Decentralized damping for sound control systems

Abstract

The present invention is in the field of sound control within an open, partially open, or closed volume, with internal or external sound sources and which comprises an active sound control system. The fluid in the volume may be static or moving across the volume boundaries. Applications of the present invention include active control of fluid oscillations in ducted systems where fluid flow and density lead to fluid oscillation effects.

Description

[0001] P100963PC00

[0002] DECENTRALIZED DAMPING FOR SOUND CONTROL SYSTEMS

[0003] FIELD OF THE INVENTION

[0004] The present invention is in the field of sound control within an open, partially open, or closed volume, with internal or external sound sources and which comprises an active sound control system. The fluid in the volume may be static or moving across the volume boundaries. Applications of the present invention include active control of fluid oscillations in ducted systems where fluid flow and density lead to fluid oscillation effects.

[0005] RELATED APPLICATIONS

[0006] The present application claims the benefit of priority from Dutch Patent Application NL2038603, filed on September 9, 2024, in the name of Novio Sound B.V., The Netherlands.

[0007] The entire contents of the above-referenced applications and of all priority documents referenced in the Application Data Sheet filed herewith are hereby incorporated by reference for all purposes.

[0008] BACKGROUND OF THE INVENTION

[0009] The present invention relates to sound control. Sound is a considered to be a vibration that propagates as an acoustic wave, through a transmission medium such as a fluid, gas, liquid or solid. Humans and animals can perceive sound. Sound is the reception of such waves and their perception by the brain. Only acoustic waves that have frequencies lying between about 20 Hz and about 20 kHz, which is typically referred to as audio frequency range, can be perceived by humans. Different animal species have varying hearing ranges, for instance a dog is able to perceive sound in a range of 10Hz-35 kHz, and a bat even in arrange of 100Hz- 100 kHz.

[0010] Sound can propagate through a medium such as air, water or solids as longitudinal waves. A sound source creates vibrations in the surrounding medium. As the source continues to vibrate the medium, the vibrations propagate away from the source at the speed of sound, thus forming the sound wave. Sound pressure is the difference, in a given medium, between average local pressure and the pressure in the sound wave. Although the unit Pa could be used, typically the logarithmic unit dB is used. For the unit dB a reference sound pressure is used. Commonly used reference sound pressures, defined in the standard ANSI SI.1-1994, are 20 pPa in air and 1 pPa in water. For the present application said reference value is typically that of water or air, respectively.

[0011] Example applications of this sound control are noise reduction in rooms or ducts where the natural shape of the volume leads to frequency resonances or reverberations that amplify sounds and where the decentralised damping introduced via the actuator systems can significantly reduce sound amplitude at resonant frequencies. This may be used to shape sound in the volume to achieve a desired response, or may be used to reduce overall unwanted sound within the volume.

[0012] Exposure to unwanted sound or noise is associated with several negative health outcomes and, it is an objective, for this application, to reduce noise, in particular in fluid ducts. Noise is associated with hearing loss, high blood pressure, ischemic heart disease, sleep disturbances, injuries, decreased performance, annoyance, psychiatric disorders, and effects on psychosocial well-being. Therefore, noise exposure has increasingly been identified as a public health issue, especially in an occupational setting. Noise pollution is rated as the second most significant pollutant in Europe by the European Environmental Agency in 2023.

[0013] As mentioned, noise is unwanted sound. It is often perceived as unpleasant, for both humans and animals. Noise is not directly distinguishable from desired sound, as both relate to vibrations through a medium, such as air or water. In the present case however, when considering a duct for instance, any sound may be considered unwanted, and therefore considered as noise. The noise can typically be distributed over a frequency range. Acoustic noise is any sound in the acoustic domain, either deliberate, or accidental; in the present case mainly unintended.

[0014] A further example of this sound control in fluids is reduction of hammer in fluid based pipe networks. The decentralised damping using actuator systems may be used to remove energy in fluid systems so that the oscillating movement of the fluid causing the hammer effect may be damped or prevented. Feedback control around actuators have been used to improve linearisation of actuator response as due to actuator non-linearities as well as enclosure and room dynamics with the intention of providing controlled sound for normal listening which normally entails a flat response as perceived by the listener. A manner in which this has been achieved has been by frequency equalisation for overall response and with filters for reducing amplitude and phase distortion in cross over filters for multi actuator sound systems such as found in traditional moving coil loudspeakers.

[0015] Incidentally US 4,677,677 A and US 6,005,952 A recite sound systems. US 4,677,677 A recites an active acoustic attenuation system for attenuating an undesirable output acoustic wave by introducing a cancelling acoustic wave from an omnidirectional speaker at the output, and for adaptively compensating for feedback from the speaker to the input for both broad band and narrow band acoustic waves, without pre-training. The feedback path is modelled with a single filter model adaptively modelling the acoustic system on-line without dedicated off-line pre-training, and also adaptively modelling the feedback path from the speaker to the input microphone on-line for both broad band and narrow band acoustic waves without dedicated off-line pre-training, and outputting a correction signal to the speaker to introduce a cancelling acoustic wave. US 6,005,952 A recites a method and an arrangement for attenuating undesirable high amplitude output sound of an acoustic system 2 by injecting a cancelling sound produced by a controller 10 and radiated by a loudspeaker 12. The controller 10 has a nonlinear transfer behaviour between the control input 20 provided with the sensed input noise and the output 26 connected to the loudspeaker 12. The architecture of the nonlinear controller 10 is directly derived from an acoustic model describing the nonlinear sound propagation in the acoustic system 2. The control architecture can be implemented in a digital signal processor with a minimum of elements and improves the efficiency of the active attenuation so that the fundamental and the nonlinear distortions of the sound can be cancelled and the output noise level is lower than by using a linear control system.

[0016] The present invention relates in particular to an alternative active sound-control system and various aspects thereof which overcomes one or more of the above disadvantages, in particular in situations where high 'Q' factor at resonant frequencies can be difficult to control without jeopardizing functionality and advantages.

[0017] SUMMARY OF THE INVENTION

[0018] The present invention is in the field of sound control within an open, partially open, or closed volume, e.g. for a fluid-duct, wherein a duct is considered to relate to a volume in which radio or sound waves are confined to a restricted path with internal or external sound sources and which comprises an active sound control system comprising of a sound control computer programme that co-ordinates actuator systems, and sensor systems which may be physical or virtual. The sound control system uses actuators that are configured to absorb acoustic energy using decentralised damping of the actuator systems so that the resonance and reverberation within the volume are managed so that the performance quality and system robustness is achieved in the presence of environmental and component variations. The present invention relates in a first aspect to an active sound-control system, which in a preferred embodiment may be regarded as a multi-input-multi-output system, in a second aspect to a fluid-duct comprising such an active sound-control system, such as an air duct, and in a third aspect to an active sound-control computer program comprising instructions for operating such an active sound-control system, as well as to an according method, and in a fourth aspect to a novel use of transducer actuator systems comprising collocated sensors and actuator configured to provide decentralised feedback that absorbs acoustic energy from the sound field and dampens the acoustic response. The active sound-control system reduces unwanted sound significantly, such as noise. The present invention in particular relates to a use of a transducer pair comprising of a collocated sensor and actuator providing decentralised feedback (see e.g. fig. 3). Figure 01 represents a sound control system in a duct, and is considered applicable for any likewise volume. The system is enclosed in a volume. In this example an enclosed volume 101 is represented which may be open or closed at one or both ends. The one or more sound sources 102, typically not being part of the invention, represent any type of acoustic sound source. The sound source(s) may be internal to the enclosed volume (as shown) or may be external to the enclosed volume. An active acoustic array 105 senses and controls the sound between the sound source(s) and the error sensor(s). The controller 106 is electrically connected to the acoustic array, It may be external or internal to the enclosed volume 101 or may be placed independent of the duct. The performance monitoring sensing 104 system(s) may be contained within the enclosed volume or external to enclosed volume. Figure 02 shows an acoustic array in a simple application context, wherein one reference sensing system measures sound, which is input to the controller 106, and wherein the controller drives an actuator system 202. The acoustic array represented 105 may be generalised to include one or more reference sensing systems 201, one or more actuator systems 202, and one or more error sensing systems 203. The sensing systems may be physical or virtual. In some applications the volume occupied by the acoustic array may contain the sound sources and the error sensing system(s) as shown in the simulated example Figure 16. So, in a first aspect the present invention relates to an active sound-damping system 601 for a fluid-duct configured for receiving sound input and to damp said sound input, comprising an actuator system controller 501 and an actuator system 202 in functional connection with each other, wherein the actuator system controller 501 comprises a first signal loop, the first signal loop comprising a first signal adder 503 configured for receiving the sound signal input and for receiving internal sensor 402 signal input, and for proving summed signal output to an actuator 403 of an actuator system, and a second signal loop, wherein the second signal loop comprising a second signal adder 502 configured to receive a sound signal and to receive a near field sensor signal, and to provide an output signal to the first signal adder, in particular wherein the first signal loop is configured for increasing acoustic damping of the loudspeaker, in particular at a fundamental resonance frequency or frequencies thereof, , and wherein the second loop is configured for acoustic damping e.g. of the actuator 202 response in the duct, and wherein the actuator system 202 comprises the first internal sensor 402 for providing feedback to the first signal adder, an actuator 403, such as a speaker, configured for receiving signal input from the first signal adder, and a second sensor 404 for receiving signal input from the speaker and for providing feedback to the second signal adder, in particular wherein the actuator system is provided in an enclosure substantially enclosing the sensor(s) (see e.g. fig. 13). With the phrase “substantially enclosing” an enclosure of >90% is indicated, typically >95%, and sometimes 99-100%. The skilled person will understand than an adder can also subtract, that is, add negative signals, or vice versa. Details of loudspeaker design can e.g. be found in Small, IEEE Transactions on audio and electroacoustics, Vol. AU-19, No. 4, December 1971.

[0019] In a second aspect the present invention relates to an array 105 of active sound damping systems according to the invention, in particular a multiple input multiple output (MIMO) array. The MIMO may comprise two or more of the present sound damping systems, and independently thereof two or more (additional) sensors per present sound damping system, in particular 3-12 sensors per sound damping system, more in particular 6-10 sensors per sound damping system, and / or and independently thereof one or more (additional) actuators per present sound damping system, in particular 2-6 actuators per sound damping system, more in particular 3-5 actuators per sound damping system, such as depicted in fig. 17. The sensors each independently may be directional sensors or not. They sensors and actuators independently provided may be considered to be additional to the sensor(s) and actuator)s) of he present active sound damping system, such as those of fig. 3. The array may further comprise at least one directional sensor system 201,203, wherein the directional sensor system is located at at least one position upstream of the actuator system 202, and downstream of the actuator system, wherein a upstream sensor system is configured to provide at least one of feedforward input, feedback input, noise addition, noise cancelling and spectral shaping to an upstream active sound damping system, and wherein a downstream sensor system is configured to provide at least one of feedforward input, feedback input, noise addition, noise cancelling and spectral shaping to an downstream active sound damping system (see e.g. fig. 17). The terms “upstream” and “downstream” are taken in view of a primary sound source. If more sound sources are present the terms may be taken in view of any of the sound sources, preferably the primary sound source.

[0020] In a third aspect the present invention relates to a fluid duct 101 comprising the active sound damping system according to the invention or the array according to the invention (see e.g. fig. 16).

[0021] In general, design and robustness of a high performance optimal acoustic control system is challenging, in particular when the properties and dimensions of the enclosed volume and the properties of the fluid lead to strong resonances and reverberation. Factors include (amongst others) key attributes of the enclosure, such as size, surface, absorption, transmission, and reflection of sound at boundaries, as well as fluid related properties, such as pressure, pressure gradient, particle velocity, volume velocity, density, flow, viscosity, and humidity (where applicable). The resonances and reverberation^) may significantly extend the time domain response to transients requiring additional complexity and processing power. The resonances and reverberation(s), which extend transient responses, may apply equally to the primary sound sources and to the actuator systems when working together with sensing systems to control sound. Reducing the quality factor 'Q' of the acoustic system has the beneficial effects of damping the acoustic response of the system at the eigen-frequencies of the volume which reduces resonance and reverberation. The length of the time domain response to transient also reduces. A consequence of the reduced 'Q' factor is that an optimal solution may be achieved using fewer transfer functions, and the impulse responses of each transfer function will be shorter and require fewer hardware and software resources allowing a more efficient design and saving costs. The present invention is specifically dedicated to reducing said Q-factor. The Q-factor (or Quality-factor) is a measure of how good of a resonator a given system is. A high Q-factor means the system resonates strongly while a low Q-factor is the opposite. This invention describes a new method for reducing the overall system quality factor 'Q' at each resonant frequency based on decentralised damping. The proposed method achieves this by applying damping to a actuator system. The actuator system typically includes actuators and sensors that are co-located and configured to provide appropriate feedback signals to the actuator controller which determines the appropriate acoustic output from he actuator or actuators so that the 'Q' factor of resonances is reduced and the required damping characteristic and resulting sound control achieved. An example of a actuator system 202 and actuator controller 501 is shown in Figure 03. The actuator controller 501 adjusts the actuator 403 performance. The Internal sensor 401 enables feedback of the speaker response while the near field sensor 404 enables compensation for low frequency speaker performance. Figure 06 shows a typical sound controller configuration where W drives the actuator systems to achieve sound control. An example simulation, where the sound control is optimised for residual noise, is described below. Figure 18 and Figure 19a show the effect of absorbing energy and the solution has the following benefits:-the overall performance achieved is improved by adding the decentralised damping using this technique; the number of transfer function required to implement a noise control system can be reduced to a subset of a fully dimensioned control matrix W without compromising performance; the impulse response of the control filters in W may be implemented with fewer terms reducing computation resources. A power or likewise pressure level of the actuator may be adapted to a pressure level in the duct, hence larger and smaller actuators are envisaged. Output activates an actuator to reduce sound in a frequency domain of 10Hz-100 kHz. Ducts may have branches and may typically have multiple openings and closed ends and may include half closed ends. They may be irregular along the duct length. They may have irregular cross section and irregular 3D transitions between section. A duct may be a series of transitions between irregular 3D shapes (as in an ear canal). Also, conical or horn shaped structures where opening is much larger than average diameter are considered as duct. In the present invention transducers may be used. Transducers may be a moving coil, a balanced armature, a piezo-effect, a MEMS, an electrostatic, a thermo-acoustic, etc. An actuator element (typically) at the axial location may be implemented using an array of m actuators, such as in a XMEMs comprising 3 x 2 array in a single package. The array of m units may also be configured across a plane orthogonal to the axial di-rection or off the orthogonal plane to account for local structure, fixing and acoustic phasing optimisation. Actuators at different locations in an array may cover different frequencies. Actuator elements may include additional sensors where practical (causal micro-phones or local feedback microphones. Actuators may be directional or omni-directional. Examples of actuators are an omni-directi on spherical actuator (piezo electric spheres), a directional moving coil with back volume, a Mems with back volume, and a (Graphene) Thermo-acoustic without back volume. It is noted that the terms “actuator” and “transducer” may be used interchangeably. It is considered that a transducer isn’t always an actuator, whereas an actuator is always a transducer, so the terms are not fully interchangeable. Transducers are considered to transfer or convert energy, whereas an actuator is configured to move something. Likewise a loudspeaker is considered to convert electric energy to sound energy; it vibrates air, but doesn’t move it. An actuator would also convert electric energy to kinetic, and would move a valve. A sensor may be a transducer. Fig. 19b shows about 12 dB damping, and it shows a limitation at around 700 Hz where active damping would reduce the resonances and likely lead to improved performance. A typical duct system may have strong resonances. An example of a typical power to power spectral density curve (2010log(Verrormic / (10 V)) (-12- to -30 dB / Hz) versus frequency (0-1200 Hz) is shown in figure 19b. (top: Transfer function of duct system from noise source to and error microphone). The lower curve shows an example of typical and easily achievable reduction of approximately 12 dB using a simple single input single output controller, whereas the present invention provides a reduction of 18 dB, typically up to 20-25 dB, and for e.g. a small duct without air flow up to 34 dB. The figure shows a clear noise reduction at the first three duct eigenfrequencies (approximately at 100, 280, and 440 Hz). It also shows minimal reduction at the 4th and 5th eigenfrequencies 660 and 900 Hz. The controller implementation is in this exemplary embodiment unable to optimisation further due to the noise at 700Hz. The performance is therefore limited by the duct dynamics at 700 Hz. If the 4th duct eigen-frequency were reduced, then this specific limitation on the controller would be removed allowing further performance. This present invention offers such a method by using an active technique to dampen the natural frequency response of an open duct, which then allows improved controller performance.

[0022] The present invention goes beyond prior art measures by absorbing sound, so that e.g. specific resonant behaviour is reduced. This may increase distortion as perceived by the listener by exaggerating damping at key frequencies. This is useful for general sound control which is different to equalisation for normalised listening. It is noted that noise cancellation or reduction is a specific case of sound control. The reduction of resonances caused by the present invention is often not desirable for normal listening. In addition, e.g. the prior art for MIMO sound control and noise reduction relies heavily on IMC and regularisation where the presence of resonances and reverberation cause complications in the design process which limit the achievable performance. The present invention reduces the impact of those complications and improves the overall outcome.

[0023] Thereby the present invention provides a solution to one or more of the above-mentioned problems.

[0024] Advantages of the present description are detailed throughout the description. References to the figures are not limiting, and are only intended to guide the person skilled in the art through details of the present invention.

[0025] DETAILED DESCRIPTION OF THE INVENTION

[0026] The invention describes a general single input single output (SISO) system, which is also extended to a multiple input, multiple output (MIMO), acoustic system with sound controller. First a general model is introduced as background information to aid interpretation of the patent application. A three dimensional dynamic acoustic field will be established in any three dimensional open or closed volume where internal or external primary time varying primary sound sources are active. Acoustic field properties measured within the volume may fall into three groups: reference measurement points whose primary purpose is to provide sound field information; Error sensor measurement points where the performance of a system can be monitored directly and optionally used for example for system identification, feedback control or adaptive control; Performance measurement points which are only used to measure the resulting performance and which are not directly used in control process. As mentioned, sensor systems may be physical and / or virtual and may measure any acoustic field properties which may include pressure, pressure gradients, particle velocity, volume velocity and density. Sound within the volume may be controlled via actuators. A typical sound control system may include one or more sound sources, one or more reference sensing systems, one or more error sensing systems, and one or more performances monitoring systems, and one or more actuator systems. The positions of the sensing systems and actuator systems may arise from the application or may be optimised during the design process to support a sound control objective. Table 01 reflect a total number or each transducer system present in such a generalised sound control system. The table also defines variables used in this document. The primary acoustic paths represented in Figure 04 and the secondary acoustic paths represented in Figure 05 are represented in the sound system shown in Figure 06 which combines the acoustic paths F, and G, and the electrical signal paths W. The positions of the sensing systems and the actuator systems and the controller transfer functions may all be optimised to achieve the required sound control. Though it may not always be possible to place error sensing systems at the point where performance minimisation is required, performance monitoring systems may still estimate performance using a combination of physical and virtual performance monitoring systems. Each such system will have its own primary and secondary acoustic paths. Fpand Gp.

[0027] Feedforward Control

[0028] A feedforward controller W without decentralised feedback may be implemented using the system defined in Figure 06. Figure 08 shows an additional acoustic feedback path shown by Gxufrom actuator system output u to reference sensing system input x. The measurement of the primary sound source at px may be compromised when secondary sound sources are active.

[0029] Unwanted feedback may be removed by using a computer programme to predict the effect of all secondary sources on each reference sensing system, and to compensate for this effect. This additional electrical compensation signal path is represented in Figure 08 by the matrix of all compensation signals GAXu.

[0030] An exemplary application is for noise reduction at an error sensing position where a common method uses least means squared minimisation. The feedforward controller matrix (W) may then be designed for the required sound control by minimising the squared sum of the error signal 'e'.

[0031] In a sampled system let 'n' denote time and 'z' the unit shift operator, then the controller is defined by u(n) and the causal W that minimises the cost function J evaluated in the time domain. The equations are shown in Figure 11.

[0032] Adding Decentralised feedback to the Generalised Model

[0033] Figure 05 represents a secondary acoustic transfer function Geufrom the 'mthlactuator system to the 'jthlerror sensing system. This is represented in Figure 07 a.

[0034] The addition of feedback Hue from position pe to pu is shown in Figure 07 b. The effect of this feedback is to modify the transfer function Gejum to create a new transfer function G'ejum as shown in Figure 07 c. The relationships between the transfer function and input signal with and without feedback is shown in Figure 07 d.

[0035] The transfer functions may be generalised to represent feedback around all acoustic paths from actuator systems to sensing systems as shown in Figure 07 e.

[0036] The equations in Figure 07 d then generalise to those shown in Figure 07 f. The size of matrix G remains the same as for Figure 06. The feedback matrix H has size (K *J).

[0037] The feedforward sound control system shown in Figure 08 transforms to the system shown in Figure 10. The revised system variables and transfer functions are represented by prime variables and transfer functions. These transformations follow the same process used to determine the equation in Figure 07 d and are outlined in a to be published paper by amongst other A.P. Berkhoff. The matrices of the revised actuator system paths G to the error sensing systems are shown in Figure 09.

[0038] Elaboration of key components defined in Figure 01 and Figure 02.

[0039] Active Acoustic Array 105

[0040] Figure 01 shows an example system containing active acoustic array 105.

[0041] Figure 02 is a simplified example of an acoustic array 105 containing a single reference sensing system, a single error sensing system, and a single actuator system and a single sound source. The acoustic paths are represented by the arrows.

[0042] Such a system may be generalised to contain one or more reference sensing system^) 201, one or more actuator systems 202, and one or more error sensing systems 202, and where there are one or more sound sources 102. The elements 102, 201, 202, and 203 may be arranged in any configuration and spacing and in any order within the volume.

[0043] The acoustic array will normally be open to the environment in which it is placed however configurations that are partially open or enclosed with transducer ports may also be practical especially for a simple system.

[0044] The volume lOlmay be open, partially open or closed and will be application dependant.

[0045] Sensing Systems 201, 203, and 104 A sensing system is used for measuring sound field properties to be used for reference sensing, error sensing, and performance monitoring. A sensing system may be virtual or physical. A virtual sensing system uses a computer programme based on known design data or from a model that has been determined by a computer programme based on simulations or actual data measurements, and optionally optimised by a computer programme. A physical sensing system may sense one or more sound properties and optionally related fluid properties. Example measurements include pressure, differential pressure, particle velocity, volume velocity, fluid density, fluid viscosity, fluid temperature and humidity if relevant.

[0046] The use of directional information on the acoustic properties at selected points provides additional information to the controller on sound prorogation at any point within the volume 101. Sound waves are reflected by the volume boundaries. Error sensing systems 203, and reference sensing systems may be placed anywhere within the volume 101. An example of use of direction sensing systems is shown in Figure 16 where the geometry of the specific application has informed position of the differential sensing systems and is used to differentiate between the forward sound wave x2 and the reverse sound wave xl .

[0047] In a generalised application reference sensing systems 201 and error sensing systems 203 may include one or more sensor elements configured to detect directional sound in a mix of both planar and axial directions. A further example of such a more complex acoustic array is shown in Figure 16 and Figure 17.

[0048] Figure 12 shows a possible implementation of a directional sensing system 300 which may be used as the sensing system for implementing a reference sensing system 201, an error sensing system 203, or a performance sensing system 104. The figure shows two omnidirectional sensors 301 measuring pressure at points (p_{ 1 }) and (p_{2 }). In an even more general version an integration over the time may be used, which is computationally more expensive. It avoids a limitation of a sample period having to align with separation of the two mics and is more flexible (can change the sample rate without changing a physical arrangement of actuators and sensors. If the sensors 301 in a time sampled system are placed a sample distance apart then the acoustic wave travelling from (p_{2}) to (p_{ 1 }) may be cancelled or minimised by the subtraction (P_{ 1 }(n+l)>P_{2}(n)). The remaining measurement must then represent the acoustic wave travelling in the opposite direction from (p_{ 1 }) to (p_{2}). This implementation of a directional sensor attenuates low frequencies. Compensation using an integration filter could be used to ensure a flat frequency response.

[0049] Actuator System 202

[0050] Actuator systems 202 may include one or more actuators and one or more sensors configured to control sound across a range of frequencies to provide the required control as determined by the system application. An example of an actuator system 202 and the local decentralised actuator controller 501 was shown in Figure 03. The actuator 403 response is sensed by internal sensor 402 within the actuator enclosure 401, and by near field sensor 404 outside the actuator enclosure but within the near field of actuator 403.

[0051] Multiple actuators and sensor may optionally be used for each purpose when the desired sound control requires sound pressure levels across a wide frequency range. In such cases cross over filters (not shown) may be used with two or more actuators. The multiple actuators may also be directly driven by the Controller for Actuator System 501.

[0052] The enclosure 401 may be completely enclosed and air sealed or may include openings or ports which may be specifically designed to achieve specific acoustic properties.

[0053] Placing a sensor within the enclosure allows measurement of actuator system properties without needing to take account of the external sound pressure contribution from primary sources and other actuator systems. Such a sensor provides strongly correlated feedback to the Actuator Controller 501.

[0054] The near field sensor must be placed close enough to the sound actuator so that it can provide feedback on the frequency response of the transducer so that the actuator controller can compensate from frequency dependant behaviour of the transducer. In an example where the actuator is a typical moving coil loudspeaker then the near field sensor enables equalisation of the loudspeaker response below resonance.

[0055] Actuator Controller 501

[0056] The impact of adding feedback around the actuator to error sensing system path was shown in Figure 07 first by considering feedback around each acoustic path, and then how transformations due to the decentralised feedback Figure 09 affects sound control system behaviour in a feedforward sound control system Figure 10. This general case with decentralised feedback may be significantly simplified by constraining the feedback paths from all error sensing systems to all actuator systems to feedback paths from a single error sensing system to a single actuator system and requiring that each actuator system is associated with one error sensing system for the specific purpose of decentralised feedback control. With this simplification the feedback matrix H simplifies to a diagonal matrix.

[0057] An example is shown in Figure 16.

[0058] The acoustic transfer function from the actuator system to the sensing system may also be simplified by co-location of the near-field error sensor 404 within the near field of the actuator 403 system. This reduces the influence of all other acoustic paths since the local near-field error sensor 404 measurement is dominated by its near field actuator 403 allowing feedback of the actuator 403 performance.

[0059] These two simplifications allow for a decentralised actuator controller able to control the sound performance of the system by absorbing sound energy which dampens the system response. If the gain H for all the feedback paths is the same for all actuator systems and sensing system pairs, then the feedback matrix simplifies further to a H = I x h where I is the identify matrix and h is a scalar.

[0060] The design of such an actuator controller 501 may now be considered for the simplified diagonalised H.

[0061] The design of the controller 501 for a moving coil actuator is shown in detail in Figure 13. Balanced armature actuators, and emerging MEMs actuators can also be controlled using similar techniques with the blocks shown in Figure 13 adapted for the specific actuator. Similarly the sensors using in Figure 03 would be chosen to be appropriate for the actuator type.

[0062] Acoustic damping is added by feeding back near-field pressure to control a volume velocity source. In this example a loudspeaker actuator is used. Loudspeakers are not volume velocity sources and therefore the loop response should be modified.

[0063] Equalisation may be applied to the loudspeaker. It is assumed that the transfer function GU from input voltage u of the loudspeaker to volume velocity U can be measured. Figure 14. An equaliser HU based on a causal inverse of GU is implemented, i.e. HU = KG-1, U in which K is a constant, which has the objective that the transfer function from u to volume velocity U, i.e., GHU, does not depend on frequency.

[0064] The scheme including feedback is shown in Figure 14. If GHU is free from nonminimum phase behaviour and has a constant frequency response, then H may be implemented as a static gain.

[0065] For loudspeakers with voltage drive it can be difficult to obtain a suitable HU. In Figure 13 a different scheme is shown. It contains a transconductance amplifier gm to eliminate the influence of self inductance and resistance. The acceleration of the loudspeaker cone may be fed back and integrated to give the required volume velocity feedback. Feedback gain Ha dampens the undamped resonance. Alternatively, instead of the acceleration of the loudspeaker the pressure pb 402 inside the volume enclosing the rear of the loudspeaker can be used, as shown in Figure 03.

[0066] Low frequency compensation may be added to extend the low frequency response. For a moving coil loudspeaker a Linkwitz correction filter is preferably used.

[0067] The near field pressure is fed back for active acoustic damping, which, in addition to the feedback gain Hp, uses a modified control loop. The modified control loop contains a differentiator because the loudspeaker now does not work as a volume velocity source but as a volume acceleration source except for the low frequencies as defined by the low frequency compensation filter. This is considered similar to simple amplification or gain. The Ha and Hp refers to the gains for the acceleration feedback (inner loop) and the pressure feedback (outer loop).

[0068] The control loop therefore contains an additional filter (1 / HPF) to correct for the combined high-pass behaviour of the low frequency compensation filter and the loudspeaker. In practice the amplification at low frequencies and high frequencies has to be limited to a finite value beyond certain frequencies.

[0069] For an example moving coil loudspeaker the final feedback filter for combining the differentiator and the compensator for the high-pass filter (1 / HPF) with limited amplification may be written as shown in Figure 15 in which s is the Laplace-domain variable, a is the highest angular frequency of the open loop gain, y is the lowest angular frequency of the open loop gain, T' is the time constant of the desired loudspeaker defined by the Linkwitz filter, and Q't is the total damping of the desired loudspeaker according to the Linkwitz filter.

[0070] The use of current drive with feedback to control the fundamental resonance may lead to a relatively stable system, which, for example, is insensitive to temperature of the voice coil or non-linear effects resulting from displacement dependent force factor and damping, and therefore may lead to more predictable behaviour than voltage drive without feedback.

[0071] In an exemplary embodiment of the present active sound damping system the at least one audio sensor is capable of receiving audio-signals at a frequency of 5-100000 Hz, or at least parts of said range, such as in view of a certain application, specific parts in said range.

[0072] In an exemplary embodiment of the present active sound damping system the at least one actuator, e.g. at least one transducer, is capable of providing audio-signals at a frequency of 5-100000 Hz, or at least parts of said range, such as in view of a certain application, specific parts in said range.

[0073] In an exemplary embodiment of the present active sound damping system the at least one sensor each individually is configured to sample at a sample frequency of 100Hz- 100MHz, in particular of IkHz-l MHz, more in particular of 5-500 kHz.

[0074] In an exemplary embodiment of the present active sound damping system the at least one actuator each individually is configured to provide active sound damping at a damping frequency of 20 Hz-500 kHz, in particular 50 Hz-200 kHz, more in particular 100 Hz-100 kHz, such as 1-50 kHz, e.g. 10-20 kHz.

[0075] In an exemplary embodiment of the present active sound damping system the at least one actuator each individually is configured to provide a sound pressure of 20-150 dB, in particular of 30-120 dB.

[0076] In an exemplary embodiment of the present active sound damping system the transducer is selected from a MEMS, a moving coil, a permanent magnet transducer, a balanced armature transducer, a thermo-acoustic device, and a piezo-element.

[0077] In an exemplary embodiment of the present active sound damping system the second signal adder provides an output signal to the first signal adder through a low frequency compensator 511 (see e.g. fig. 13). In an exemplary embodiment of the present active sound damping system the first internal sensor 402 provides an input signal to a first differentiator 514, wherein the first differentiator provides an input signal to the first signal adder, in particular wherein the first differentiator provides an input signal to the first signal adder via a gain Ha 513, in particular wherein the gain Ha is configured for critical damping of a lowest resonance frequency of the actuator system 202.

[0078] In an exemplary embodiment of the present active sound damping system the second sensor 404 provides signal input to a second differentiator 517, wherein the second differentiator provides signal input to the second signal adder, in particular wherein the second differentiator provides input to the second signal adder via a finite gain filter at both high and low frequencies, wherein a high frequency in particular is 20-100 kHz, and wherein a low frequency in particular is 10-400 Hz, more in particular wherein the finite gain filter comprises an inverse High Pass filter (1 / HPF 516), and a high pass filter (Hp 515).

[0079] In an exemplary embodiment the present active sound damping system comprises at least one performance monitoring system 104 configured to be downstream of the actuator system in the duct and to receive acoustic input from the actuator system.

[0080] In an exemplary embodiment the present active sound damping system comprises an acoustic error sensing system 203 which is selected from a physical error correction system, and a virtual error sensing system, such as a computer implemented program, and a combination thereof, in particular wherein said acoustic error sensing system 203 is configured to measure an acoustic error, to predict an acoustic error, and to perceive an acoustic error.

[0081] In an exemplary embodiment the present active sound damping system comprises one controller 501 per actuator system 202 for providing localized adjustment of the actuator performance.

[0082] In an exemplary embodiment the present active sound damping system comprises an error sensing system 203 [fig. 17 uses this as well) downstream of the actuator system for providing feedback to the actuator system 202 and or an optional reference sensing system 201 [see e.g. figs. 2&17). Non directional sensing systems may also be used with this decentralised damping control.

[0083] In an exemplary embodiment of the present active sound damping system the controller 602 is configured for identification of a sound signal and for adaptation for said sound signal (see e.g. fig. 13).

[0084] In an exemplary embodiment of the present active sound damping in an axial array, each individually, each array element is provided with at least one sensor and at least one actuator, respectively, or wherein in an axial array, each individually, 50-99% of array elements is provided with a sensor and an actuator, respectively, in particular 80-95% of array element, such as in an asymmetric provision. Typically more sensors than actuators are provided, such as 1.5-5 times as many. It is noted that 80-95% implies mostly fully configured with sensors and actuators. The term “axial” in general relates amongst others to actuators in the plane (like in the side a T pocket). An actuator(s) and sensor(s) (such as 402, 404) configuration may be combined at each location in the axial array as speaker (actuator) and / or microphone (sensor). Therewith an array may be largely or nearly fully populated with sensors and actuators, respectively.

[0085] In an exemplary embodiment the present active sound damping may comprise 2-10 axial arrays.

[0086] In an exemplary embodiment of the present array each directional sensor system comprises at least two sensors, wherein each second sensor is downstream of each first sensor [see e.g. fig. 12], wherein the directional sensor system is configured for sensing a direction of a sound wave, in particular wherein the directional sensor system is provided in an enclosure substantially enclosing the sensors or outside said enclosure.

[0087] In an exemplary embodiment the present array comprises a controller system (106) for controlling the array of actuator systems, in particular for controlling the acoustic array 105, the at least one acoustic error sensing system 203, and the at least one performance monitoring system 104.

[0088] In an exemplary embodiment of the present array the acoustic array is configured to sense and control the sound between the said at least one sound source and the at least one acoustic error sensing system, in particular wherein the acoustic array, the at least one acoustic error sensing system and the sound source are provided within one physical body enclosing a volume, and / or wherein the controller is electrically connected to the acoustic array, and / or wherein the controller is provided external or internal to the duct, in particular independent of the duct, in particular wherein the at least one acoustic error sensing system is provided external or internal to the duct.

[0089] In an exemplary embodiment the present fluid duct comprises at least one sound source 102.

[0090] In an exemplary embodiment of the present fluid duct the duct is selected from a duct that is open at two opposite sides thereof, a duct that is open at one side thereof, a duct that comprises at least one acoustic port, and a duct that is closed.

[0091] In an exemplary embodiment the present fluid duct the cross-section of the duct is substantially tubular, substantially rectangular, substantially multigonal, or a combination thereof.

[0092] In an exemplary embodiment the present fluid duct the duct is selected from a heating system, a cooling system, a ventilation system, an air condition system, an air supply system, an exhaust system, an air intake system, a particle collector system, such as a dust collector, an engine, such as an air craft engine, and an intra ear canal, and combinations thereof, such as an air intake of an engine.

[0093] The invention is further detailed by the accompanying figures and examples, which are exemplary and explanatory of nature and are not limiting the scope of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims.

[0094] SUMMARY OF FIGURES

[0095] Figures 1-23 show details of technical features.

[0096] DETAILED DESCRIPTION OF FIGURES

[0097] Details of the figures are at least partly given above.

[0098] Further also tables 1-3 are included in figure 23.

[0099] The figures are further detailed in the description.

[0100] The invention although described in detailed explanatory context may be best understood in conjunction with the accompanying figures.

[0101] Experimental results

[0102] The distributed damping control was demonstrated with feedforward control in an example configuration and the performance contribution of the decentralised control and feedforward control measured and summed.

[0103] Configuration for simulation in a duct

[0104] The duct configuration of Figure 16 was used to simulate the addition of decentralised damping and the subsequent use of feedforward control. The distances used for placement of the acoustic elements is summarised in Table 02.

[0105] The following parameters were used: RO=-O.75,RL=O.9, speed of sound (c) = 343 m / s, density (p) = 1.21 kg / m3. The length of the duct is 1.37m. The sampling frequency for feedforward control is 2 kHz. Regularization was set at P = 10'2, which is normalized. The feedback gain is H = 7 • 10-6.

[0106] The connection of the acoustic elements to the controller is shown in Figure 17.

[0107] Directional sensors

[0108] The error sensors (e = (e_{4), e_{ 5 }, e_{6})T ) are directional sensors which measure the pressure constituent in a specific direction. The reference signals (x = (x_{l}, x_{2}, x_{3} )T )also measure the pressure in a specific direction. The direction is shown in Figure 16, above the symbols (x_{l}, x_{2}, x_{3} ) and ( e_{4}, e_{5 }, e_{6}. ) In this example the directional sensors use two microphones, with pressure signals (p_{ 1 }) and (p_{2}). The microphones are spaced a sample distance apart. The wave travelling from (p_{2}) to (p_{ 1 }) is determined by the subtraction (p_{ 1 }(n + 1) - p_{2 } (n).) The error signal then consists of waves travelling from (p_{ 1 }) to (p_{2 }). This operation attenuates the low frequencies. An integration filter was used to ensure that the frequency response is flat.

[0109] Damping by decentralized feedback

[0110] In this example sources (s_{ 1 }) and (s_{2 }) are used for decentralized feedback. There is no benefit for using (s_{3 }). The transfer functions from the primary sources to the three error sensors (e_{4},e_{ 5 },e_{6}) are shown in Figure 18 with (dashed line), and without (solid line), distributed feedback control. These graphs show the effect of the damping in reducing resonance. Both curves show the primary paths without any feedforward control. Although it may be connected there is no drive signal from the controller 602 to the second adder 502. W is in fact not connecting inputs to outputs.

[0111] The transfer function from the primary source (s) to the performance sensor (e_{p}) is shown in Figure 19. The performance achieved at the epsensor due to the decentralised feedback is a 4.9 dB reduction.

[0112] Reduction at the performance sensor using feedforward control

[0113] The feed forward controller was designed using four independent Gaussian random data sets.

[0114] The signals were filtered with a first-order low pass filter having a cut off frequency at half the Nyquist frequency. Initially, all three sources (s_{ 1 },s_{2},s_{3 }) were used. The control coefficients are shown in Figure 20. Figure 21 shows the transfer functions from the three actuators to the error sensors. The solid lines show the transfer functions without decentralised feedback control. The dashed lines show the transfer function with the decentralised feedback control enabled. The benefit of the decentralised feedback on damping the transfer functions from actuators to reference sensors is evident.

[0115] The feedforward controller (W)was designed by minimizing the squared sum of the error signal (e) where (n) denotes time and (z) the unit shift operator. The controller output is defined by (u(n)). The equations are shown in Figure 11.

[0116] The decentralised feedback performance improvement measured at epwas 4.9dB. The feedforward controller achieves a performance improvement of 20.9 dB of reduction. The combined achieved performance from the decentralised feedback control and the feedforward control is a 25.8 dB reduction.

[0117] In Figure 20 it can be seen that several control filters have very small coefficients. A selection was made of the control filters. A total of three filters were used instead of nine, using the combinations {si, x2}, {sl, x3} and {s2, x2}. As with feedback control for damping, only si and s2 were used. In this case the performance at ep is 4.9 dB reduction for feedback and 23.2 dB for feedforward, which combined gives 28.1 dB reduction. Therefore, by reducing the number of the control filters the performance increases. The explanation is that sources between e5, e6 and ep leads to increased noise at ep because in this case the noise reduction at ep depends on the noise reduction controlled at e5,e6, which is only exact if there is no other source between e5, e6 and ep.

[0118] The noise reductions are collected in Table 03.

[0119] The impact of the feedback control in damping the transfer response of the system to error sensing systems e4,e5,e6 is shown in Figure 21 and show for ep in Figure 22.

[0120] It should be appreciated that for commercial application it may be preferable to use one or more variations of the present system, which would be similar to the ones disclosed in the present application and are within the spirit of the invention.

Claims

CLAIMS1. An active sound-damping system (601) for a fluid-duct configured for receiving sound input and to damp said sound input, comprising an actuator system controller (501) and an actuator system (202) in functional connection with each other, wherein the actuator system controller (501) comprises a first signal loop, the first signal loop comprising a first signal adder (503) configured for receiving the sound signal input and for receiving internal sensor (402) signal input, and for providing summed signal output to an actuator (403) of an actuator system, and a second signal loop, wherein the second signal loop comprising a second signal adder (502) configured to receive a sound signal and to receive a near field sensor signal (404), and to provide an output signal to the first signal adder, in particular wherein the first signal loop is configured for increasing acoustic damping, and wherein the second loop is configured for acoustic damping in the duct, and wherein the actuator system (202) comprises the first internal sensor (402) for providing feedback to the first signal adder, an actuator (403), such as a speaker, configured for receiving signal input from the first signal adder, within an actuator enclosure (401), and a second sensor (404) for receiving signal input from the speaker and for providing feedback to the second signal adder, in particular wherein the actuator system is provided in an enclosure substantially enclosing the sensors.

2. The active sound-damping system according to claim 1, wherein the second signal adder provides an output signal to the first signal adder through a low frequency compensator (511).

3. The active sound-damping system according to any of claims 1-2, wherein the first internal sensor (402) provides an input signal to a first differentiator (514), wherein the first differentiator provides an input signal to the first signal adder, in particular wherein the first differentiator provides an input signal to the first signal adder via a gain Ha (513), in particular wherein the gain Ha is configured for critical damping of a lowest resonance frequency of the actuator system (202).

4. The active sound-damping system according to any of claims 1-3, wherein the second sensor (404) provides signal input to a second differentiator (517), wherein the second differentiator provides signal input to the second signal adder, in particular wherein the second differentiator provides input to the second signal adder via a finite gain filter at both high and low frequencies, wherein a high frequency in particular is 20-100 kHz, and wherein a low frequency in particular is 10-400 Hz, more in particular wherein the finite gain filter comprises an inverse High Pass filter (1 / HPF 516), and a gain (Hp 515).

5. The active sound-damping system according to any of claims 1-4, comprising at least one performance monitoring system (104) configured to be downstream of the actuator system in the duct and to receive acoustic input from the actuator system.

6. The active sound-damping system according to any of claims 1-5, comprising an acousticerror sensing system (203) which is selected from a physical error sensing system, and a virtual error sensing system, such as a computer implemented program, and a combination thereof, in particular wherein said acoustic error sensing system (203) is configured to measure an acoustic error, to predict an acoustic error, and to perceive an acoustic error.

7. The active sound-damping system according to any of claims 1-6, comprising one controller per actuator system for providing localized adjustment of the actuator performance.

8. The active sound-damping system according to any of claims 1-7, comprising an error sensing system (203) downstream of the actuator system for providing feedback to the actuator system (202) and or an optional reference sensing system (201).

9. The active sound-damping system according to any of claims 1-8, wherein the controller is configured for identification of a sound signal and for adaptation for said sound signal.

10. An array (105) of active sound damping systems according to any of claims 1-9, in particular a multiple input multiple output (MIMO) array, wherein the MIMO comprises two or more of the present sound damping systems, and independently thereof two or more (additional) sensors per present sound damping system, and / or and independently thereof comprises one or more actuators per present sound damping system, and / or in particular further comprising at least one directional sensor system (201,203), wherein the directional sensor system is located at at least one position upstream of the actuator system (202), and downstream of the actuator system, wherein a upstream sensor system is configured to provide at least one of feedforward input, feedback input, noise addition, noise cancelling and spectral shaping to a downstream active sound damping system, and wherein a downstream sensor system is configured to provide at least one of feedforward input, feedback input, noise addition, noise cancelling and spectral shaping to an upstream active sound damping system.

11. The array according to claim 10, wherein each directional sensor system comprises at least two sensors, wherein each second sensor is downstream of each first sensor, wherein the directional sensor system is configured for sensing a direction of a sound wave, in particular wherein the directional sensor system is provided in an enclosure substantially enclosing the sensors.

12. The array according to any of claims 10-11, comprising a controller system (106) for controlling the array of actuator systems, in particular for controlling the acoustic array (105), the at least one acoustic error sensing system (203), and the at least one performance monitoring system (104).

13. The array according to any of claims 10-12, wherein the acoustic array is configured to sense and control the sound between the said at least one sound source and the at least one acoustic error sensing system, in particular wherein the acoustic array, the at least one acoustic error sensing system and the sound source are provided within one physical body enclosing a volume.

14. The array according to any of claims 10-13, wherein the controller (106) is electrically connected to the acoustic array, and / orwherein the controller is provided external or internal to the duct, in particular independent of the duct, in particular wherein the at least one acoustic error sensing system is provided external or internal to the duct.

15. A fluid duct (101) comprising the active sound damping system according to any of claims 1-9 or the array according to any of claims 10-14.

16. The fluid duct according to claim 15, comprising at least one sound source (102).

17. The fluid duct according to any of claims 15-16, wherein the duct is selected from a duct that is open at two opposite sides thereof, a duct that is open at one side thereof, a duct that comprises at least one acoustic port, and a duct that is closed.

18. The fluid duct according to any of claims 15-17, wherein a cross-section of the duct is substantially tubular substantially rectangular, substantially multigonal, or a combination thereof.

19. The fluid duct according to any of claims 15-18, wherein the duct is selected from a heat- ing system, a cooling system, a ventilation system, an air condition system, an air supply system, an exhaust system, an air intake system, a particle collector system, such as a dust collector, an engine, such as an aircraft engine, and an intra ear canal.

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