oscillation at one or multiple frequencies in the audible frequency range of 20 Hz - 20 kHz, and / or rhythmic pulsations in the sub-audible frequency range of less than 20 Hz; ii) either no significant changes or steady and repetitive periodic changes in the frequency spectrum and / or rhythmic components of the signal for the duration of the signal; and / or a rise and decay of the frequency spectrum and rhythmic components of the signal with relatively steady periodicities; iii) a variability in the periodic changes in the frequency spectrum and / or rhythmic components of the signal, the variability preferably being between 0-200 ms; iv) tempered amplitude levels, e.g., amplitude levels of the signal being less than 85 dB and / or no substantial amplitude peaks; v) spaciousness, e.g., reverberation of the signal in an environment which may be a recorded environment, a simulated virtual environment and / or the playback environment itself; and vi) a duration of at least one minute, at least two minutes, or more typically, at least five minutes.

[0043] An example of a signal that matches such characterizations is the repeated tone caused by the repeated striking of a singing bowl. Each strike results in an audio signal with a periodically modulating frequency (caused by the heterodyne interference of the overtones, also known as the beat frequency) and an approximately exponentially decaying amplitude. The fundamental tone and the overtones generally have frequencies in the audible range. The beat frequency is typically in the order of a few Hz, and thus causes a rhythmic pulsation. The repeated striking at regular intervals causes a repetitive periodic change. The slight difference in time intervals and amplitudes between subsequent strikes by a human striker results in the desired variability.

[0044] Other examples of entrainment signals include the repeated tone or tones of an instrument in relatively regular, repeated time-intervals, e.g., a striking of varying chords on a guitar in a regular rhythm, repeated electronically generated waves fading in and out, preferably with a (programmed) degree of variability, etc.; soundscapes of natural environments, e.g. a meadow with various types of crickets and chirping insects, the ebbing and flowing of waves on a shore, etc.; the rhythms of the human body, i.e. recording of the heartbeat and blood flow, the inhale and exhale of breathing, (sonification of) brain wave activity, etc.

[0045] Thus, the embodiments in this disclosure account for the bidirectional effects of entrainment, i.e., the effect of the stimulus on the subject and vice versa, and incorporate accurate auto-response to these effects so as to enhance the process of entrainment.

[0046] The flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present embodiments. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the blocks may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and / or flowchart illustrations, and combinations of blocks in the block diagrams and / or flowchart illustrations, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

[0047] The embodiments will be further illustrated with reference to the attached drawings, which schematically will show embodiments according to the invention. It will be understood that the invention is not in any way restricted to these specific embodiments. Identical reference signs refer to identical, or at least similar elements.

[0048] Brief description of the drawings

[0049] Fig. 1A-1D schematically depict signal processing modules according to various embodiments; Fig. 2A schematically depicts a wearable signal playback system according to an embodiment and Fig. 2B schematically depicts a corresponding signal processing module;

[0050] Fig. 3A and 3B schematically depict static signal playback systems according to embodiments and Fig. 3C schematically depicts a corresponding signal processing module;

[0051] Fig. 4A-4C schematically depict entrainment systems according to various embodiments;

[0052] Fig. 5A and 5B schematically depict an entrainment system according to an embodiment;

[0053] Fig. 6 schematically depicts a signal processing and analysis module according to an embodiment;

[0054] Fig. 7A-F are graphs showing the difference in measured signals between various embodiments;

[0055] Fig. 8 is a flowchart of a method according to an embodiment;

[0056] Fig. 9A-E show the effect of the presence of a human subject on the amplitude of a stimulus signal;

[0057] Fig. 10A-C show some experimental results correlating physical and mental subjectspecific responses to entrainment stimuli; and

[0058] Fig. 11A-D are graphs comparing effect on acoustic signals of a circuit according to an embodiment with an active noise-cancelling circuit.

[0059] Detailed description

[0060] Fig. 1A-1D schematically depict signal processing modules according to various embodiments.

[0061] Fig. 1A schematically illustrates a signal processing module according to an embodiment. The signal processing module 100 comprises a first signal input 102, a second signal input 106, and a signal output 104. The first signal input is configured to receive an input signal xin. The input signal represents a target signal, e.g., an entrainment signal. The input signal can be, for instance, a computer-generated signal, a recorded signal, or a processed recorded signal.

[0062] The signal output 104 is configured to provide an output signal xoutto (and is therefore connectable to) an output device 120. The output device can be a transducer, e.g. a loudspeaker, or a different type of signal generator such as a controllable electromagnetic radiation source (e.g., an antenna, a light array, et cetera). The output device is configured to generate a signal in an environment, e.g., a room. The environment is typically a closed environment, e.g., a room or hall, but can also be an open environment. In a typical embodiment, the environment comprises a living subject, usually a human subject. Of course, a controlled acoustic environment gives opportunity to more precisely detect and compensate for differences caused by the human subject, in the absence of other factors such as environmental noise and / or reflections / reverberations of the room. An example of a closed environment is shown in Fig. 5A.

[0063] The output device can also be configured as a headphone, earbud, or similar device. In that case, the environment comprises the space covered by the ear cushion or earbud and at least part of the head of the wearer of the device. An example of a system configured as a pair of headphones is discussed in more detail below with reference to Fig. 2.

[0064] The second signal input 106 is configured for receiving a recorded signal y from (and is therefore connectable to) an input device 122. The input device can be a microphone, camera, receiver, or other type of sensor configured to record the output generated by the output device 120. Thus, the recorded signal represents a recording of the output generated by the output device 120. The recorded signal has interacted with the environment, and may comprise, e.g., reflections of the outputted signal. The recorded signal y may be considered an environmentally modified version of the output signal xout.

[0065] The signal processing module further comprises signal processing circuitry 108 configured to generate the output signal xout. In general, the output signal xoutis obtained by adding the input signal x and an (optionally attenuated) difference a(xref- y) between a reference signal xrefand the recorded signal y. In other words, xout= xin+ a(xref- y). In the example depicted in Fig. 1A, the reference signal is the input signal, i.e. , xref= xin.

[0066] In this example, the recorded signal y is modified by an inverter 110, which applies an inverting operation to the signal, i.e., the inverter multiplies the signal with a factor of -1, resulting in an output y * -1 = -y; this is equal to a (frequency-independent) phase shift of 180° of the signal. The inverted version of the recorded signal is combined with (added to) the reference signal, in this case the input signal xin, by a first adder 112. The result of this operation is a difference signal xin- y comprising the difference between the input signal and the environmentally modified output signal.

[0067] Optionally, the difference signal is further modified by an (optional) attenuator 114 which applies an attenuation operation to the difference signal, i.e., the attenuator multiplies the difference signal xin- y with a (frequency-independent) attenuation factor a, resulting in an output a(xin- y). In the current example, the attenuation factor a is typically smaller than or equal to 1 , preferably equal to 1.0 (i.e., no attenuation). An attenuation factor a > 1.0 may result in overcorrection, an attenuation factor a < 1.0 may result in undercorrection. In some embodiments, the attenuator may have a fixed (static) attenuation factor, whereas in other embodiments, the attenuation factor is controllable. In some embodiments, the attenuator is omitted, effectively resulting in an attenuation factor of 1. The attenuated version of the difference signal a(xin- y) is combined with (added to) the input signal xinby a second adder 116. The result is the output signal xout= xin+ a(xin- y), with typically 0 < a < 1.

[0068] The output signal xoutobtained using the described circuitry 108 may be referred to as subject-dependent stimulus. The output signal may be provided to a subject by means of output device 120, typically a transducer, e.g. a loudspeaker.

[0069] The recording device 122 is typically positioned in close proximity to the given output device, for instance within a distance of 10 cm or less, 5 cm or less, 2 cm or less, or even 1 cm or less. The distance may be determined between a centre of the recording device and a centre of the output device, or between an outer edge of the recording device and an outer edge of the output device.

[0070] The depicted circuitry 108 is fully analogue, and therefore essentially instantaneous or real-time, i.e., with a time delay that is substantially zero. The same is true for the embodiments depicted in, e.g., Fig. 1B-D, 2B, 3C, and 6. In some embodiments, in order to ensure that the time delay remains close to zero (i.e., effectively real-time), the distance between output device and recording device is minimized, or otherwise cancelled in the recorded signal, such as in an environmental sound loudspeaker set-up.

[0071] Thus, the depicted signal processing module 100 generates a signal that incorporates the environment’s response function in real time. As will be discussed in more detail below with reference to Fig. 9A-D, if the environment comprises a (living) subject, e.g., a human being, this response-function is subject-dependent. In the context of entrainment, the output signal may be referred to as the stimulus.

[0072] The circuitry described herein acts as a frequency-dependent filter modifying amplitude and phase differences, such that when y > xref, y is diminished and when y < xref, y is increased. Effectively,y + out= xrefif the phase difference ip = 0 and / or if the amplitude difference AA = 0; andy + out« xref, or at least |xref-y + out| « xrefif both p #= 0 and AA #= 0. As a result, modifications due to the subject-dependent response function are counterbalanced in the stimulus leading to an entrainment process where the recorded signal y is naturally shifted towards equilibrium with the output signal xout, and eventually xout« xref, without the need for obtaining and interpreting (digital) data to provide an engineered response. This allows operation in real-time, which is optimally effective and accurate.

[0073] Fig. 1B-D differ from Fig. 1A mainly in the choice of reference signal.

[0074] In the example depicted in Fig. 1B, the reference signal is the output signal (which is therefore recursively defined): xref= xout. The difference signal then is (xout- y). In this case, as the circuit comprises a feedback loop, the attenuation factor a is typically smaller than or equal t noted that for a = 1 / 2, this expression mathematically converges to xout= xin+ (xin- y).

[0075] Fig. 1C schematically depicts a signal processing module according to a different embodiment. The signal processing module is mostly identical to the signal processing module described above with reference to Fig. 1A. However, the signal processing module further comprises a third signal input 132 for receiving an external reference signal xext. Thus, in this embodiment, the reference signal is the external reference signal rather than the input or output signal, i.e., %ref= xext. As shown in Fig. 1C, this leads to a difference signalXcxt - y and an output signal xout= xin+ a (xext- y), with typically 0 < a < 1, preferably a « 1. Again, the attenuator may be omitted.

[0076] The external reference signal can be, e.g., a pre-recorded or stored version of the input signal. If the external reference signal is a pre-recorded signal, it is typically recorded using the same hardware (except, possibly, for the use of a signal processing module as described herein) and in the same environment. The external reference signal is time- synchronised with the input signal. The external reference signal may, e.g., represent a recording under neutral or ideal conditions, or a recording in the absence of a subject undergoing entrainment, or a recording with the subject (or a different subject) in a well- entrained state, et cetera.

[0077] The signal processing may comprise a second signal output 134 for providing the recorded signal to a recording device. Optionally, the signal processing module may comprise a switch 136 to switch between a recording state and a signal processing state.

[0078] Naturally, many further variations can obtain essentially the same result. One example of such a variation is depicted in Fig. 1D. The signal processing module of Fig. 1D is mostly identical to the signal processing module of Fig. 1B. However, in this embodiment, instead of inverting the recorded signal y to obtain an inverted recorded signal -y, the reference signal (in this case, the output signal xout) is inverted before being added to the recorded signal. Thus, the output of first adder 112 is a difference signal y - xref= y - xout, rather than xout- y as was the case in the embodiment of Fig. 1 B. In order to obtain the same output signal, the difference signal is inverted before, during, or after the attenuation; in the depicted example, the inversion and attenuation are combined in a single operation by attenuator 114.

[0079] Otherwise, the embodiment shown in Fig. 1D is identical to the one shown in Fig. 1B. The skilled person can envisage many more modifications that are functionally equivalent to the depicted embodiments.

[0080] The embodiments depicted in Fig. 1A-D show the combination of one output device (e.g., loudspeaker or transducer), and one input device (e.g., microphone, receiver or sensor). Advantageously, the input device is an omnidirectional input device, e.g., an omnidirectional microphone. Several such systems (comprising a single output device and a group of one or more input devices) may be combined.

[0081] In the following, it is assumed that the output devices are transducers or loudspeakers, and that the input devices are microphones, typically omnidirectional microphones. However, it is understood that different embodiments may use different input and output devices.

[0082] Fig. 2A depicts an embodiment of a system configured as a headphone. In this example, the headphone 200 comprises two earcups 202I,2 and a headband 204 connecting the two earcups. In principle, each earcup may comprise a subsystem as depicted in any of Fig. 1 A-D or a variation thereof. An explicit example is provided in Fig. 2B. The earcups can be e.g., supra-aural or circumaural earcups. In general, there is little direct interaction between the left and right ear subsystems. As the used components are relatively small, the microphone can easily be positioned in close proximity to the transducer, e.g., at a distance of one or a few centimetres, or even less, e.g., a few millimetres.

[0083] The headphone may further be configured to connect in a wired or wireless way with an external signal source. The headphone may also comprise a power source or be connectable to a power source.

[0084] As depicted, the headphone 200 comprises two transducers (headphone outputs) 120I,2, forming a stereo headphone system, and two microphones 122I,2, preferably omnidirectional microphones. In this embodiment, each microphone is preferably positioned next to the respective transducer, more preferable in between the transducer and an ear of the subject 206, most preferably centred between the transducer and the ear of the subject. In such an embodiment, the subject 206 is considered part of the environment for both earcups.

[0085] The system may also be configured as a pair of earbuds. In this case, the microphone may be configured to be inserted into the wearer’s ear canal.

[0086] Fig. 2B depicts a variation of the embodiment shown in Fig. 1 A for a dual output system, which may be provided either with a mono signal (i.e., xL in= xfl in) or with a stereo signal (i.e., xL in#= xR,in)- Incase°famono signal, signal inputs 102i and 1022 may be the same signal input. It is understood that the variations as depicted in, e.g., Fig. 1B-D may be applied, mutatis mutandis, to the embodiment depicted in Fig. 2B.

[0087] When the signal processing is used in combination with one or more loudspeakers, each of the loudspeakers is associated with a group of one or more microphones. If the loudspeaker is associated with a plurality of microphones, they may be arranged as a so- called environmental sound loudspeaker as described in WO 2023 / 287291 A1. Specific examples of environmental sound loudspeakers are provided, in particular, in Fig. 1 and Fig. 5A-5D and the accompanying description of WO 2023 / 287291 A1. Fig. 3A depicts an embodiment with two loudspeakers 302I,2, forming a stereo system 300. The loudspeakers may define a central position in which the subject 306 is to take place. This position is typically equidistant from the two loudspeakers.

[0088] Fig. 3B depicts an embodiment with four loudspeakers 302I-4, forming a quadraphonic or surround sound system. The loudspeakers may define a central position in which the subject 306 is to take place. This position is typically equidistant from the four loudspeakers.

[0089] Similar configuration can also be extended to three dimensions. For example, the loudspeakers may be positioned on the vertices of a Platonic solid.

[0090] It may be understood that for a configuration as depicted in Fig. 3A or 3B, each loudspeaker is provided with an audio output signal generated in dependence of a recorded signal by a microphone associated with the loudspeaker. For each loudspeaker and microphone pair, one of the signal processing modules as depicted in Fig. 1A-D or a variation thereof may be applied. Each microphone may be positioned close to the associated loudspeaker. A microphone and a loudspeaker may be said to be associated with each other if they are connected to the same signal processing module.

[0091] Fig. 3C schematically shows an environmental sound loudspeaker according to an embodiment. The environmental sound loudspeaker 320 comprises a loudspeaker driver 120, for example a coaxial loudspeaker driver, and one or more pairs of microphones 122I-4. In the depicted example, there are two pairs of microphones, the first pair consisting of microphones 122i and 122s and the second pair consisting of microphones 122a and 1224. The microphones in a pair of microphones are placed diametrically and in an equidistant manner relative to the centre of the loudspeaker driver. The microphones in a pair of microphones are identical to each other. Preferably, the microphones are omnidirectional microphones. Preferably, when an environmental sound loudspeaker comprises a plurality of microphone pairs, all microphones are identical. Preferably, when an environmental sound loudspeaker comprises a plurality of microphone pairs, all microphones are placed at the same distance from the centre of the loudspeaker driver. Preferably, when an environmental sound loudspeaker comprises a plurality of microphone pairs, the pairs are distributed equally around the loudspeaker driver; for example, a first pair may be placed at angles of 0° and 180°, and a second pair may be placed at angles of 90° and 270°.

[0092] In some embodiments, the environmental sound loudspeaker may comprise a plurality of loudspeaker drivers, preferably identical loudspeaker drivers, and the microphones may be arranged symmetrically around the centre of mass of the loudspeaker drivers.

[0093] In some embodiments, the microphones and the loudspeaker driver(s) may be integrated in a single device, whereas in other embodiments, the microphones and the loudspeaker driver(s) may be implemented as a system of separate devices. The environmental sound loudspeaker further comprises a signal processor 320. The signal processor is arranged to combine input signals from the microphones, typically into a single output signal 339 that can be provided to the loudspeaker driver. The signal processor is further arranged to provide feedback reduction. The signal processor is furthermore arranged to maintain or restore the fidelity of the combined input signals.

[0094] The signal processor 320 can be integrated into the loudspeaker device, or it can be (part of) a separate device. As depicted, the signal processor comprises a plurality of microphone signal inputs IO61-4 for receiving input signals from microphones 122I-4. In the depicted embodiment, there is one signal input for each individual microphone. In some embodiments, several inputs can be physically or logically combined. In this example, each microphone is connected via a dedicated wire to the corresponding input. It should be noted that pairs of microphones are connected to pairs of inputs; in this example, inputs IO61 and IO62 form a first pair, receiving input from microphone pair 122i and 122s, and inputs IO63 and IO64 form a second pair, receiving input from microphone pair 122a and 1224.

[0095] One input 1062,4 from each pair of inputs is connected to an inverter 324I,2 which inverts the input signal 3232,4 received from the corresponding microphone. Inverting a signal corresponds to applying a phase shift of 180° to the signal or, equivalently, to multiplying a signal with a factor -1. After inversion of one of the signals from an input pair, the two signals from a pair of microphones are combined, typically added or summed, by an adder 326I,2, resulting in combined signals 327I,2. Combining a first signal and an inverted second signal can be understood as subtracting the second signal from the first signal, or as determining a difference between the first signal and second signal. Because the microphones in a microphone pair are positioned equidistant from the loudspeaker driver, the contributions of the loudspeaker driver at the respective microphones are in phase with each other, and are thus cancelled out. Sounds from different sources will typically arrive at the microphones of a microphone pair with a phase difference, and will thus not cancel out by subtracting the signals from each other.

[0096] The combined signal 3272 from one or more microphone pairs may be phase-shifted 328 with a phase shift A<p. In the depicted example with two pairs of microphones, one pair may be phase-shifted with a 90° phase shift, for example by applying a Hilbert transform. The combined, and optionally phase-shifted, signals from the microphone pairs are then again combined by a further signal combiner 330 resulting in a further combined signal 331.

[0097] The further combined signal 331 is provided to a signal amplifier 332 for restoring, or at least approximating, the frequency spectrum (or loudness balance) of the input signal representing environmental sounds, i.e. sounds from other sources than directly from the loudspeaker driver 120. The signal amplifier typically comprises components for boosting the low frequencies and for attenuating the high frequencies. For example, the amplifier may comprise one or more high-shelf filters for attenuating the high-frequency signals and / or one or more low-shelf filters for boosting the low-frequency signals.

[0098] Preferably, the amplifier attenuates frequencies above a first transition frequency with -3 dB per doubling of the number of microphones, for example, -3 dB for one microphone pair, -6 dB for two microphone pairs, -7.8 dB for three microphone pairs, -9 dB for four microphone pairs, -10 dB for five microphone pairs, -10.8 dB for six microphone pairs, -12 dB for eight microphone pairs, or -13 dB for ten microphone pairs. In general, the attenuation a(f) may be given or approximated by where N is the number of microphones. The amplifier may comprise a high-shelf filter to implement the attenuation.

[0099] Preferably, the amplifier boosts frequencies below a second transition frequency with +6 dB per octave. In general, the boost b(f) may be given or approximated by where ft 2is the second transition frequency. The amplifier may comprise a plurality of low- shelf filters in series.

[0100] The first and second transition frequencies can be the same frequency. The first and second transition frequencies typically depend on the distance between the microphones in a microphone pair, on the number of microphones pairs, and on the parameters of the filters.

[0101] The output of the amplifier 122, i.e. signal 333 corresponds to the recorded signal y of the circuitry 108 as described above with reference to Fig. 1A-D. The circuitry thus determines a difference signal based on the recorded signal and a reference signal, which in the depicted embodiment is the input signal provided by a further sound input 102. In other embodiments, the output signal or an external reference signal may be used, as described above. The further sound input may be a wired or wireless connection. The signal processor 320 may comprise a further amplifier to amplify the input signal received by the further sound input.

[0102] The signal processor 320 further comprises an output 104 for providing an output signal 339. The output is connected to the loudspeaker driver 120. The connection may be wired or wireless.

[0103] The signal processor 320 may further comprise a power source 340 for providing power to components of the signal processor. The components may be implemented using fully analogue components. The power source may comprise an integrated power source such as a battery and / or a connector to connect the device to an external power source.

[0104] Because microphones from a microphone pair are positioned at equal distance from the loudspeaker driver 120, and signals from the microphones are inverted before being combined, sounds originating from the loudspeaker driver may efficiently be removed from the output that is fed to the loudspeaker driver. Thus, unwanted feedback may be prevented or at least minimised. The amplification ensures that the environmental sounds, i.e. , sounds recorded by the microphones that are not coming (directly) from the loudspeaker driver, may be played back with high fidelity. Sounds coming from the loudspeaker driver that have been reflected by an object in the environment, are considered environmental sounds and will typically not be cancelled by the signal processing.

[0105] Fig. 4A depicts an embodiment with seven transducer / sensor pairs 402I-7, each pair (indicated with an asterisk) comprising a vibrational transducer, e.g., piezo-electric transducers or electrodes, and a receiver or sensor associated with the transducer. The transducer and associated sensor are preferably positioned as close as possible to each other. The transducers are positioned on different parts of a body of a human subject 406, preferably in direct contact with the skin of the subject. In the depicted example, the transducers are positioned on or near the crown 402i, left shoulder 4022, right shoulder 402s, sternum 4024, sacrum 402s, left knee 402s, and right knee 402?. Other embodiments may use a subset of one or more of the locations, and / or additional locations. In this embodiment, the human subject is understood to be part of the environment for each module.

[0106] For a configuration as depicted in Fig. 4A, a vibrational transducer is provided with an (audio) output signal generated in dependence of a recorded signal by a receiver or sensor associated with the vibrational transducer. For example, for each transducer and receiver or sensor pair, one of the embodiments as depicted in Fig. 1 A-D may be applied.

[0107] Fig. 4B shows a back view of an embodiment of an entrainment system for supporting a ‘lotus-position’. The depicted chair 410 contains a plurality of integrated loudspeaker / microphone pairs 4122-7 corresponding to the locations 4022-7 denoted in Fig. 4A, respectively (a loudspeaker / microphone pair above the crown of the head is not included in this design). The loudspeaker configuration may be configured for sound shape projection. The chair may be placed in the centre of a spherical pod to contain a sound system to improve a physiological condition of a subject, as shown in Fig. 5A-B.

[0108] Fig. 4C shows an oblique side view of the entrainment chair shown in Fig. 4B. for the model shows a human subject 406 in lotus position, supported by the chair. The loudspeaker / microphone pairs have been integrated at points with reliable contact between subject 406 and chair 410.

[0109] Fig. 5A shows an embodiment of an entrainment system comprising a loudspeaker configuration with eight preferably omnidirectional loudspeakers 502i-s. The loudspeakers are arranged surrounding a mostly acoustically transparent spherical shell 504, at equal radius from a centre of the sphere 500 and equidistant to one another, forming a ‘tilted cube’ or ‘star-tetrahedron’ shape. Each loudspeaker is an environmental sound loudspeaker, e.g., as described with reference to Fig. 3C. Other variations may use a different configuration, e.g., corresponding to one of the other Platonic solids. The loudspeaker configuration may be configured for sound shape projections.

[0110] Fig. 5B shows the same entrainment system as Fig. 5A, but from the bottom side. Additionally, on the outside of the sound system’s circumference a sound-proof shell 508 enclosing the sound system within a spherical pod is shown.

[0111] The experiments described below with reference to Fig. 9A-D were performed with a chair as shown in Fig. 4B-C, centrally positioned in a spherical pod as shown in Fig. 5B. The inner shell 504 had an inner diameter of 2.0 m, and the distance between inner and outer shell, in which the loudspeakers were positioned, was 0.2 m. The pod could be closed with a lid. Inside the pod, atmospheric noise levels are reduced by -30 dB (when the lid is closed). The temperature and humidity inside the pod were regulated and monitored during all trials to be -22 °C and -40% relative humidity.

[0112] In the embodiment depicted in Fig. 6, the signal processing module is connected to a real-time or non-real-time data analysis device. The signal processing module has the same basic structure as the one shown in Fig. 1C. Signal inputs / and outputs are not implicitly shown, but assumed to be present as needed. In this example, analytical data are obtained by generating a time-based Fast Fourier Transform (FFT) 602I-3 of the recorded signal y, the external reference signal xcxtand the difference signal u = xext- y, respectively, for a subset of time fractions n and corresponding frequency regions. In embodiments using a different reference signal, the input signal xinor the output signal xoutmay be analysed instead of the external reference signal %ext. The output data of each subset comprises the amplitude A(n) for each frequency region and time fraction n. Based on these data, an amplitude difference AA can be generated by computing: A — A(y, tn, fn) ~ (xex, to, fo) (3) and a phase difference can be generated by computing:

[0113] The measurements may be obtained relevant to signal generation (in particular entrainment) with and without the subject-specific response function modifying the stimulus, by controlling the attenuation operation of the attenuator 114, where the attenuation factor a of the difference signal may be varied from a = 0 (min) 1.0 (max) for non-recursive embodiments, and a = 0 (min) 0.5 (max) for recursive embodiments. Fig. 7A-F are graphs showing the difference between various embodiments as described herein. The graphs are obtained using signal processing as described with reference to Fig. 6.

[0114] Fig. 7A is a graph showing the effect of the signal processing modules as described with reference to Fig. 1A-C on the amplitude difference as defined in eq. (3), in a set-up wherein a microphone is positioned 1 m in front of a loudspeaker. The curves are obtained by computing (xout- y) / 2 - xref, where xout= xin+ u.

[0115] The solid grey curve 702 represent the situation where u = 0 (so xout= xin, i.e., without the signal processing as described herein), and xref= xout. There are clear frequency-dependent deviations in amplitude.

[0116] The dashed curve 704 represents the situation with xref= xinand u = xin- y, as shown in Fig. 1A (with a = 1); the solid black curve 706 represents the situation with xref= xoutand u = 0.5 (xout- y), as shown in Fig. 1B (with a = 1 / 2); and the dotted curve 708 represents the situation with xref= xextand u = xext- y, as shown in Fig. 1C (with a = 1). It can be seen that for all three configurations, the mean of the output signal and the recorded signal is much closer to the reference signal than for the configuration without signal correction. For frequencies < 1000 Hz, the configurations behave substantially the same, whereas for frequencies > 1000 Hz, the configuration of Fig. 1A performs slightly better than the configuration of Fig. 1B, which in turn performs slightly better than the configuration of Fig. 1C. For the highest frequencies (around 10 kHz), the difference disappears again.

[0117] Fig. 7B shows the same measurements as Fig. 7A with curves 712-718 corresponding to curves 702-708. respectively, but shows the phase difference as defined in eq. (4) instead of the amplitude difference. The behaviour is qualitatively similar. However, it may be noted that the human ear is, generally, less sensitive to phase differences than to amplitude differences. Moreover, the role of subject-dependent phase differences — and hence the importance of compensating for phase differences — is less well-understood than the role of subject-dependent amplitude differences, as will be discussed below with reference to Fig. 9A-D.

[0118] Thus, Fig. 7A and 7B show that the described signal processing circuits achieve the goal of creating a sound at a reference position that corresponds closely to a reference signal.

[0119] Fig. 7C and 7D show, for amplitude difference and phase difference, respectively, that the changes in the unmodified output signal (i.e., the grey curves 702,712 where u = 0) seen in Fig. 7A and 7B are due mostly to the time delay of about 2.92 ms due to the distance between loudspeaker and microphone. For these graphs, the reference signal in the computation (xout- y) / 2 - xref(t + 2.92 ms) has been delayed with 2.92 ms. Again, the dashed curves 724,734 represent xrcf= xin; the solid black curves 726,736 represent xrcf= xout; and the dotted curves 728,738 represent xref= xext.

[0120] Fig. 7C clearly shows the amplitude compensation of the circuit in the black solid 724, dashed 726, and dotted 728 lines, which correspond for the main part to an inverted difference of the grey curve 702 in Fig. 7A. This shows that the circuit effectively corrects the difference, but that in this case the circuit predominantly compensates for the time delay due to the distance of 1 m. The same can be seen for the phase differences in Fig. 7B (grey line 712) compared to Fig. 7D (black solid 734, dashed 736, and dotted 738 lines) where the phase difference is mostly similar between these lines.

[0121] These two figures show that the circuit indeed corrects for changes in the signal. However, in this case, the time delay due to the distance between the microphone and loudspeaker dominates the correction signal from the circuit, which may effectively suppress any effects caused by interaction with the environment. To minimise these time-delay effects, the distance between output device and recording device may be minimized or otherwise compensated for.

[0122] Fig. 7E and 7F show the normalised amplitude difference and phase difference, respectively, for various relative configurations of loudspeaker and microphone(s). The curves represent the value y - xref, with xref= xin. The solid grey curves 742,752 represent a configuration with a single microphone at 1 m distance and u = 0; these curves represent the same measurement as shown in Fig. 7C and 7E (curves 722 and 732, respectively). The black dashed curves 744,754 represent a configuration with a single microphone at 0.0005 m distance and u = xin- y, and the black dotted curves 746,756 represent a configuration with two microphones configured as an environmental sound loudspeaker and u = xin- y. It can be seen that the modification of the input signal is no longer dominated by the time-delay effects, and that the circuit effectively compensates for the interaction with the environment. The last configuration gives the most true results, for both phase and amplitude.

[0123] Fig. 8 is a flowchart of a method according to an embodiment. A step 802 comprises receiving or generating an input signal xin. The input signal may represent an entrainment signal. Generating an input signal may comprise generating an analogue signal based on a (digitally or analogously) stored input signal. Generating an input signal may also comprise modifying a previously recorded sound, e.g., to generate a so-called soundscape or geometrical sound.

[0124] Many different (kinds of) signals can be used as entrainment signals. An aural entrainment signal typically has at least one main component that has one or more (usually all, or at least most) of the following characteristics: i) a steady oscillation at one or multiple frequencies in the audible frequency range of 20 Hz - 20 kHz, and / or rhythmic pulsations in the sub-audible frequency range of less than 20 Hz; ii) either no significant changes or steady and repetitive periodic changes in the frequency spectrum and / or rhythmic components of the signal for the duration of the signal; and / or a rise and decay of the frequency spectrum and rhythmic components of the signal with relatively steady periodicities; iii) a variability in the periodic changes in the frequency spectrum and / or rhythmic components of the signal, the variability preferably being between 0-200 ms; iv) tempered amplitude levels, e.g., amplitude levels of the signal being less than 85 dB and / or no substantial amplitude peaks; v) spaciousness, e.g., reverberation of the signal in an environment which may be a recorded environment, a simulated virtual environment and / or the playback environment itself; and vi) a duration of at least one minute, at least two minutes, or more typically, at least five minutes.

[0125] A step 804 comprises generating, in an environment, an output signal xout, based on the input signal xin. The output signal may be obtained by processing the input signal, e.g. as described in step 808. The output signal can be an acoustic signal or a different kind of signal, e.g., an electromagnetic signal. Generating the output signal may comprise providing a signal to an output device, e.g., a transducer or loudspeaker, for instance an omnidirectional loudspeaker. The environment may comprise a subject, typically a human subject.

[0126] A step 806 comprises recording a recorded signal y. The recorded signal represents the output signal having interacted with the environment. The recorded signal may be obtained using a recording device such as a microphone, e.g., an omnidirectional microphone.

[0127] A step 808 comprises modifying the output signal by adding a difference (xref- y) between a reference signal and the recorded signal to the input signal. The reference signal can be the input signal (xref= xin), the output signal (xref= xout), or an external reference signal (xref= xext). The difference may be an attenuated difference, such that xout= xin+ a (xref - y). If the reference signal is an external reference signal, the external reference signal may be based on the input signal, the output signal, or the recorded signal (at a different moment in time and / or in different circumstances). The external reference signal may be synchronised with the input signal.

[0128] The method may be applied in a therapeutic or non-therapeutic way. Fig. 9A-D show some experimental results correlating physical and mental subjectspecific responses to entrainment stimuli. A full description is provided in P. Oomen et al., ‘Deviations in sound waves associated with physiological and psychological states’ (forthcoming), which is hereby incorporated in its entirety by reference.

[0129] Sound waves are affected by the presence of a (human) subject in the acoustic environment. Deviations in sound waves are dependent on the position, posture and body mass of the subject, and show consistent variance dependent on the subject and the sound stimulus. Deviations in sound waves in the presence of a human subject were measured in an acoustic test environment, in conjunction with measurements reporting the physiological and psychological state of the subject. The acoustic measurements were performed using a set-up as described with reference to Figs. 4B-C and 5B.

[0130] The absorption or amplification of sound waves in response to the subject is directly proportional to the amplitude of the brain activity of the subject, where the amplification of both sound waves and brain activity is associated with increased positive valence and decreased negative valence experienced by the subject.

[0131] Trials were conducted with 15 participating subjects (labelled s1-15; 7 male, 8 female) who are 18 years or older, healthy, sober, proficient in English, non-pregnant with normal hearing and no history of neurological or psychiatric disease, e.g., no seizures, epilepsy and especially sound-induced epilepsy, nor claustrophobia or other issues that could interfere with the valid interpretation of results, and do not have an artificial heart pacemaker, cerebral shunt and / or prosthetic joints. Participants provided informed consent prior to enrolling in the experimental procedure.

[0132] Trials were conducted using a digital sound sample of a singing bowl as the audio input signal (AIS), with a fundamental frequency of 40 Hz (harmonic 1; hi) and the most prominent harmonics at 121 Hz (h2), 362 Hz (h3) 1008 Hz (h4), 1847 Hz (h5), 2849 Hz (h6), hereinafter also referred to as h1-6. The sound sample has a total decay time of 16.55 seconds. For each recorded trial, the sample is repeated 17 times, providing a total duration of ~5 minutes for each trial. The average measured sound pressure level (dBA) of the signal is -40.8 dBFS with a maximum peak level of -24.8 dBFS, with FS (Full Scale) = 0 dB.

[0133] The stimuli were provided to the subjects in a condition of a spatial sound projection, i.e. spatially distributed across 8 loudspeakers (integrated in the pod as shown in Fig. 5) and 6 bass transducer channels (integrated in the chair as shown in Fig. 4B). The spatial sound projections comprise the acoustic simulation of a sound source reflecting in an equilateral pyramid of small (2 m base width), medium (8 m base width) and large (18 m base width), labelled conditions C1-C3. The reflections are computed following absorption characteristics of limestone walls, with recursive absorption of the sound energy per octave bands: 67.5 Hz = 0.02; 125 Hz = 0.02; 250 Hz = 0.02; 500 Hz = 0.03; 1 kHz = 0.04; 2 kHz = 0.05; 4 kHz = 0.05; 8 kHz = 0.05; 16 kHz = 0.05. The acoustic simulation adds a generated total reverberation to the sound sample of ~2 s, ~8 s, and ~12 s, respectively for conditions C1- C3. The virtual sound source is positioned to be in the centre of a pyramid coinciding with the centre of the spherical pod. The resulting stimulus is an example of an entrainment signal.

[0134] The acoustic simulations were computed by a software configuration built in 4DSOUND v2.0 audio framework, wherein the audio signal is associated with a virtual sound source having a shape and a position relative to a centre, where in this case the centre is also the position of the subject. The virtual sound source is defined by a plurality of virtual points, comprising audio signal components determined by the positions of the associated virtual points such that the audio signal is perceived by a subject as originating from the virtual sound source having said shape and said position relative to the subject. For the acoustic simulations of the pyramids, a point source resolution of 770 points was used.

[0135] A reference recording (REF) is obtained while the spherical pod is empty, i.e. , without the presence of a human subject in the acoustic environment. The REF is chosen from 3 identically reproduced trials, after determining deviations of reference trials do not exceed the mean uncertainty of ± 0.1 dB amplitude difference and ± 1° phase shift.

[0136] Measurements were made when the subjects were exposed to the described stimulus and in a resting state (without acoustic stimulus, but otherwise identical). Both states were monitored for a duration of 5 minutes each. All measurements were taken while subjects were comfortably seated within the spherical pod, with their eyes closed, and their sternum located at the centre of the spherical pod and centred within the projected virtual sound source. All subjects were seated in an identical position supported by the chair and were requested not to move during the trial. The order of stimuli was randomly alternated between participants and recorded in a single blind paradigm. All trials took place on consecutive days between 10 a.m. - 6 p.m.

[0137] Electroencephalography (EEG) measurements address different aspects of cortical auditory processing. In order to achieve a viable balance between the time needed for EEG preparation and the achievable spatial resolution, EEG was recorded at 64 channels.

[0138] Before exposure to the sound stimuli, evoked responses following test tone presentation were recorded as a functional localiser of the auditory cortex. Pure tones at 500 Hz with a total duration of 100 ms, 3 ms cosine envelope function and repetition rate of 1 / .75 were played 200 times in randomized order from the left and right speakers on the horizontal plane at ear level.

[0139] Each EEG examination consisted of 2 blocks: test tones and stimuli. The stimuli consisted of: (i) base condition (Base) with the subject seated in a resting state with eyes closed in silence for a duration of 5 minutes. The resting state brain network was recorded to measure functional connectivity between the auditory cortex and possibly other regions of the brain; and, (ii) sound stimuli in the described condition.

[0140] In support of physiological measurements, a combination of questionnaires were provided to the subjects, prior to and immediately after exposure to the sound stimuli, to monitor emotional and psychological response. The POMS (Profile of Mood States) questionnaire includes 35 items (short version) and is used in many fields, for subjects 13 years of age and older, to understand the emotional state of the subjects that are analysed. POMS allows a quick assessment of transient, fluctuating feelings and enduring affect states, and is an applicable tool in medical research and athletic settings, where its sensitivity to change makes the assessment suitable for treatment, monitoring and evaluation, as well as clinical trials. MDMQ is an English version of the Multidimensional Mood State Questionnaire and includes 30 items with 5 answer categories. The original version is in German and has been published by Hogrefe Publishers (Steyer et al., 1997). Both the POMS and MDMQ questionnaires were administered in this same order prior to and immediately after exposure to the experimental procedure and sound stimuli.

[0141] Analysis of the audio data was performed as described above with reference to Fig. 6. All EEG analysis was conducted on the source level to localize brain activity in two paradigms: i) frequency analysis, and ii) statistical analysis. All EEG data were analysed in MATLAB using FieldTrip.

[0142] A Fourier analysis in the range from 4-100 Hz in steps of 2 Hz was computed by implementing a multitaper frequency transformation with Hanning tapers. The power and amplitude were then averaged across trials corresponding to each condition.

[0143] In order to assess significant differences among conditions, a cluster-based nonparametric permutation test with a cluster-alpha of 0.05 and minimum cluster size of 2 neighbouring channels was implemented. To investigate significant differences in amplitude, the mean amplitude of all subjects for each of the conditions were compared against each other as Stimulus - Base. These comparisons enabled the evaluation of significant differences between conditions in any given frequency bin between 4-100 Hz and for any group of EEG channels.

[0144] Statistical analysis was performed on all items of the POMS and MDMQ questionnaires for each participant in pre- and post-exposure to the sound stimuli. After determining items with significant differences in pre and post, these items were categorized according to their weighted position in a two-dimensional valence and arousal model. Statistical analysis was furthermore performed per category: negative valence; positive valence; low arousal; high arousal; after which the significant differences per category in pre and post were determined. In conjunction with the statistical analysis, the mean values for each subject were determined for all categories pre- and post-exposure to the sound stimuli, where all items from the questionnaires were weighted in relation to each of the categories.

[0145] Fig. 9A-C show the amplitude difference AD (dB) for the stimulus condition for conditions C1-C3, respectively, compared to the reference signal (REF), for each subject (s1 — 15) . The mean amplitude difference was AA = 3.61 dB (a = 2.491 , p = 0.00008) and was found to be significantly greater than the mean uncertainty. The maximum measured amplification was AA = 6.98 dB (s2, h3) and the maximum absorption was AA = -1.47 dB (s7, h2). For comparison, Fig. 9D and 9E show results of a similar trial with 6 subjects in a similar set-up, with a different stimulus. The phase difference (not shown here) shows similar behaviour, in the sense that the measured effects differ strongly per subject, and that different stimuli have significantly different responses.

[0146] This shows that the deviations for each condition are caused by a dynamic, non-linear interaction between the subject and the stimulus. Therefore, if the stimulus is adjusted (both in amplitude and phase) based on the response of the subject, with a circuit as described herein, this will, in turn cause a change in the response of the subject. This interaction is known as the entrainment principle.

[0147] Fig. 10A shows significant mean amplitude differences in brain activity when comparing the stimulus condition to the resting state (Base) for all subjects (s1— 15). The mean amplitude difference of Stimulus - Base was AA = 1.13 mV (a = 0.756, p = 0.03881) and was found to be significant for frequency bins in the Theta range (9: 5-8 Hz, AA = 2.42, <j = 0.004, p = 0.03598) and the Gamma range (y: 39-42 Hz, AA = 2.35, a = 0.004, p = 0.04197).

[0148] This shows that the sound stimuli have a measurable, significant effect on brain activity, in particular in the theta and gamma ranges.

[0149] Fig. 10B shows the means of significant items from both questionnaires relating to positive and negative valence pre- and post-exposure. The mean negative valence preexposure was found to be significantly greater than post-exposure (A / z = 0.85, a = 0.83, p = 0.00737). The mean positive valence pre-exposure was found to be significantly smaller than post-exposure (AJu = -0.83, a = 0.88, p = 0.01069). When comparing significant items relating to low and high arousal, the mean differences pre- and post-exposure were not found to be significant. (Low Arousal Pre - Post : A z = 0.16, o = 1.41 , p = 0.38429; High Arousal Pre - Post : A / z = 0.56, a = 1 .22, p = 0.11309).

[0150] This shows that the stimuli have a significant positive effect on the well-being of the subjects

[0151] Fig. 10C shows a linear regression fit of the mean amplitude difference (AA) of sound waves (in dB) vs brain activity (in mV) (left) and of the mean amplitude difference of high frequency sound waves vs Gamma brain activity (right). Each point represents an average over the 15 subjects for different pairs of conditions. The brain activity was measured over the whole gamma range (30-100 Hz)

[0152] The results obtained from the audio analysis were compared to the results obtained from the EEG analysis and the questionnaires for correlation across all significant measurements. A fairly strong correlation was found between the mean amplitude difference of sound waves for various stimulus conditions compared to the reference (REF) and the mean amplitude difference of brain activity for various stimulus conditions compared to the resting state (Base) as measured for all subjects (s1 - 15), as can be seen in the left graph (R = 0.80). A very strong correlation was found between the mean amplitude difference of high frequency sound waves (>1 kHz, h5-6) and the mean amplitude difference of brain activity in the Gamma range (y: 30-100 Hz), as can be observed in the right graph (R = 0.99).

[0153] A low to moderate correlation was found between the maximum sound absorption and negative valence per subject pre-exposure to the sound stimuli (R=0.47) and the mean amplitude difference and mean valence per subject pre-exposure to the sound stimuli (R = 0.44). No correlation was found between the phase shift of sound waves in response to the subjects, as obtained from the audio analysis, and the results obtained from the EEG analysis or questionnaires.

[0154] A strong correlation was found between the amplitude difference of sound waves, i.e., the amplification or absorption of the sound energy in response to the subjects, and the amplitude of the brain activity of the subjects. It has previously been shown that the maximum absorption is associated with the body mass of the subject, while the degree of either absorption or amplification per frequency showed consistent variance in response to the subject and changes in the stimulus. In the above-described experiments, it is observed that a greater amplification of sound waves in response to the subject is associated with greater amplification of the brain activity of the subjects.

[0155] With regards to the amplification of the brain activity of subjects, a significant increase in Gamma activity was found during exposure to the described entrainment sound stimulus compared to the resting state. Research studies reported that Gamma activity is associated with attention, working memory, and long-term memory processes. When the brain produces high levels of Gamma waves, this indicates a happier and more receptive state of the subject. Increased activity in the higher Gamma range may also be associated with active engagement, focus, creativity and positive mood states.

[0156] By adjusting the stimulus based on the subject-response, this effect can be further improved. A significant amplification was also found in Theta, and low- and middle-range Beta activity during exposure to the medium sized stimulus compared to resting state. Theta activity is thought to help coordinate neural activity between the regions of the brain that are involved in learning and memory, and also appears when subjects encounter emotional stimuli, which suggests a role in social cognition. An increase in Theta activity is indicative of a drowsy state prior to sleep onset, and deep meditative or hypnotic states. Increased activity in higher Theta waves is often associated with increased creativity, stress reduction, restfulness, and contentment.

[0157] The measured increase of the brain activity in respective frequency regions may indicate higher levels of awareness, attention and focus of the subjects, which are associated with a high level of arousal, while at the same time they may indicate relaxation, quiet and calm, which are associated with low arousal. As such, we observe contradictory effects with regards to the level of arousal of the subjects in response to the stimuli. These diverting levels of arousal may have been experienced alternating or simultaneously by subjects during exposure to the different sizes of the sound stimuli, and this is confirmed by the difference in amplitude of the brain activity in response to the size of the sound stimulus.

[0158] A shift towards a more calm, happy and receptive state as indicated by the amplified brain activity in response to the entrainment sound stimuli is reflected in the psychoemotional response reported by subjects. These results indicate a shift in the spectrum of emotions from more negative and less positive valence before exposure to the sound stimuli compared to less negative and more positive valence after exposure to the sound stimuli. A neutral level of arousal after exposure to the various stimuli was also reflected in the reported psycho-emotional response of the subjects. When comparing categories of items relating to high and low arousal, they were not found to be significantly different before and after exposure to the sound stimuli. Thus, the entrainment sound stimuli used in this experiment had an explicit effect on positive valence, i.e. , increasing pleasantness of emotions, and a less explicit, or more ambiguous effect on the level of arousal. This does not exclude that other sound stimuli could have significant effect on arousal;, however this was not observed in the described study.

[0159] Fig. 11A-D are graphs comparing effect on acoustic signals of a circuit according to an embodiment with an active noise-cancelling circuit. The graphs show simulated output of an entrainment circuit as shown in Fig. 1A (with a = 1) and an active noise-cancelling circuit, with the inputs being i) a pre-recorded audio input signal, in this case a 100 Hz sine wave, which was used as the input signal for both circuits; and ii) an acoustic noise source. The acoustic noise source is the reference signal of the active noise-cancelling circuit, and the pre-recorded audio input signal is the reference signal of the entrainment circuit. Fig. 11A shows the acoustic signals y as measured at a microphone in an environment of acoustic noise (solid line 1102) and acoustic noise with a 100 Hz sine wave as an added input signal (dotted line 1104) without the noise cancelling and entrainment circuits being active.

[0160] Fig. 11B shows the acoustic signals y of the same acoustic noise (solid line 1102), as well as the effect of the active noise-cancelling circuit (dotted line 1118) and the entrainment circuit (dashed line 1116). As can be seen, the noise-cancelling circuit effectively cancels acoustic noise as measured by the microphone, while the entrainment circuit has only minor effect on the acoustic noise signal and does not cancel the noise.

[0161] Fig. 11C shows the acoustic signals y of acoustic noise with a 100 Hz sine wave as an added input signal (solid line 1104), as well as the effect of the active noise-cancelling circuit (dotted line 1128) and the entrainment circuit (dashed line 1126). The noise-cancelling circuit used the acoustic noise signal as reference signal and the 100 Hz sine wave signal as (added) input signal. The entrainment signal used the combination of the acoustic noise signal and the 100 Hz sine wave signal as input and reference signal. As can be seen, the noise-cancelling circuit effectively cancels the acoustic noise at the area of the microphone, while the output of the 100 Hz sine wave is unchanged. The entrainment circuit, however, does not cancel the acoustic noise, but slightly changes the noise signal and significantly modifies the output of the 100 Hz sine wave.

[0162] The entrainment circuit modifies the output signal so that the acoustic signal becomes more similar to the reference (input) signal; in this example, the reference signal is a 100 Hz sine wave on top of an acoustic noise signal. Fig. 11 D shows that the acoustic output y at 100 Hz (triangle 1134) has an amplitude difference of ~ -6 dB and a phase shift of — 20.7° compared to the input signal x. Adding an entrainment circuit (circle 1136) effectively entrains the acoustic output, reducing the amplitude difference (+0.6 dB) and phase shift (-10.4°) of y compared to input signal x , while adding a noise-cancelling circuit (square 1138) does not entrain the output signal to the added input signal — in fact, the response of y is unchanged in amplitude and phase.

[0163] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and / or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present embodiments has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

Claims

CLAIMS1. An analogue signal processing module for processing an entrainment signal, the signal processing module comprising: a first signal input for receiving an input signal; a signal output for providing an output signal to an output device; a second signal input, different from the first signal input, for receiving a recorded signal, the recorded signal representing a recording of output generated by the output device in response to receiving the output signal; and signal processing circuitry configured to generate the output signal by adding a difference between a reference signal and the recorded signal to the input signal, the reference signal being one of: the output signal, the input signal, or an external signal, the external signal being a recording of the output generated by the output device in response to receiving the output signal and being synchronised with the input signal.

2. The signal processing module as claimed in claim 1, wherein the output signal represents an audio signal, and wherein the output device comprises one or more loudspeakers, and wherein the recorded signal is recorded by a microphone.

3. The signal processing module as claimed in any one of the preceding claims, wherein the input signal, the output signal, the reference signal, and the recorded signal are analogue signals.

4. The signal processing module as claimed in any one of the preceding claims, wherein the circuitry comprises: an inverter for inverting the recorded signal, a first adder for creating a difference signal by adding the inverted recorded signal and the reference signal, and a second adder for adding the difference signal and the input signal; or a first inverter for inverting the reference signal, a first adder for creating a difference signal by adding the recorded signal and the inverted reference signal, a second inverter for inverting the difference signal, and a second adder for adding the inverted difference signal and the input signal.

5. A system comprising: one or more signal processing modules as claimed in any one of claims 1-4; one or more output devices, each output device being connected to the signal output of a respective signal processing module; andone or more groups of one or more recording devices, each group of one or more recording devices being connected to the second signal input of the respective signal processing module; wherein the group of one or more recording devices connected to the same signal processing module as a given output device is positioned in close proximity to the given output device.

6. The system as claimed in claim 5, the system being configured: as a headphones comprising a set of earcups, wherein each earcup comprises one output device and one recording device on the inside of the earcup; or as a set of earbuds wherein each earcup comprises one output device and one recording device configured to be inserted into a wearer’s ear canal.

7. The system as claimed in claim 5, wherein each output device and each group of one or more recording devices that are connected to the same signal processing module are configured as an environmental sound loudspeaker.

8. The system as claimed in claim 5, wherein the one or more output devices and the one or more groups of one or more recording devices are configured to be worn on or to be in contact with a predetermined set of body parts, preferably the predetermined set of body parts corresponding at least approximately to one or more of: crown, left shoulder, right shoulder, left hand, right hand, sternum, chest bone, sacrum, left knee, and right knee, left foot, and right foot.

9. The system as claimed in any one of claims 5-8, further comprising a signal generator for generating an input signal, preferably the input signal being an entrainment signal, preferably the entrainment signal having at least one main component with one or more of the following properties: i) a steady oscillation at one or multiple frequencies in the audible frequency range of 20 Hz - 20 kHz, and / or rhythmic pulsations in the sub-audible frequency range of less than 20 Hz; ii) either no significant changes or steady and repetitive periodic changes in the frequency spectrum and / or rhythmic components of the signal for the duration of the signal; and / or a rise and decay of the frequency spectrum and rhythmic components of the signal with relatively steady periodicities;iii) a variability in the periodic changes in the frequency spectrum and / or rhythmic components of the signal, the variability preferably being between 0-200 ms; iv) amplitude levels of the signal being less than 85 dB; v) reverberation of the signal in an environment which may be a recorded environment, a simulated virtual environment and / or the playback environment itself; and vi) a duration of at least one minute, preferably at least two minutes, more preferably at least five minutes.

10. A method for processing a signal, the method comprising: receiving or generating an input signal, preferably the input signal representing an entrainment signal; generating, in an environment, an output signal, based on the input signal; recording a recorded signal, the recorded signal representing the output signal having interacted with the environment; and modifying the output signal by adding a difference between a reference signal and the recorded signal to the input signal, the reference signal being one of: the output signal, the input signal, or an external signal, the external signal being a recording of the output generated by the output device in response to receiving the output signal and being synchronised with the input signal.

11. The method as claimed in claim 10, wherein the output signal is an acoustic signal generated by one or more loudspeakers, and wherein the recorded signal is recorded by a microphone.

12. The method as claimed in claim 10, wherein the input signal is an entrainment signal, preferably the entrainment signal having one or more of the following properties: i) a steady oscillation at one or multiple frequencies in the audible frequency range of 20 Hz - 20 kHz, and / or rhythmic pulsations in the sub-audible frequency range of less than 20 Hz; ii) either no significant changes or steady and repetitive periodic changes in the frequency spectrum and / or rhythmic components of the signal for the duration of the signal; and / or a rise and decay of the frequency spectrum and rhythmic components of the signal with relatively steady periodicities; iii) a variability in the periodic changes in the frequency spectrum and / or rhythmic components of the signal, the variability preferably being between 0-200 ms; iv) amplitude levels of the signal being less than 85 dB;v) reverberation of the signal in an environment which may be a recorded environment, a simulated virtual environment and / or the playback environment itself; and vi) a duration of at least one minute, preferably at least two minutes, more preferably at least five minutes.

13. The method as defined in claim 10 or 12, wherein a subject is present in the environment, preferably the subject being an organism, more preferably a human, and wherein interaction with the environment comprises interaction with the subject.

14. The method as claimed in any one of claims 10-13, wherein the method is a non-therapeutic method.

15. The method as claimed in any one of claims 10-14, further comprising obtaining the reference signal, the obtaining of the reference signal comprising: receiving the input signal; generating, in a reference environment, a reference output signal, while the reference environment does not comprise the subject; and recording the reference signal, the recorded signal representing the reference output signal having interacted with the reference environment without the subject.

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