THEREOF

TECHNICAL FIELD The present disclosure relates to a tunable acoustic absorber and method of operating the tunable acoustic absorber.

BACKGROUND

Noise pollution can be a big issue in urban environments. Example of noise sources include vehicles and building construction for an outdoor setting, and loudspeakers, and air conditioning systems for an indoor setting. The noise that is heard can be transmitted directly from a source, or indirectly from sound that has reflected off surfaces. Furthermore, these noises can have a variety of audible frequencies as the dominant frequency of the noise being produced by each source varies based on the type and state of operation of the source. For example, the frequencies of the noise produced by vehicles vary with engine speed and loading (a lighter vehicle produces noise of higher frequencies than a heavier vehicle). Therefore, traffic noise during off-peak hours tend to be skewed towards higher frequencies compared to traffic noise in traffic jams.

Noise barriers/absorbers that are employed to reduce noise pollution can be made from either hard or soft materials. Noise barriers made from hard materials such as concrete walls, metal slabs, and window glasses, are able to reflect most of the incident sound to quieten a target region but causes echoes and increase noise levels in another region. In an indoor setting with an enclosed space, high acoustic reflection from concrete walls and window glasses causes too much echoes. This indoor sound reverberation in turn degrades intelligibility of speech. On the other hand, noise barriers made from soft materials such as foams and fiberglass, are able to absorb high frequency sound but are inefficient in absorbing low frequency sounds. Therefore, the efficiency of such noise barriers varies widely depending on the dominant frequencies of the noise being produced.

A variety of tunable acoustic absorbers have been proposed. In one example, a tunable acoustic absorber is realized by providing a micro-perforated membrane (MPM) absorber having an adjustable back-cavity depth to tune the peak absorption frequency of the MPM absorber. The MPM absorber includes a front plate having about 400 holes for receiving incident sound waves. Each hole has a diameter of about 100pm. The MPM absorber further includes a back rigid wall that is movable using a stepping motor. By sliding the rigid wall backwards, the back-cavity depth of the MPM absorber is increased, which in turn decreases the peak absorption frequency of the MPM absorber. However, the sliding of the rigid wall produces screeching noises. Furthermore, the stepping motor has to exert significant power in order to move the rigid wall, especially for an MPM absorber of considerable size. As such, the MPM absorber is limited for use for discrete adjustments, but not for real-time tuning.

In another example, a silent tunable acoustic absorber may be realized using a single microperforated dielectric elastomer actuator (MPDEA) having a plurality of holes. The MPDEA is voltage activated to cause the hole sizes to shrink. This in turn leads to a shift in the acoustic absorption spectrum of the acoustic absorber. However, such devices do not function well in dusty environments such as dusty traffic and construction sites. Moreover, reliability and high- voltage requirement pose issues for such devices.

Therefore, it is desirable to provide a tunable acoustic absorber that addresses at least one of the problems mentioned in existing prior art, and/or to provide the public with a useful alternative.

SUMMARY

Various aspects of the present disclosure are described here. It is intended that a general overview of the present disclosure is provided and this, by no means, delineate the scope of the invention.

According to a first aspect, there is provided a tunable acoustic absorber including a first perforated panel having a first array of first apertures, a second perforated panel having a second array of second apertures, and an acoustic aperture array with each acoustic aperture having an effective diameter for receiving sound waves. Each acoustic aperture is formed by relative positions of the first apertures and the second apertures. The tunable acoustic absorber further includes a spacing member arranged to support and space one of the first perforated panel and the second perforated panel from a support structure. The spacing member is further operable to adjust the relative positions of the first apertures and the second apertures in order to control the effective diameter of each acoustic aperture.

In a described embodiment, the spacing member may have two functions. The first function is to space the acoustic absorber member from the support structure. The second function is to produce a displacement of the first perforated panel relative to the second perforated panel. This reduces the number of elements required for setting up the tunable acoustic absorber. Advantageously, this allows the tunable acoustic absorber to have a simple yet effective configuration.

Preferably, the tunable acoustic absorber may further include a resiliently deformable layer arranged between the first perforated panel and the second perforated panel.

Preferably, the resiliently deformable layer may include an elastomer layer arranged to resiliently stretch when the first perforated panel is displaced relative to the second perforated panel Preferably, the tunable acoustic absorber may further include a feedback control system. The feedback control system may include an audio receiver for converting ambient acoustics into a digital signal, and a controller configured to identify a dominant noise frequency of the digital signal, and to operate the spacing member to adjust the relative positions of the first apertures and the second apertures to match a peak absorption frequency of the tunable acoustic absorber to the dominant noise frequency.

Advantageously, the tunable acoustic absorber can be tuned dynamically in real-time. In other words, the tunable acoustic absorber is able to adapt to changes in ambient noise to optimize the performance of the tunable acoustic absorber.

The controller may be configured to identify the dominant noise frequency from a frequency domain representation of the digital signal.

Further, the frequency domain representation may be obtained by performing a Fast Fourier Transform analysis on the digital signal. In a described embodiment, the spacing member may include a bimorph actuator arranged to produce a displacement of the second perforated panel relative to the first perforated panel to adjust the relative positions of the first apertures and the second apertures. In another described embodiment, the first perforated panel may instead include a sloped edge, and the spacing member may include a tapered portion arranged to mate with the sloped edge of the first perforated panel. The spacing member may be operable to move linearly to produce a displacement of the first perforated panel with respect to the second perforated panel that is orthogonal to the linear movement to adjust the relative positions of the first apertures and the second apertures.

Preferably, the tapered portion may have a tapered angle of 5 degrees. Or more generally, the tapered angle may be between 3 degrees and 10 degrees, or 4 degrees to 9 degrees or any possible angle within this range.

Preferably, a stepping motor may be arranged to operate the spacing member to move linearly.

According to a second aspect, there is provided a tunable acoustic absorber including a first perforated panel having a first array of first apertures, a second perforated panel having a second array of second apertures, and a resiliently deformable layer arranged between the first perforated panel and the second perforated panel. The resiliently deformable layer has a third array of third apertures. The tunable acoustic absorber further includes an acoustic aperture array with each acoustic aperture having an effective diameter for receiving sound waves. Each acoustic aperture is formed by relative positions of the first apertures, the second apertures, and the third apertures. The first perforated panel is displaceable relative to the second perforated panel to adjust the relative positions of the first apertures, the second apertures, and the third apertures in order to control the effective diameter of each acoustic aperture. The resiliently deformable layer arranged to be resiliently stretched when the first perforated panel is displaced relative to the second perforated panel. In a described embodiment, the elastomer layer aids in smoother movement of the first perforated panel relative to the second perforated panel by reducing friction between the first and second perforated panel. The elastomer layer further ensures good sealing between the first and second perforated panel. Advantageously, the elastomer layer reduces noise generation when the first perforated panel is displaced relative to the second perforated panel.

According to a third aspect, there is provided a method of operating a tunable acoustic absorber including a first perforated panel having a first array of first apertures, a second perforated panel having a second array of second apertures, and an acoustic aperture array with each acoustic aperture having an effective diameter for receiving sound waves. Each acoustic aperture is formed by relative positions of the first apertures and the second apertures. The tunable acoustic absorber further including a spacing member arranged to support and space one of the first perforated panel and the second perforated panel from a support structure. The method includes operating the spacing member to adjust the relative positions of the first apertures and the second apertures to control the effective diameter of each acoustic aperture.

According to an fourth aspect, there is provided a method of operating a tunable acoustic absorber including a first perforated panel having a first array of first apertures, a second perforated panel having a second array of second apertures, and a resiliently deformable layer formed between the first perforated panel and the second perforated panel. The resiliently deformable layer further has a third array of third apertures. The tunable acoustic absorber further includes an acoustic aperture array with each acoustic aperture having an effective diameter for receiving sound waves. Each acoustic aperture is formed by relative positions of the first apertures, the second apertures, and the third apertures. The method includes displacing the first perforated panel with respect to the second perforated panel, thereby resiliently stretching the resiliently deformable layer, and adjusting the relative positions of the first apertures, the second apertures, and third apertures in order to control the effective diameter of each acoustic aperture.

BRIEF DESCRIPTION OF FIGURES

Exemplary embodiments will be described with reference to the accompanying drawings in which: Figure 1 is a front view of a tunable acoustic absorber according to a first embodiment.

Figure 2 is a cross-sectional side view of the tunable acoustic absorber of Figure 1.

Figure 3A is a cross-sectional view of an acoustic aperture when the tunable acoustic absorber of Figure 1 in an inactive state.

Figure 3B is a cross-sectional view of the acoustic aperture of Figure 3A when the tunable acoustic absorber in an active state.

Figure 4 is a block diagram of a system architecture of the tunable acoustic absorber of Figure 1. Figure 5 is a perspective view of a tunable acoustic absorber according to a second embodiment.

Figure 6 is a side view of the tunable acoustic absorber of Figure 5.

Figure 7 A is a cross-sectional view of an acoustic aperture when the tunable acoustic absorber of Figure 5 is in an inactive state.

Figure 7B is a cross-sectional view of the acoustic aperture when the tunable acoustic absorber of Figure 5 is in an active state.

Figure 8 is a block diagram of a system architecture of the tunable acoustic absorber of Figure 5. Figure 9 is a line graph showing an acoustic performance of the tunable acoustic absorber of Figure 1 and Figure 5.

Figure 10A is a cross-sectional view of an alternative embodiment of the acoustic aperture of Figure 7A when the tunable acoustic absorber is in an inactive state. Figure 10B is a cross-sectional view of an alternative embodiment of the acoustic aperture of Figure 7B in a first configuration when the tunable acoustic absorber is in an active state.

Figure 10C is a cross-sectional view of an alternative embodiment of the acoustic aperture of Figure 7B in a second configuration when the tunable acoustic absorber is in an active state.

DETAILED DESCRIPTION

The following description contains specific examples for illustrative purposes. The person skilled in the art would appreciate that variations and alterations to the specific examples are possible and within the scope of the present disclosure. The figures and the following description of the particular embodiments should not take away from the generality of the preceding summary.

Figure 1 illustrates a front view of a tunable acoustic absorber 100 according to a first embodiment. The tunable acoustic absorber 100 includes a first perforated panel 110 having a first array of first apertures 112. In this embodiment, each of the first apertures 112 has an average diameter of 0.5mm. The first apertures are spaced equally apart at a distance of 5mm from each other and disposed on a planar first panel body.

The tunable acoustic absorber 100 also includes a second perforated panel 120, and a resiliently deformable layer in the form of an elastomer layer 130 that is formed between and bonded to the first perforated panel 110 and the second perforated panel 120 (refer to Figure 2). The second perforated panel 120 includes a second array of second apertures 122 disposed on a planar second panel body and having a similar structure and arrangement as the first apertures 112 of the first perforated panel 110. That is to say, each of the second apertures 122 has an average diameter of 0.5mm. The second apertures 122 are also spaced equally apart at a distance of 5mm from each other. Similarly, the elastomer layer 130 includes a third array of third apertures 132 which are arranged in a similar manner as the first and second apertures

112,122. The tunable acoustic absorber 100 further includes a spacing member in the form of a wedge 140 having a tapered portion 142, a bottom end 144, and an upstanding end 146 which has a greater thickness or height compared to the other end to create a tapered effect. The first perforated panel 110 further includes a sloped edge 116 that mates with the tapered portion 142 of the wedge 140 and the wedge 140 thus supports the first perforated panel 110 in a manner that a longitudinal axis of the bottom end 144 is substantially orthogonal to a vertical axis of the first perforated panel 110. The bottom end 144 of the wedge 140 rests on a support structure 145. In this embodiment, the support structure 145 is the floor. Thus, the wedge 140 also spaces the first perforated panel 110 from the support structure 145 i.e. the floor.

A stepping motor 150 is arranged to be attached to the upstanding end 146 of the wedge 140 and is configured to move the wedge 140 linearly in a direction 152. The linear movement of the wedge 140 in the direction 152 causes the tapered portion 142 to abut or push against the sloped edge 116, thereby producing a displacement of the first perforated panel 110 with respect to the second perforated panel 120 in an orthogonal direction 154. At the same time, the elastomer layer 130 resiliently stretches to accommodate the displacement of the first perforated panel 110 relative to the second perforated panel 120. By controlling the displacement of the first perforated panel 110 relative to the second perforated panel 120, the relative positions of the first apertures 112, second apertures 122, and third apertures 132 can be adjusted. In this embodiment, the wedge 140 has a tapered angle 148 of 5° (i.e. angle between the tapered portion 142 and the bottom end 144). The low angle of the wedge 140 translates a large motion generated by the stepping motor 150 into a small and precise displacement (a few microns) of the first perforated panel 110 relative to the second perforated panel 120.

The tunable acoustic absorber 100 further includes an acoustic aperture array 158 (see Figure 2) with each acoustic aperture 160 having an effective diameter for receiving sound waves. The effective diameter of each acoustic aperture 160 is determined by relative positions of the first apertures 112, the second apertures 122, and the third apertures 132 and it would be apparent that each acoustic aperture 160 is formed when the first apertures 112, the second apertures 122 and the third apertures 132 are in registration with each other. By adjusting the displacement of the first perforated panel 110 relative to the second perforated panel 120, the relative positions of the first apertures 112, second apertures 122, and third apertures 132 can be adjusted to achieve a desired effective diameter. Adjusting the effective diameter of each acoustic aperture 160 has the effect of shifting the peak absorption frequency of the tunable acoustic absorber 100. The smaller the effective diameter of each acoustic aperture 160, the lower the peak absorption frequency of the tunable acoustic absorber 100. When the second panel’s body fully blocks the first apertures 112, the effective diameter of acoustic apertures 160 is thus zero. Figure 2 provides a side view of the tunable acoustic absorber 100. As can be seen, the elastomer layer 130 is sandwiched between the first perforated panel 110 and the second perforated panel 120. Specifically, the elastomer layer 130 includes a front side 134 and a back side 136. The front side 134 of the elastomer layer 130 is attached to the first perforated panel 110, while the back side 136 of the elastomer layer 130 is attached to the second perforated panel 120. The second perforated panel 120 and the elastomer layer 130 have respective bottom ends 124,134 that are flushed with the bottom end 144 of the wedge 140. Notably, the elastomer layer 130 is arranged to resiliently hold the first perforated panel 110 relative to the second perforated panel 120. At the same time, the elastomer layer 130 is also able to resiliently stretch to accommodate movement of the first perforated panel 110 relative to the second perforated panel 120. The elastomer layer 130 is able to reduce friction between the first perforated panel 110 and the second perforated panel 120 which reduces noise generated when the first perforated panel 110 is displaced relative to the second perforated panel 120. Furthermore, the elastomer layer 130 also helps to dampen panel vibrations which can be useful in vehicle applications. For ease of reference, when the first perforated panel 110 is in its original position, i.e. when the first perforated panel 110 is aligned to the second perforated panel 120 (and the first, second and third apertures 112,122,132 are aligned or in registration), the tunable acoustic absorber 100 (and the wedge 140) is referred to as being in an inactive state. Conversely, when the first perforated panel is displaced from its original position, the tunable acoustic absorber 100 (and the wedge 140) is referred to as being in an active state.

Figure 3A illustrates a cross-sectional view of an exemplary acoustic aperture 360 when the tunable acoustic absorber 100 is in an inactive state. The acoustic aperture 360 is formed by relative positions of an exemplary first aperture 312, an exemplary second aperture 322, and an exemplary third aperture 332. In the inactive state, the first aperture 312, the second aperture 322, and the third aperture 332 are all aligned, and the acoustic aperture 360 has an effective diameter 310 equal to the diameter of the first aperture 312 (or the second aperture 322, or the third aperture 332). The effective diameter 310 of the acoustic aperture 360 is at its maximum in the inactive state.

Figure 3B illustrates a cross-sectional view of the acoustic aperture 360 when the tunable acoustic absorber is in an active state. In the active state, the first perforated panel 110 is displaced relative to the second perforated panel 120 in the direction 154, and the elastomer layer 130 is resiliently stretched. The first aperture 312 is similarly displaced relative to the second aperture 322, while the third aperture 332 is being resiliently stretched. While the acoustic aperture is formed by relative positions of the first aperture 312, the second aperture 322, and the third aperture 332, in the active state, the effective diameter 310 of the acoustic aperture 360 is determined only by the relative positions of the first aperture 312 and the second aperture 322. Due to the overlap between the first aperture 312 and the second aperture 322, the effective diameter 310 of the acoustic aperture 360 is decreased in the active state. By controlling a degree of displacement of the first perforated panel 110 relative to the second perforated panel 120, the effective diameter 310 of the acoustic aperture 360 can be correspondingly adjusted. The tunable acoustic absorber 100 also includes a feedback control system 170 for adjusting the effective diameter of the acoustic aperture 160,360. The feedback control system 170 is described next with reference to Figure 4 which illustrates a system architecture 400 of the tunable acoustic absorber 100. The feedback control system 170 includes an audio receiver in the form of a microphone 172 for converting ambient acoustics, i.e. sound waves into a digital signal 174, and a controller 176 configured to analyze the digital signal, and to control the stepping motor 150 to operate the wedge 140 to move linearly by a specific distance.

Specifically, the microphone 172 is communicatively coupled to the controller 176, and is arranged to transmit the digital signal 174 to the controller 176. The controller 176 is configured to perform a Fast Fourier Transform (FFT) analysis on the digital signal to obtain a frequency domain representation of the captured sound waves. The controller 176 is then configured to identify a dominant noise frequency 178 of the captured sound wave from the frequency domain representation. The controller 176 is further configured to determine the specific distance that the wedge 140 has to move linearly in order for the peak absorption frequency of the tunable acoustic absorber 100 to match the dominant noise frequency 178.

The controller 176 is communicatively coupled to the stepping motor 150, and is arranged to transmit a control signal 178 to the stepping motor 150. The control signal 178 includes instructions for the stepping motor 150 to operate the wedge 140 to move linearly by the specific distance, thus adjusting relative positions of the first apertures 110 and the second apertures 120 in order to control the effective diameter of the acoustic aperture 160,360. Advantageously, the tunable acoustic absorber 100 is tuned in real-time and adapts to changes in the dominant noise frequency 178 of the ambient noise. In this way, performance of the tunable acoustic absorber 100 is optimized.

It should be noted that the description of the first embodiment is not meant to be limitative. For example, the elastomer layer 130 may be made of rubber or any suitable resiliently deformable material. Alternatively, the tunable acoustic absorber 100 can also function without the elastomer layer 130, in which case each acoustic aperture will then be formed by relative positions of the first apertures 112 and the second apertures 122. Further, the first and second perforated panels 110,120 may be made of a metal. Alternatively, the first and second perforated panels 110,120 may be perforated membranes made of a suitable soft and pliable material based on the particular application. Depending on the specific application, the tunable acoustic absorber 100 may be required to operate at a higher (or lower) frequency range. In this case, the average diameters of the first perforated panel 110, the second perforated panel 120, and the elastomer layer 130 may be adjusted accordingly. For example, the average diameters may be 1mm or 0.01mm, depending on the requirements.

Similarly, depending on the level of precision that is required, or the average diameters of the first perforated panel 110, the second perforated panel 120, and the elastomer layer 130, the wedge 140 may instead have a smaller (or larger) tapered angle 148. For example, the wedge 140 may have a tapered angle 148 of 3° for applications that require a higher degree of precision, or if the average diameters are very small. Alternatively, the wedge 140 may have a tapered angle 148 of 6° for lower precision applications, or if the average diameters are comparatively larger.

Still further, the tapered angle 148 may be dependent on a maximum displacement of the wedge 140, and the frequency range that the tunable acoustic absorber 100 is expected to operate in. Given a maximum displacement of 10mm of the wedge 140, for low frequency absorption, the average diameters of the first perforated panel 110, the second perforated panel 120, and the elastomer layer 130 can be as small as 0.2mm. As such, the tapered angle 148 can be approximately 1°. On the other hand, for high frequency absorption, the average diameters of the first perforated panel 110, the second perforated panel 120, and the elastomer layer 130 can be 1mm. In this case, the tapered angle 148 can be approximately 6°. In general, for low frequency applications, the tapered angle 148 may be 1° - 2°, while for high frequency applications, the tapered angle 148 may be 5° - 6°.

Clearly, the tapered angle 148, the maximum displacement of the wedge 140, and the average diameters of the first perforated panel 110, the second perforated panel 120, and the elastomer layer 130 can vary widely according to the specific application. The person skilled in the art would appreciate that the various alterations required for the specific applications are possible and within the scope of the present disclosure.

Figure 4 illustrates a perspective view of a tunable acoustic absorber 1000 according to a second embodiment. The tunable acoustic absorber 1000 has similar components as the tunable acoustic absorber 100, and like components in this embodiment uses the same reference numerals as the first embodiment with an addition of 1000. Similar to the tunable acoustic absorber 100 of the first embodiment, the tunable acoustic absorber 1000 includes a first perforated panel 1100, a second perforated panel 1120, and an elastomer layer 1300 that is formed between and bonded to the first perforated panel 110 and the second perforated panel 120. Like the first perforated panel 100 in the first embodiment, the first perforated panel 1100 includes a first array of first apertures 1112. Each of the first apertures 1112 has an average diameter of 0.5mm. The first apertures are spaced equally apart at a distance of 5mm from each other. Likewise, the second perforated panel 1120 includes a second array of second apertures 1122, with each of the second apertures 1122 having an average diameter of

0.5mm. The second apertures 1122 are also spaced equally apart at a distance of 5mm from each other. Similarly, the elastomer layer 130 includes a third array of third apertures 1132 which are arranged in a similar manner as the first and second apertures 1112,1122. For reference purposes, the first perforated panel 1110, the second perforated panel 1120, and the elastomer layer 1130 is collectively referred to as an acoustic absorber member 600.

Similar to the first embodiment, the tunable acoustic absorber 1000 further includes an acoustic aperture array 1158 (see Figure 6) with each acoustic aperture 1160 having an effective diameter for receiving sound waves. The effective diameter of each acoustic aperture 1160 is determined by relative positions of the first apertures 1112, the second apertures 1122, and the third apertures 1132 and it would be apparent that each acoustic aperture 1160 is formed when the first apertures 1112, the second apertures 1122 and the third apertures 1132 are in registration with each other. By adjusting the displacement of the first perforated panel 1110 relative to the second perforated panel 1120, the relative positions of the first apertures 1112, and the second apertures 1122 can be adjusted to achieve a desired effective diameter. Just like in the first embodiment, the smaller the effective diameter of each acoustic aperture 1160, the lower the peak absorption frequency of the tunable acoustic absorber 1000.

In this embodiment, the tunable acoustic absorber 1000 further includes a support structure in the form of a wall mount 400 for mounting to a structural wall for example, and a spacing member in the form of four bimorph actuators. In this embodiment, the bimorph actuators are piezoelectric bimorphs 420. The wall mount 400 includes a back panel or wall 410, and the bimorphs 420 are arranged to support and space the acoustic absorber member 600 from the wall 410 to form a partially enclosed space 430. Specifically, the bimorphs 420 are located at and adhered to each corner 1124 of the second perforated panel 1120, to support and space the second perforated panel 1120 (and consequently, the elastomer layer 1130 and the first perforated panel 1110) from the wall 410. At the same time, the bimorphs 420 are arranged to produce a displacement of the second perforated panel 1120 relative to the first perforated panel 1110 upon electrical activation of the bimorphs 420.

Figure 6 illustrates a side view of the tunable acoustic absorber 1000 including the acoustic absorber member 600, and the wall mount 400. The bimorphs 420 are arranged to space the acoustic absorber member 600 from the wall 410 to form the partially enclosed space 430. The partially enclosed space 430 is defined by the second perforated panel 1120 and the wall 410. The partially enclosed space 430 has a depth of 40mm between the second perforated panel 1120 and the wall 410. Unlike the first embodiment, the first perforated panel 1110 and the second perforated panel 1120 in the second embodiment have respective bottom ends 1114,1124 that are flushed with each other.

The operation of the bimorphs will be described next with reference to Figures 7A and 7B. For ease of reference, when the bimorphs 420 are not being electrically activated, the tunable acoustic absorber 1000 (and the bimorphs 420) is referred to as being in an inactive state. Conversely, when the bimorphs 420 are being electrically activated, the tunable acoustic absorber 1000 (and the bimorphs 420) is referred to as being in an active state.

Figure 7A illustrates a cross-section view of an exemplary acoustic aperture 1360 when the tunable acoustic absorber 1000 is in the inactive state. The acoustic aperture 1360 is formed by relative positions of an exemplary first aperture 1312, an exemplary second aperture 1322, and an exemplary third aperture 1332. In the inactive state, the first aperture 1312, the second aperture 1322, and the third aperture 1332 are all aligned, and the acoustic aperture 1360 has an effective diameter 1310 equal to the diameter of the first aperture 1312 (or the second aperture 1322, or the third aperture 1332). The effective diameter 1310 of the acoustic aperture 1360 is at its maximum in the inactive state.

Figure 7B illustrates a cross-sectional view of the acoustic aperture 1360 when the tunable acoustic absorber 1000 is in an active state. In the active state, the bimorphs 420 are electrically activated such that the bimorphs 420 bend in direction 1154. This results in the second perforated panel 1120 to be displaced relative to the first perforated panel 1110 in the direction 1154 as well. Similar to the first embodiment, the elastomer layer 1130 resiliently stretches to accommodate the displacement of the second perforated panel 1120 relative to the first perforated panel 1110. While the acoustic aperture 1360 is formed by relative positions of the first aperture 1312, the second aperture 1322, and the third aperture 1332, in the active state, the effective diameter 1310 of the acoustic aperture 1360 is determined only by the relative positions of the first aperture 1312 and the second aperture 1322. Due to the overlap between the first aperture 1312 and the second aperture 1322, the effective diameter 1310 of the acoustic aperture 360 is decreased in the active state.

The tunable acoustic absorber 1000 also includes a feedback control system 1170 for adjusting the effective diameter of the acoustic aperture 1160,1360. The feedback control system 1170 is described next with reference to Figure 8 which illustrates a system architecture 800 of the tunable acoustic absorber

1000.

The feedback control system 1170 includes an audio receiver in the form of a microphone 1172 for converting ambient acoustics, i.e. sound waves into a digital signal 1174, and a controller 1176 configured to analyze the digital signal, and to send an electrical current to the bimorphs 420 to cause the bimorphs 420 to bend by a specific degree. Specifically, the microphone 1172 is communicatively coupled to the controller 1176, and is arranged to transmit the digital signal 1174 to the controller 1176. The controller 1176 is configured to perform a Fast Fourier Transform (FFT) analysis on the digital signal to obtain a frequency domain representation of the captured sound waves. The controller 1176 is then configured to identify a dominant noise frequency 1178 of the captured sound wave from the frequency domain representation.

The controller 1176 is further configured to determine a specific degree that the bimorphs 420 has to bend in order for the peak absorption frequency of the tunable acoustic absorber 1000 to match the dominant noise frequency 1178. Thus, the amount of electric current that is needed to bend the bimorphs 420 by the specific degree can also be determined by the controller 1176. The controller 1176 is electrically coupled to the bimorphs 420, and is arranged to transmit the electrical current 1178 to the bimorphs 420 to bend the bimorphs 420 by the specific degree, thus adjusting relative positions of the first apertures 1110 and the second apertures 1120 in order to control the effective diameter of the acoustic aperture 1160,1360. Advantageously, the tunable acoustic absorber 1000 is tuned in real-time and adapts to changes in the dominant noise frequency 1178 of the ambient noise. In this way, performance of the tunable acoustic absorber 1000 is optimized. It should be noted that the description of the second embodiment is also not meant to be limitative. For example, it should be understood that the acoustic aperture 1360 is not limited to the first and second configurations. The acoustic aperture 1360 can have any number of configurations depending on the relative positions of the first and second apertures 1112, 1122. Further, while the tunable acoustic absorber 1000 is described with the elastomer layer 130, it should be clear that the tunable acoustic absorber 1000 may be formed without the elastomer layer 130. Still further, depending on a desired frequency range of the tunable acoustic absorber 1000, the partially enclosed space 430 may have a depth of 20mm, instead of 40mm. Although the tunable acoustic absorber 1000 is described as having four bimorphs 420, depending on a force required to displace the second perforated panel 1120 relative to the first perforated panel 1110, the tunable acoustic absorber 1000 may also have less than four bimorphs 420 (as long as there is at least one bimorph 420). Alternatively, the tunable acoustic absorber 1000 may also have more than four bimorphs 420.

Further, while the spacers in the second embodiment are described as being bimorph actuators, and more specifically piezoelectric bimorphs, the spacers may also be other bimorph actuators, or even other kinds of actuators. For example, the piezoelectric bimorphs 420 are generally able to produce a displacement of the first perforated panel 1110 relative to the second perforated panel 1120 by 0.1mm to 0.5mm. If more displacement is required, the piezoelectric bimorphs 420 may be replaced with a more suitable spacer. Indeed, the tunable acoustic absorber 1000 may not include the back panel or wall 410 if the bimorph actuators 420 are mounted directly to a structural wall.

Figure 9 is a line graph 700 showing an acoustic absorption performance of the tunable acoustic absorber 100,1000. Specifically, line graph 700 includes eleven line graphs 701,702,703,704,705,706,707,708,709,710,711. The line graphs show how an acoustic absorption performance of the tunable acoustic absorber 100,1000 varies with the acoustic aperture 160,360,1160,1360 at a different effective diameter 310,1310. The effective diameter 310,1310 of the acoustic aperture 160,360,1160,1360 that each line graph represent is summarized in Table 1 below.

Table 1

When the effective diameter 310,1310 of the acoustic aperture 160,360,1160,1360 is reduced to 0.01mm, the tunable acoustic absorber

100,1000 has a peak absorption frequency of 505Hz. On the other hand, the tunable acoustic absorber 100,1000 has a peak absorption frequency of 1750Hz, when the effective diameter 310,1310 of the acoustic aperture 160,360,1160,1360 is increased to 1mm. The tunable acoustic absorber 100,1000 is able to efficiently absorb noise across a wide frequency range of 1478Hz. The tunable acoustic absorber 100,1000 has a maximum sound absorption coefficient (SAC) of 97.4% @ 934Hz. Further, the bandwidth at which the tunable acoustic absorber 100,1000 can operate with a 55% SAC is 800Hz (from 621 Hz to 1421 Hz)

The advantages of the tunable acoustic absorber 100,1000 are summarized herein.

(1) Thin design: The first perforated panel 110,1110, the second perforated panel 120,1120, and the elastomer layer 130 has a combined thickness of only

0.25mm which allows the tunable acoustic absorber 100 to be fitted to narrow spaces.

(2) Tunable peak absorption frequency: The tunable acoustic absorber 100,1000 enables optimum absorption of sound since a dominant frequency of a noise can be targeted by adjusting the effective diameter of each acoustic aperture in real-time.

(3) Large tuning range: The tunable acoustic absorber 100,1000 can cover all the typical noise sources in the urban city since the absorption band of the tunable acoustic absorber 100,1000 extends from 404Hz to 1828Hz (bandwidth of 1478Hz).

(4) Solid construction: Other than the elastomer layer 130,1130, the tunable acoustic absorber 100,1000 has a solid construction made of hard materials which allows easy cleaning and has a long lifetime. (5) Low angled wedge design: The low angle of the wedge 140 converts the large motion generated by the stepping motor 150 into a small and precise displacement of the first perforated panel 110 relative to the second perforated panel 120. This enables the use of low power, low precision and low cost stepping motors in an application which requires a high level of precision control (a few microns).

It should be clear that although the present disclosure has been described with reference to specific exemplary embodiments, various modifications may be made to the embodiments without departing from the scope of the invention as laid out in the claims. For example, the tunable acoustic absorber 100,100 can be deployed in many different applications due to its broadband noise attenuation and noise adaptive features. Its simple design allows for the tunable acoustic absorber 100,1000 to be repurposed for different applications. For example, the tunable acoustic absorber 100,1000 may have a compact, low height structure, or tunable acoustic absorber 100,1000 may be designed to have good ventilation.

Additionally, in the first embodiment, the first perforated panel 110 is described as being displaced relative to the second perforated panel 120. Flowever, it can also be the case that the second perforated panel 120 is displaced relative to the first perforated panel 110 by having the wedge 140 support the second perforated panel 120 instead of the first perforated panel 110. In the same way, in the second embodiment, the second perforated panel 1120 is described as being displaced relative to the first perforated panel 1110. However, it is also possible for the first perforated panel 1110 to be displaced relative to the second perforated panel 1120. For example, this can be realized by attaching the bimorphs to the first perforated panel 1110 instead of the second perforated panel 1120.

Further, although the tunable acoustic absorber 1000 is described as including the wall mount 400 and the wall 410, the tunable acoustic absorber 1000 may also be provided without the wall 410. In such a case, the tunable acoustic absorber 1000 may simply include the bimorphs 420, and the bimorphs 420 can then be attached to an existing wall structure on site.

The tunable acoustic absorber 100 in the first embodiment may also include a wall mount having spacers and a back panel. Alternatively, the tunable acoustic absorber 100 may simply include spacers which, like the second embodiment, are then attached to an existing wall structure on site.

Still further, it may not be necessary for the effective diameter 1310 of the acoustic aperture 1360 to be at a maximum when the tunable acoustic absorber 1000 is in the inactive state. For example, the first perforated membrane 1110 may be slightly displaced from the second perforated membrane 1120 so that the first aperture 1112 and the second aperture 1122 are slightly misaligned. The acoustic aperture 1360 would thus have an effective diameter 1310 that is between a maximum and a minimum when the tunable acoustic absorber is in the inactive state. This is illustrated in Figure 10A which depicts a cross- sectional view of the acoustic aperture 1360 (although this time without the elastomer layer 1130 and the wall mount 400 for simplicity) when the tunable acoustic absorber 1000 is in the inactive state.

Figure 10B illustrates a cross-sectional view of the acoustic aperture 1360 in a first configuration when the tunable acoustic absorber 1000 is in the active state. In the first configuration, the first perforated membrane 1110 is displaced in a first direction 910 relative to the second perforated membrane 1120 to align the first and second apertures 1112,1122. The effective diameter 1310 of the acoustic aperture 1360 is at its maximum in the first configuration.

Figure 10C illustrates a cross-sectional view of the acoustic aperture 1360 in a second configuration when the tunable acoustic absorber 1000 is in the active state. In the second configuration, the first perforated membrane 1110 is then displaced in a second direction 920 relative to the second perforated membrane 1120. Notably, the second direction 920 is opposite the first direction 910. Since the second direction 920 is opposite the first direction 910, the misalignment between the first and second apertures 1112,1122 is increased to the point that the first and second apertures 1112,1122 are completely misaligned. The effective diameter 1310 of the acoustic aperture 1360 is at its minimum i.e. zero in the second configuration. Additionally, each component of the tunable acoustic absorber 100,1000 may be made from a transparent material. For example, the wall 410 in the second embodiment may be made of a material that is transparent. Alternatively, the wall 410 may also be made of a non-transparent material such as plaster, or any other suitable wall materials. Preferably, the elastomer layer 130 is an acrylic elastomer. However, the elastomer layer 130 may also be made from other resiliently deformable materials. It is often the case that a sound barrier such as the tunable acoustic absorber 100,1000 is needed to reduce noise in an urban environment such as at bus stops, or along train tracks where aesthetic may play an important role. A transparent tunable acoustic absorber 100,1000 has the advantage of being visually discreet.

Various embodiments as discussed above may be practiced using any means available to the skilled person without departing from the scope of the invention as laid out in the claims. Modifications and alternative constructions apparent to the skilled person are understood to be within the scope of the disclosure.