Multilayer interference tube microphone with acoustic metamaterials

Abstract

An acoustic device comprising an interference tube (2) with a tube section (AMM1) and a flange (AMM2) connected to the tube section (AMM1), the interference tube (2) comprising a first acoustic metamaterial for enhancing sound directivity, and the flange (AMM2) comprising a second acoustic metamaterial configured to transform on-axis waves travelling in the tube section (AMM1) in such a way that they recombine coherently.

Description

MULTILAYER INTERFERENCE TUBE MICROPHONE WITH ACOUSTIC METAMATERIALSTECHNICAL FIELDThe present disclosure generally pertains to the field of audio technology and sound engineering.TECHNICAL BACKGROUNDAcoustic devices transport sound, convert sound signals into electrical signals, process these electrical signals, convert electrical signals into sound. Microphones, for example, are devices that convert sound waves into electrical signals. They are used to capture sound and transmit it to other devices such as amplifiers, speakers, and recording equipment. The diaphragm of a microphone vibrates when it is exposed to sound waves, which in turn generates an electrical signal that corresponds to the sound wave.There are different types of microphones, including dynamic microphones and condenser microphones, which work according to different principles. A specific type of microphone, called superdirectional, or interference tube microphone, aims at providing high directivity. It is also known as a shotgun microphone because of its long, narrow shape that resembles a shotgun barrel. A superdirectional microphone is highly sensitive to sounds coming from a specific direction. It is used to capture sound from a specific direction while minimizing the sound from other directions.Superdirectional microphones are used to pick up sounds at a long distance. The superdirectional microphone has a high directivity and can pick up sounds in front of the microphone while cancelling out surrounding sounds.Superdirectional microphones are commonly used in film and television production, where they are mounted on boom poles and pointed at the actors to capture their dialogue while minimizing background noise. They are also used in live sound reinforcement, where they can be aimed at specific instruments or performers on stage to isolate their sound from other sources.A problem with prior art superdirectional microphones is that their gain in directivity of the microphone increases with the length of the aperture (i.e., interference tube). A longer aperture leads to higher directivity and effective performance at even lower frequencies. However, in applications where the microphone’s dimensions may interfere with other sensors, this inevitably limits the maximum dimensions (particularly with respect to length) and thus the maximum possible directivity gain of the interference tube. For example, for a camera microphone mountedclose to the camera lens, a long interference tube is not acceptable, although it would still benefit from high directivity.Another aspect that causes problems with known microphones is the coloration of waves impinging off-axis. This often leads to unnatural-sounding results, when aiming towards professional level recording, and such microphones are therefore often a last resort and not necessarily a preferred option.Although there exist techniques for increasing the directivity of a microphone, it is generally desirable to improve on the techniques for enhancing directivity of a microphone.SUMMARYAccording to an aspect, the disclosure provides an acoustic device comprising an interference tube with a tube section and a flange connected to the tube section, the interference tube comprising an acoustic metamaterial for enhancing sound directivity, and the flange comprising an acoustic metamaterial configured to transform on-axis waves travelling in the tube section in such a way that they recombine coherently.Further aspects are set forth in the dependent claims, the drawings and the following description.BRIEF DESCRIPTION OF THE DRAWINGSEmbodiments are explained by way of example with respect to the accompanying drawings, in which:Fig. 1 shows an example of an interference tube microphone;Figs. 2a-c show an interference tube provided with a plurality of openings in its side wall, providing directionality;Fig. 3 schematically shows, in a longitudinal sectional view, an interference tube microphone using acoustic metamaterial for enhancing sound directivity;Fig. 4 shows, in a perspective view, an embodiment of an interference tube microphone corresponding to the principle described in Fig. 3;In Fig. 5 shows a further perspective view of the interference tube microphone of Fig. 4;Fig. 6 shows in more detail the cylindrical shells of the tube section AMM1 of the interference tube microphone of Fig. 4;Fig. 7 shows in more detail the flange AMM2, which connects the tube section AMM1 to the microphone capsule and which guides the sound waves to the microphone capsule;Fig. 8 shows in more detail the rear end of the flange AMM2, which connects to the microphone capsule;Fig. 9 shows in more detail the front end of the flange AMM2, which connects to the tube section AMM1;Fig. 10 shows an exemplifying design of an interference tube with a uniform spacing between the shells of the tube section AMM1;Fig. 1 la shows an exemplifying design of a flange AMM2 with a uniform spacing between the shells;Fig. 1 lb shows the exemplifying design of the flange AMM2 of Fig. 1 la in a front view;Figs. 12a, b, and c schematically describe a process of determining helically revolving slopes of a flange AMM2;Fig. 13 shows in a diagram the directivity expected with an interference tube with uniform spacing as shown in Figs. 10, and 1 la, b;Fig. 14 shows an exemplifying design of an interference tube with a non-uniform shell spacing;Fig. 15 shows in a diagram the directivity expected with an interference tube with non-uniform shell spacing as shown in Fig. 14;Fig. 16 shows a design option of an interference tube with non-uniform shell spacing, in which the tube section AMM1 is configured shorter but wider than in the example of Fig. 14;Fig. 17 shows in a diagram the directivity expected with the interference tube of Fig. 16 with non-uniform shell spacing and shorter but wider design;Fig. 18 shows a variant of the design option of Fig. 16 with a non-uniform shell length in the tube section AMM1;Fig. 19 shows in a diagram the directivity expected with the interference tube of Fig. 18 with non-uniform shell length;Figs. 20a and 20b compare the on-axis sound pressure level expected with the interference tube of Fig. 18 with non-uniform shell length with the sound pressure level expected with the interference tube of Fig. 16 with uniform shell length;Fig. 21a shows the truncation of air channels and slopes in the flange AMM2 according to an embodiment;Fig. 21b shows a cross-section through a 3D representation of flange AMM2 of Fig. 21a;Fig. 21c shows an enlarged view of the part of Fig. 21a in which the gaps are located (highlighted in Fig. 21a with the rectangle with rounded corners);Fig. 22a, b, and c schematically describe a process of determining helically revolving slopes of the flange AMM2 of Figs. 21a,b,c;Fig. 23 shows an enlarged view of the centre portion of flange AMM2 of Figs. 21a,b,c; andFig. 24 shows the concept of staggered cells in a cross-sectional view of the interference tube.DETAILED DESCRIPTION OF EMBODIMENTSBefore a detailed description of the embodiments under reference of Fig. 1 is given, general explanations are made.The embodiments disclose an acoustic device comprising an interference tube with a tube section and a flange connected to the tube section, the interference tube comprising a first acoustic metamaterial for enhancing sound directivity, and the flange comprises a second acoustic metamaterial configured to transform on-axis waves travelling in the tube section in such a way that they recombine coherently.The tube section may for example comprise an acoustic metamaterial with high spatial dispersion, with an emphasis on enhanced transmission in on-axis direction and strong attenuation from interference for waves travelling in off-axis directions.According to some embodiments, the tube section comprises multiple cylindrical shells of different diameter with openings. These openings may allow multi scattering and interference within the tube section for an impinging wave.The cylindrical shells of different diameter may for example be arranged along the radial axis of the interference tube. This may allow multi scattering along the radial axis that generates a significantly higher amount of interference for waves impinging onto the tube section at off-axis angles than a single interference tube.According to an embodiment, the interference tube has a uniform spacing between the shells of the tube section.According to another embodiment, the interference tube has a non-uniform spacing between the shells of the tube section. A non-uniform spacing of the shells in the tube section may permit for a more even distribution of acoustic energy between the shells for waves impinging in the target direction.The flange may for example be configured to transform the on-axis waves travelling between the shells in such a way that they recombine coherently before a capsule, i.e the sensor, of the acoustic device.The flange may serve to ensure a coherent transmission of the on-axis wave impinging through the various channels (defined by the shells) between the cylindrical shells when they are then redirected towards the capsule. In that way, a potential pathlength difference cannot lead to destructive interference for the on-axis wave before the capsule.The flange may be designed through space-coiling by artificially equalising the pathlength for the various channels while aiming to preserve impedance matching and avoiding reflection of the wave at the entrance of the flange.The diameter of the tube section may be larger than that of the capsule of the acoustic device. A larger diameter may yield an even higher directivity as it increases the aperture surface of the tube section, permitting even more interference.According to some embodiments, the flange comprises coaxial conical shells between which the sound propagates.The flange may be configured so that the effective speed of sound within the various channels is adjusted such that the waves recombine in-phase at the end of the flange.According to some embodiments, the flange comprises a number of helically revolving slopes that artificially create a longer than the straight path for the sound waves to reach the end of the flange.According to an embodiment, the length of the interference tube including the tube section and the flange is 75 mm and the radius of the tube section is 35 mm.According to some embodiments, the length of the shells in the tube section is non-uniform. For example, the shells may be increasingly shorter towards the centre of the tube section.The acoustic device may comprise resonators (e.g. absorbers, metamaterials, loudspeakers, headphones, microphones, etc.).According to some embodiments, the acoustic device is a microphone, for example a superdirectional microphone.Superdirectional microphoneThe embodiments described below disclose superdirectional microphones that are used to capture sound from a specific direction while minimizing the sound from other directions. The embodiments achieve high directivity without having to resort to a microphone array.Fig. 1 shows an example of an interference tube microphone. The interference tube microphone is mounted on a microphone stand 4 by means of a microphone clip 5. The interference tube microphone combines a directional sensor (e.g. a cardioid, a super- or hypercardioid) with a narrow and elongated columnar interference tube 2 extending from the capsule. The interference tube microphone converts sound waves into electrical signals. A cable 7 is connected via an XLR connector 6 to the interference tube microphone. The electrical signals are sent via a cable 7 to an external device (not shown in Fig. 1) that receives the electrical signals.The directivity of the interference tube microphone is achieved through the use of interference tube 2, which is a long, narrow tube with openings 3 (slots) cut into it. The openings 3 allow sound waves to enter the interference tube 2 from the front, while blocking sound waves that arrive from other directions. The sound waves that enter the tube 2 are then combined in such a way that they interfere with each other, resulting in a highly directional pickup pattern. The interference tube 2 works by creating an acoustic labyrinth that yields the lobar polar pattern. The interference tube concentrates the polar pattern of the microphone to a very narrow acceptance angle. The wanted on-axis sound passes straight down the length of the tube to the capsule diaphragm unimpeded, while the unwanted off-axis sound has to reach the diaphragm by entering the side slots.As shown in Figs. 2a-c, an interference tube 2 extending from a diaphragm 1 is provided with a plurality of openings 3 in its side wall, providing directionality in which the microphone is highly sensitive to a sound coming from its front and along the centre line of the tube 2, or the opposite side of the diaphragm 1.To be more specific, as shown in Fig. 2a, acoustic waves coming from ahead of the microphone (the right-hand in the figure) have the same path length to the diaphragm 1, regardless of whether they arrive at it from the top of the interference tube 2 or any one of the openings 3, so that they arrive in the same phase to be added together.In contrast, as shown in Fig. 2b, an acoustic wave coming from a side of the interference tube 2 through different openings 3 differ in phase because their path lengths from the through-holes, or incident positions, to the diaphragm 1 are different.Likewise, as shown in Fig. 2c, an acoustic wave coming to the microphone from a rearward direction arrives via different openings 3 at the diaphragm 1, causing a phase difference in the acoustic wave, or an incident signal. A plurality of openings 3 in the interference tube 2 are arranged so that incident acoustic signals weaken each other.That is, the interference tube is designed in such a way that, for a wave impinging on-axis, the wave reaches the microphone almost undisturbed, i.e., as if the tube were not there. But when a wave impinges from an off-axis direction, it enters the tube at different places with different phase, and that naturally leads to interference within the tube and at the sensor. Since it is usually not given that this interference is constructive, the consequence is that the resulting pressure at the sensor is lower than for a wave impinging on-axis. This causes off-axis sounds to cancel out as they reach the diaphragm with varying amounts of phase shift. Thus, the interference tube increases the directivity of the microphone.Interference tube microphone with acoustic metamaterial (AMM)The embodiments described below in more detail disclose an interference tube microphone that provides an acoustic metamaterial (AMM) that, combined with a suitable acoustic sensor, yields a directivity comparable to or higher than that of known superdirectional microphones, without the need for multiple sensors and achieving high directivity through beamforming.Fig. 3 schematically shows, in a longitudinal sectional view, an interference tube microphone using an acoustic metamaterial (AMM) for enhancing sound directivity. The interference tube comprises a section AMM1 that generates an acoustic metamaterial with high spatial dispersion, with an emphasis on enhanced transmission in on-axis direction and strong attenuation from interference for waves travelling in off-axis directions. The section AMM1 of the tube is obtained by creating multiple cylindrical shells 11-1, 11-2, ..., 11-5 with suitably sized and distributed openings 3 that allow multi scattering and interference within the section AMM1 for an impinging wave. With cylindrical shells 11-1, 11-2, ..., 11-5, the interference tube has multiple layers along the radial axis. These layers generate an acoustic metamaterial (AMM) with multi scattering along the radial axis that generates a significantly higher amount of interference for waves impinging onto section AMM1 at off-axis angles than a single interference tube.A flange AMM2 is configured to transform the on-axis waves travelling between the shells 11-1, 11-2, ..., 11-5 in such a way that they recombine coherently immediately before the capsule. The flange AMM2 serves to ensure a coherent transmission of the impinging on-axis wave through the various channels between the cylindrical shells 11-1, 11-2, ..., 11-5 when they are redirectedtowards the microphone capsule 1, that is of smaller diameter than the tube section AMM1. In that way, a potential pathlength difference cannot lead to destructive interference for the on-axis wave before the capsule. The flange AMM2 can for example be designed through space-coiling by artificially equalising the pathlength for the various channels while aiming to preserve impedance matching and avoiding reflection of the wave at the entrance of AMM2. Any other processes known to the skilled person may likewise be used.The length L of the interference tube including the flange AMM2 may, for example, be 10 cm. The embodiments are, however, not restricted to this example value.The diameter D of the tube section AMM1 may be much larger than that of the microphone capsule 1. A larger diameter D yields an even higher directivity as it increases the aperture surface of AMM1, permitting even more interference. It can be expected that the diameter and the length of section AMM1 enable a trade-off, so that the section AMM1 can be made shorter, if required, but to preserve the directivity may need to be made with a larger diameter.Fig. 4 shows, in a perspective view, an embodiment of an interference tube microphone corresponding to the principle described in Fig. 3. The tube section AMM1 with cylindrical shells 11-1, H-2, ..., 11-5 is visible. At the rear, the flange AMM2 and the microphone capsule 1 (i.e., microphone) are visible.Figs. 5, 6, 7, and 8 show four further perspective views of the interference tube microphone of Fig. 4.In Fig. 5, the interference tube section AMM1 with the cylindrical shells 11-1, 11-2, ..., 11-5 and the flange AMM2 at the rear end of the tube which guides the sound waves to the microphone capsule (1 in Figs. 2a, b, c) are visible.Fig. 6 shows in more detail the cylindrical shells 11-1, 11-2, ..., 11-5 of the tube section AMM1. As shown in Fig. 6, the interference tube has a uniform spacing between the shells of the tube section (AMM1).Fig. 7 shows in more detail the flange AMM2 which connects the tube section AMM1 to the microphone capsule and which guides the sound waves to the microphone capsule.Fig. 8 shows in more detail the rear end of the flange AMM2 which connects the flange to the microphone capsule.Fig. 9 shows in more detail the front end of the flange AMM2 which connects to the tube section AMM1. The arrows indicate sound waves that are guided within the cylindrical shells 11-1, 11- 2, ..., 11-5 of the tube section AMM1 to the entry of the flange AMM2. The flange AMM2serves to ensure a coherent transmission of the impinging on-axis waves through the various channels between the cylindrical shells 11-1, 11-2, ..., 11-5 when they are redirected towards the microphone capsule 1. In that way, a potential pathlength difference cannot lead to destructive interference for the on-axis wave before the capsule. The flange AMM2 can be designed through space-coiling by artificially equalising the pathlength for the various channels while aiming to preserve impedance matching and avoiding reflection of the wave at the entrance of AMM2.According to this principle, the embodiments provide an interference tube microphone with high directivity and short aperture, making the device easier to wield and increasing its usability and potential interference with other sensors. High directivity is provided without having to resort to a microphone array.The tube section AMMl and flange AMM2 can be optimised not only with respect to their directivity but also to minimise coloration for off-axis sound. As such, the embodiments improve the usability, especially for the professional recording sector, but also for the amateur user.Fig. 10 shows an exemplifying design of an interference tube with uniform spacing between the shells of the tube section AMMl. In this example, the tube section AMMl has a diameter of 51.5 mm and a length of 70 mm. The thickness awof the walls that define the shells is 1.5 mm. The gap gsbetween shells is 3.5 mm. The number Nsof shells is 5. The angle acdefined by two circumferentially adjacent openings 3 is 22.5 degree, i.e., in each shell wall there are sixteen openings 3 arranged in circumferential direction around the whole tube.With the parameters given above, the outer radius Rnof each respective shell (depending on shell index ri) can be determined according to:The distance of two adjacent openings 3 in circumferential direction is chosen to be equal to the length of each opening 3 in circumferential direction. Accordingly, the size (area) of each opening 3 (depending on the shell index ri) is given by: ws2tr7?3602crcwhere the width wsof each opening 3 in the longitudinal direction of the tube section AMM1 is chosen to be 5 mm. That is, the size of openings 3 increases proportional with the outer radius Rnof each respective shell.Fig. I la shows an exemplifying design of the flange AMM2 with a uniform spacing between the shells. In this example, the flange AMM2 consists of a number Nsof 5 conical shells 13-1, 13-2, . . . 13-5 between which the sound propagates. To achieve a good performance, the speed of sound within the various channels is adjusted such that the waves recombine in-phase at the microphone end of the flange (the narrow end). This is achieved by introducing a number (e.g., here, 5) of helically revolving slopes 12-1, . . ., 12-5 that artificially create a longer than the straight path for the waves to reach the end of the flange.In the example of Fig. I la, the parameters of the flange and the helically revolving slopes are as follows: the bottom radius of AMM2 (interface with AMM1) is 25 mm and the top radius, rmic(interface with microphone capsule) is chosen to be 5 mm. The distance D from bottom to top interface is 27 mm. The gap gmbetween the top interface and the capsule diaphragm is 3 mm.The coordinates xns, yn s, zn sof the respective core sections of the helically revolving slope with index n are given by equation (A):^n,s (D 9 m) where dnRnRn+1 and where Rnis the outer radius of each respective shell, and where the outermost slope has index 1, G [0,1] and s2G [0,1] are the parameters expanding the surface.The length of the helically revolving slopes xns, yns, zn sas defined in equation (A) above is chosen such that the slopes transform on-axis waves travelling in the tube section in such a way that they recombine coherently at the location of the capsule.A typical set of parameters for the revolution angles is:<p1= 0 rad, (f>2= 0.66 rad, <p3= 1.76 rad,4= 2.68 rad, <p3= 3.11 radIn the embodiment of Fig. I la, there is provided a set of helically revolving slopes at each octant.Fig. 1 lb shows the exemplifying design of the flange of Fig. 1 la in a front view.Figs. 12a, b, and c schematically describe a process of determining helically revolving slopes 12- 1, . . . , 12-5 of the flange AMM2. Fig. 12a shows the curves (center lines) of the core sections of four helically revolving slopes as given by equation (A) above. A sweep of a given boundary along these curves of Fig. 12a provides a set of four helically revolving slopes. These are shown in Fig. 12b. This set of four helically revolving slopes is replicated at 3 further angles. In the example of Fig. 12c this results in a set of four helically revolving slopes for each of the replication angles.Fig. 13 shows in a diagram the directivity expected with an interference tube with uniform spacing as shown in Figs. 10, and 1 la, b as determined by simulation. The vertical axis shows the directivity index in the range from 1 to 7.5. The horizontal axis shows sound frequency in Hz from 0 to 10000 Hz. The directivity is shown for three different scenarios. A first scenario relates to an omnidirectional microphone capsule. A second scenario relates to a velocity type scenario, i.e. the result is for a sensor that is sensitive to the particle velocity, i.e. a 1storder directivity (figure of eight) receiver. A third scenario relates to a cardioid microphone capsule. For frequencies above 2500 Hz, the directivity is substantially the same for all three scenarios and the directivity is linearly increasing with frequency. At low frequencies, the scenarios “velocity” and “cardioid” exhibit a bump, whereas the “omni” scenario approaches zero directivity, i.e. a directivity index of one, in a substantially asymptotic way.Fig. 14 shows an exemplifying design of an interference tube with a non-uniform shell spacing. A non-uniform spacing of the shells in tube section AMM1 (and necessarily also in flange AMM2), where the cross-sectional area in between two shells is quasi-uniform for all channels, permits for a more even distribution of acoustic energy between the shells for waves impinging in the target direction. The energy density should then preferably be the same per layer. Accordingly, the thickness of the layers therefore decreases towards the outside. It was found that this improves the directivity of the acoustic device in the mid frequency region, where the exact frequency region in which the improvement occurs depends on the dimensions of the tube section AMM1.Fig. 15 shows in a diagram the directivity expected with an interference tube with non-uniform shell spacing as shown in Fig. 14. The vertical axis shows the directivity index in the range from 1 to 8. The horizontal axis shows sound frequency in Hz from 0 to 10000 Hz. As in Fig. 13, the directivity is shown for the three different scenarios “omni”, “velocity”, and “cardioid”. As compared to the result in Fig. 13, the directivity of the acoustic device increases in the mid frequency region between approximately 2000 Hz and 7000 Hz.Fig. 16 shows a design option of an interference tube with non-uniform shell spacing, in which the tube section AMM1 is configured shorter but wider than in the example of Fig. 14. The length of the overall interference tube including the tube section AMM1 and the flange AMM2 is 75 mm. The radius R of the tube section AMM1 is 35 mm. By reducing the length and increasing the radius of AMM1 (and AMM2 accordingly), the overall structure may be reduced in size, to the effect that the aperture surface of the tube section AMM1 is the same yet even larger than before. This was found to lead to an even larger directivity index, i.e., better directivity of the design.Fig. 17 shows in a diagram the directivity expected with the interference tube of Fig. 16 with non-uniform shell spacing and shorter but wider design. The vertical axis shows the directivity index in the range from 1 to 12. The horizontal axis shows sound frequency in Hz from 0 to 10000 Hz. As in Fig. 15, the directivity is shown for the three different scenarios “omni”, “velocity”, and “cardioid”. As compared to the result in Fig. 15, the directivity index of the acoustic device is increased to 12 at 10000 Hz as compared to the directivity index of around 7.5 in the case of the result in Fig. 15 for the interference tube of Fig. 14.Fig. 18 shows a variant of the design option of Fig. 16 with a non-uniform shell length in the tube section AMM1. In the following, this design variant with a non-uniform shell length is called “staggered cells”. As in the example of Fig. 16, the length of the overall interference tube including the tube section AMM1 and the flange AMM2 is 75 mm, and the radius R of the tube section AMM1 is 35 mm. The shells are increasingly shorter towards the centre of the tube.Fig. 19 shows in a diagram the directivity expected with the interference tube of Fig. 18 with non-uniform shell length (“staggered cells”). The vertical axis shows the directivity index in the range from 1 to 12. The horizontal axis shows sound frequency in Hz from 0 to 10000 Hz. As in Figs. 13, 15, and 17, the directivity is shown for the three different scenarios “omni”, “velocity”, and “cardioid”. The result of the interference tube of Fig. 18 with non-uniform shell length (and with non-uniform shell spacing and shorter but wider design as in Fig. 16) is substantially the same as the result in Fig. 17 with non-uniform shell spacing and shorter but wider design but uniform shell length. As a minor difference, for interference tube of Fig. 18 with non-uniform shell length there is a plateau visible in the directivity for frequencies between 8000 and 9000 Hz. At around 2000 Hz there is merely a light increase of the directivity index for both the velocity (figure of eight) and the cardioid sensor.Figs. 20a and 20b compare the on-axis sound pressure level expected with the interference tube of Fi g. 18 with non-uniform shell length (“staggered cells”) with the sound pressure levelexpected with the interference tube of Fig. 16 with uniform shell length. Fig. 20a shows the sound pressure level (SPL) expected with the interference tube of Fig. 16 with uniform shell length. The vertical axis shows the sound pressure level (SPL) in the longitudinal direction of the tube (phi = 0, i.e. on-axis) in the range from 94 to 134 dB. The horizontal axis shows sound frequency in Hz from 0 to 10000 Hz. The sound pressure level (SPL) is shown for the three different scenarios “omni”, “velocity”, and “cardioid”. Fig. 20b shows the sound pressure level (SPL) expected with the interference tube of Fig. 18 with non-uniform shell length (“staggered cells”). The vertical axis shows the sound pressure level (SPL) in the longitudinal direction of the tube (phi = 0, i.e. on-axis) in the range from 94 to 128 dB. The horizontal axis shows sound frequency in Hz from 0 to 10000 Hz. The sound pressure level (SPL) is shown for the three different scenarios “omni”, “velocity”, and “cardioid”. Permitting for a given design to have non-uniform shell length was found to be beneficial for a smoother on-axis response. As can be seen in Fig. 20a, for uniform shell length the fluctuations of the sound pressure level for frequency range between 1000 Hz and 4000 hz are rather strong. As can be seen in the highlighted part of Fig. 20b, for a non-uniform shell length the fluctuations of the sound pressure level in the frequency range between 1000 Hz and 4000 hz are significantly reduced. Still further, the non-uniform shell length design also improves the performance of higher-order microphone capsules (“velocity” and “cardioid”).In the following it is described an alternative design of an interference tube with non-uniform spacing between shells of the tube section AMM1.In this alternative embodiment, the tube section AMM1 is designed according to the general principle described in the embodiment of Fig. 10. Here, the tube section AMM1 has an outer radius As of 36.5 mm and a length of 45 mm. The thickness awof the walls that define the shells is 1.5 mm. The number Nsof shells is 5. The angle acdefined by two circumferentially adjacent openings 3 is 22.5 degree, i.e., in each shell wall there are sixteen openings 3 arranged in circumferential direction around the whole tube.With the parameters given above, the outer radius Rnof each respective shell (centre line, with respect to shell’s thickness) can be determined according to:The distance of two adjacent openings 3 in circumferential direction is chosen to be equal to the length of each opening 3 in circumferential direction. Accordingly, the size (area) of each opening 3 (depending on the shell index ri) is given by: ws27r / ?n360 2acwhere the width wsof each opening 3 in the longitudinal direction of the tube section AMM1 is chosen to be 5 mm. That is, the size of openings 3 increases proportional with the outer radius Rnof each respective shell.The flange AMM2 is designed according to the principle of described in Fig. I la and b. In the example described here, the flange has a length of 30 mm. The flange AMM2 consists of a number Nsof 5 conical shells 13-1, 13-2, . . . 13-5 between which the sound propagates. To achieve a good performance, the speed of sound within the various channels is adjusted such that the waves recombine in-phase at the microphone end of the flange (the narrow end). This is achieved by introducing a number (e.g., here, 5 helically revolving slopes 12-1, . . . , 12-5 that artificially create a longer than the straight path for the waves to reach the end of the flange.The parameters of the flange and the helically revolving slopes are as follows: the bottom radius of AMM2 (interface with AMM1) is 36.5 mm and the top radius, rmic(interface with microphone capsule) is chosen to be 5 mm.The coordinates xns, yn s, zn sof the respective core sections of the helically revolving slope with index n are given by equation (B):where dn— RnR n+1 and where Rnis the outer radius of each respective shell, and where the outermost slope has index 1, E [0,1] and s2E [0,1] are the parameters expanding the surface.The parameter xTE(typical -30 mm) denotes the negative distance of the large end of the flange to the diaphragm, and xttpsdenotes the negative distance of the diaphragm to the end of the conical section of the flange, where a typical value is -2.6 mm.The range of the parametersand s2is generally from 0 to 1. However, the parameterdetermines the length of the slopes, and for wall thicknesses larger than one, it may be convenient to limitto values smaller than 1.The length of the helically revolving slopes xns, yn s, zn sas defined in equation (B) above is chosen such that the slopes transform on-axis waves travelling in the tube section in such a way that they recombine coherently at the location of the capsule.A typical set of parameters for the revolution angles is:< >! = 0 rad, <p2= 0.66 rad, (f>3= 1.76 rad, (f>4= 2.68 rad, (f>5= 3.11 radFig. 21a shows the truncation of air channels and slopes in the flange AMM2 according to this embodiment. In the embodiment described here, the section of the air channels with the slopes is truncated to ensure a targeted gap width, wgap, for which a typical value is 950 um. This parameter also determines the truncation point, xgap n. xgap nindicates where to truncate the slopes and to separate the air channel. For each shell n = 0 . . . Ns- 1, xgap nis given aswhereandFor the innermost channel n = Ns, the gap location is defined through a different equation, namely:Fig. 21b shows a cross-section through a 3D representation of flange AMM2 of Fig. 21a.Fig. 21c shows an enlarged view of the part of Fig. 21a in which the gaps are located (highlighted in Fig. 21a with the rectangle with rounded corners).Fig. 22a, b, and c schematically describe a process of determining helically revolving slopes of the flange AMM2 of Figs. 21a,b,c. Fig. 22a shows five helically revolving slopes as given by equation (B) above. A sweep of these boundaries around the flange’s axis provides a set of five helically revolving slopes that focus sound at a position of a diaphragm. These slopes are shown in Fig. 22b. This set of five helically revolving slopes is replicated at three further angles. As shown in Fig. 22c this results in a set of five helically revolving slopes for each of the four angles.It has proven useful to divide the innermost slope into two parts and sweep them separately. That way, it can be avoided that the thickness of the slope near the central axis of the flange becomes very small. This may make the device easier to manufacture.Fig. 23 shows an enlarged view of the centre portion of flange AMM2 of Figs. 21a,b,c.Fig. 24 shows the concept of staggered cells in a cross-sectional view of the interference tube. According to the design option of staggered cells, the length of the shells is staggered at the open end of the interference tube AMM1. The fluctuations of the sound pressure level as observed in Fig. 20a (“ripple”) is generated (in part) by the reflection of the outgoing wave at the entrance of the tube section AMM1 due to impedance change. As can be seen in Fig. 20b above, the design option with staggered cells provides a more gradual impedance change that reduces the reflection and thus the ripple.Note that the present technology can also be configured as described below.[1] An acoustic device comprising an interference tube (2) with a tube section (AMM1) and a flange (AMM2) connected to the tube section (AMM1), the interference tube (2) comprising a first acoustic metamaterial for enhancing sound directivity, and the flange (AMM2) comprising a second acoustic metamaterial configured to transform on-axis waves travelling in the tube section (AMM1) in such a way that they recombine coherently.[2] The acoustic device of [1], the tube section (AMM1) comprising an acoustic metamaterial with high spatial dispersion, with an emphasis on enhanced transmission in on-axis direction and strong attenuation from interference for waves travelling in off-axis directions.[3] The acoustic device of [1], the tube section (AMM1) comprising multiple cylindrical shells (11-1, 11-2, ..., 11-5) with openings (3).[4] The acoustic device of [1], the cylindrical shells (11-1, 11-2, ..., 11-5) being arranged along the radial axis of the interference tube (2).[5] The acoustic device of [3] or [4], the interference tube (2) having a uniform spacing between the shells (11-1, 11-2, ..., 11-5) of the tube section (AMM1).[6] The acoustic device of [3] or [4], the interference tube (2) having a non-uniform spacing between the shells (11-1, 11-2, ..., 11-5) of the tube section (AMM1).[7] The acoustic device of [1], the flange (AMM2) being configured to transform the on-axis waves travelling between the shells (11-1, 11-2, ..., 11-5) in such a way that they recombine coherently before a capsule (1) of the acoustic device.[8] The acoustic device of [7], the flange (AMM2) serving to ensure a coherent transmission of the on-axis wave impinging through the various channels between the cylindrical shells (11-1, 11-2, ..., 11-5) when they are redirected towards the capsule (1).[9] The acoustic device of [1], the flange (AMM2) being designed through space-coiling by artificially equalising the pathlength for the various channels while aiming to preserve impedance matching and avoiding reflection of the wave at the entrance of the flange (AMM2).

[0010] The acoustic device of any one of [7] to [9], the diameter (D) of the tube section (AMM1) being larger than that of the capsule (1) of the acoustic device.

[0011] The acoustic device of any one of [1] to

[0010] , the flange (AMM2) comprising coaxial conical shells (13-1, 13-2, ... 13-5) between which the sound propagates.

[0012] The device of any one of [1] to

[0011] , the flange (AMM2) being configured so that the effective speed of sound within the various channels is adjusted such that the waves recombine in-phase at the end of the flange (AMM2).

[0013] The acoustic device of any one of [1] to

[0012] , the flange (AMM2) comprising a number of helically revolving slopes (12-1, . . . , 12-5) that artificially create a longer than the straight path for the sound waves to reach the end of the flange (AMM2).

[0014] The acoustic device of any one of [1] to

[0013] , the length of the interference tube including the tube section (AMM1) and the flange (AMM2) being 75 mm and the radius (R) of the tube section (AMM1) being 35 mm.

[0015] The acoustic device of any one of [3] to

[0014] , the length of the shells (11-1, 11-2, ..., 11- 5) in the tube section (AMM1) being non-uniform.

[0016] The acoustic device of

[0015] , the shells (11-1, 11-2, ..., 11-5) being increasingly shorter towards the centre of the tube section (AMM1).

[0017] The acoustic device of any one of [1] to

[0016] , the acoustic device comprising resonators.

[0018] The acoustic device of any one of [1] to

[0017] , the acoustic device being a microphone.Reference signs1 diaphragm (of capsule)2 interference tube3 openings4 microphone stand5 microphone clip6 XLR connector7 cable11-1 to 11-5 cylindrical shells of tube section12-1 to 12-5 helically revolving slopes13-1 to 13-5 conical shells of flange

Claims

CLAIMS1. An acoustic device comprising an interference tube with a tube section and a flange connected to the tube section, the interference tube comprising a first acoustic metamaterial for enhancing sound directivity, and the flange comprising a second acoustic metamaterial configured to transform on-axis waves travelling in the tube section in such a way that they recombine coherently.

2. The acoustic device of claim 1, the tube section comprising an acoustic metamaterial with high spatial dispersion, with an emphasis on enhanced transmission in on-axis direction and strong attenuation from interference for waves travelling in off-axis directions.

3. The acoustic device of claim 1, the tube section comprising multiple cylindrical shells with openings.

4. The acoustic device of claim 1, the cylindrical shells being arranged along the radial axis of the interference tube.

5. The acoustic device of claim 3, the interference tube having a uniform spacing between the shells of the tube section.

6. The acoustic device of claim 3, the interference tube having a non-uniform spacing between the shells of the tube section.

7. The acoustic device of claim 1, the flange being configured to transform the on-axis waves travelling between the shells in such a way that they recombine coherently before a capsule of the acoustic device.

8. The acoustic device of claim 7, the flange serving to ensure a coherent transmission of the on-axis wave impinging through the various channels between the cylindrical shells when they are redirected towards the capsule.

9. The acoustic device of claim 1, the flange being designed through space-coiling by artificially equalising the pathlength for the various channels while aiming to preserve impedance matching and avoiding reflection of the wave at the entrance of the flange.

10. The acoustic device of claim 7, the diameter of the tube section being larger than that of the capsule of the acoustic device.

11. The acoustic device of claim 1, the flange comprising coaxial conical shells between which the sound propagates.

12. The device of claim 1, the flange being configured so that the effective speed of sound within the various channels is adjusted such that the waves recombine in-phase at the end of the flange.

13. The acoustic device of claim 1, the flange comprising a number of helically revolving slopes that artificially create a longer than the straight path for the sound waves to reach the end of the flange.

14. The acoustic device of claim 1, the length of the interference tube including the tube section and the flange being 75 mm and the radius of the tube section being 35 mm.

15. The acoustic device of claim 3, the length of the shells in the tube section being non- uniform.

16. The acoustic device of claim 15, the shells being increasingly shorter towards the centre of the tube section.

17. The acoustic device of claim 1, the acoustic device comprising resonators.

18. The acoustic device of claim 1, the acoustic device being a microphone.

Images