GB2565033A

GB2565033A - Acoustic resonators

Info

Publication number: GB2565033A
Application number: GB1708245.4A
Authority: GB
Inventors: Papanagiotou Kyriakos
Original assignee: KP Acoustics Ltd
Current assignee: KP Acoustics Ltd
Priority date: 2017-05-23
Filing date: 2017-05-23
Publication date: 2019-02-06
Anticipated expiration: 2037-05-23
Also published as: GB201708245D0; GB2565033B

Abstract

An acoustic resonator comprises a cavity and a plurality of apertures S1, S2, the resonance frequency of the resonator being variable over time by changing the volume of the cavity and/or the properties of the apertures. An actuator may be used to move a moving surface 45 to change the volume of the cavity or to change the size S of the apertures by deforming a perforated sheet or membrane 20 or moving a cover to obscure parts of the apertures. Diaphragms and separators may be used to divide the cavity into different variable volumes (figure 8). The resonator may be controlled to absorb selected frequencies sensed by a microphone which detects frequencies in a room. The resonator may change its absorbed frequency by a random or predetermined pattern. The resonator may dynamically respond to the dominant frequency detected by the microphone. The resonator may form a panel attached to a wall of a building or space.

Description

This invention relates to acoustic resonators and specifically to apparatus for and methods of dynamically altering the acoustic properties of a space. This has a wide range of applications, not least in noise control.

Every space has a unique frequency response or ‘acoustic identity’, defined by its geometry as well as by the materials which comprise its surfaces (walls, floor, ceiling, furniture, etc.), which may be represented as a set of discrete frequency peaks and troughs denoting how the room amplifies or reduces acoustic energy of each frequency. Every sound that is heard in a room is filtered by this ‘equalizer’ which reduces sound energy in some frequency bands and amplifies it in others, providing a distinct colouration to any sound within the space.

Another key parameter, crucial in assisting the human ear in identifying different spaces, is the reverberation time. Reverberation time is responsible for the effect that is often described as a ‘dizzy’ sound field. This effect is more evident when there is more than one sound source in the room, for example in the dining space of a restaurant.

In other words, every sound in the space is filtered in both frequency and time domains.

Research has shown that the way we perceive sound in enclosed spaces may result in ‘acoustic fatigue’ which is directly linked to the frequency response and the reverberation time. Examples of work done on this topic include:

• Klatte, M.; Bastian, J.; Meis, M. & Noack, B. (2007). Wirkungen von Hintergrundgerauschen und Nachhall auf Sprachverstehen und Arbeitsgedachtnis in verschiedenen Altersgruppen. Fortschritte der Akustik. Beitrage zur 33. Jahrestagung fur Akustik, DAGA 2007, Stuttgart.

• Sato H, Bradley JS, Morimoto M., Using listening difficulty ratings of conditions for speech communication in rooms, Journal of the Acoustical Society of America (JASA) March 2015.

• Drgas S, Blaszak MA., Perceptual consequences of changes in vocoded speech parameters in various reverberation conditions, Journal of Speech Lang Hear Res. August 2009.

• Sato H, Morimoto M, Sato H, Wada M., Relationship between listening difficulty and acoustical objective measures in reverberant sound fields, JASA April 2008.

• Morimoto M, Sato H, Kobayashi M., Listening difficulty as a subjective measure for evaluation of speech transmission performance in public spaces, JASA September 2004.

• Sato H, Bradley JS, Morimoto M., Using listening difficulty ratings of conditions for speech communication in rooms, JASA March 2005.

• Prodi N, Visentin C, Farnetani A., Intelligibility, listening difficulty and listening efficiency in auralized classrooms, JASA July 2010

For example, a living room with highly reflective surfaces may result in undesirable ‘sharpness’, ie. a high proportion of high frequency sound.

Figure 1 shows an example of a) typical frequency response and b) typical reverberation time of different frequencies in a single-occupancy office.

One way of altering the acoustic properties of a space is by using Helmholtz resonators.

Figure 2(a) shows a simple Helmholtz resonator 2, comprising a cavity with one or more apertures or openings (each with an associated neck), which resonates at a certain frequency (f₀) determined by:

• Volume of the cavity (V) • Length of the neck(s) (L) • Cross-sectional area(s) of the neck (S)

The main principle of a Helmholtz resonator is that it performs selective acoustic absorption in a narrow frequency band in the immediate vicinity of its resonant frequency.

Figure 2(b) shows the absorption co-efficient of a Helmholtz resonator in the frequency domain, showing marked acoustic absorption at around the fundamental frequency (f₀).

Examples of Helmholtz resonators include bass-traps for recording studios, wallmounted panels for auditoria, perforated panels for offices, etc. These resonators are configured - by way of geometry and materials - to attenuate acoustic energy of specific frequencies, inherently the resonant frequencies of the resonators.

Typically, Helmholtz resonators have a specific geometry and therefore a discrete resonant frequency.

Various tuneable Helmholtz resonators are known. Examples are used for mitigating noise in industry and for tuning engine intake manifolds. There are also products such as the Vicoustic Vari Bass Tunable Helmholtz Resonator (http://arqen.com/store/vicoustic-vari-bass-helmholtz-resonator/), comprising a basstrap tuneable to a room's acoustics.

According to one aspect of the invention, there is provided an acoustic resonator, comprising: a cavity; and a plurality of apertures, the resonant frequency of the resonator dependent on the volume of the cavity and the properties of the apertures; and an actuator adapted to change, in dependence on time, the volume of the cavity and/or properties of the apertures and therefore the resonant frequency of the resonator.

In other words, an (dynamic) acoustic resonator having a resonant frequency defined by a physical dimension, comprising an actuator adapted to dynamically change the physical dimension and therefore the resonant frequency.

In yet other words, a dynamically adapting Helmholtz resonator is provided which changes its geometry in order to attenuate the acoustic energy in the appropriate frequency band.

Preferably, dynamically means in a time-varying or time-dependent way. In other words, the acoustic resonator is adapted to change its resonant frequency over time.

Preferably, the actuator is adapted to change the volume of the cavity and/or properties of the apertures by predetermined amounts at predetermined times.

As the geometry and therefore the resonant frequency of the dynamic acoustic resonator changes, the acoustic behaviour of the space or room in which it is located may be affected as a consequence. Thus, the room may as a result have a timevarying acoustic identity.

This is distinct from passive acoustic resonators which are intended to attenuate a specific noise or otherwise to manipulate the acoustics of the room a single or one-off time, say as might be done with a bass-trap to address a particular initial issue such as a specific frequency which has been identified as problematic.

This also differs from active systems, which use a speaker to generate audible sound. Such systems are intended to either kill the sound in space (which is not possible in multiple locations) or to mask the sound already in the room (which will create even more problems especially in rooms with more than one sound source).

For a given noise heard by a listener in a space with a particular acoustic identity or frequency response, use of such a dynamic acoustic resonator may mean the listener experiences the space as having an acoustic identity which changes over time, which may help reduce acoustic fatigue.

Preferably, the actuator is operable in different modes and is adapted to change the physical dimension by predetermined amounts at predetermined times. Either the amounts or times may be continuous or discontinuous. The actuator may be adapted to change the physical dimension by random amounts and/or at random times.

Preferably, the actuator is adapted to change the volume of the cavity and/or properties of the apertures by predetermined amounts at predetermined times. The amounts and/or times may be continuous or discontinuous. The actuator may be adapted to change the volume of the cavity and/or properties of the apertures by random amounts and/or at random times.

Preferably, the dynamic acoustic resonator comprises a plurality of apertures or openings, preferably of different sizes. This may mean the dynamic acoustic resonator may operate at multiple frequencies, ie. have more than one resonant frequency.

The plurality of apertures may be of different sizes. The size of wherein the size of at least some apertures may be variable. At least some apertures may be partially or fully covered. The acoustic resonator may comprise a panel, wherein at least some apertures comprise perforations in the panel. Or alternatively a membrane, wherein at least some apertures comprise perforations in the membrane, and the size of the apertures is variable due to stretching of the membrane by an actuator.

The acoustic resonator may comprise a panel, preferably a perforated panel. The perforations may be visible or non-visible. The panel may be installed in spaces such as living rooms, restaurants, cafes, etc., meaning generally in domestic or nonindustrial business spaces.

The acoustic resonator may comprise a surface or diaphragm moved by the actuator to change the volume of the cavity or a plurality of diaphragms, adapted to move dependently or interdependently. The acoustic resonator may further comprise a plurality of separators arranged between the plurality of diaphragms, thereby defining separate cavity volumes within the resonator.

The acoustic resonator may further comprise a receiver adapted to receive a signal representative of a sound; wherein the actuator is adapted to change, in dependence on the received signal, the volume of the cavity and/or properties of the apertures and therefore the resonant frequency of the resonator. The receiver may be adapted to detect the dominant frequency or frequencies in the space in which the acoustic resonator is located.

In other words, there is provided an (responsive, dynamic) acoustic resonator having a resonant frequency defined by a physical dimension, comprising an actuator adapted to change the physical dimension and further comprising a receiver adapted to receive a signal representative of a sound, wherein the actuator is adapted to change the physical dimension and therefore the resonant frequency of the acoustic resonator in response to the signal received by the receiver.

The receiver may be a microphone, preferably adapted to detect the dominant frequency or frequencies in a room or space.

In other words, the responsive dynamic acoustic resonator may change its geometry and therefore its resonant frequency according to the soundscape of the room by means of a microphone adapted to detect the dominant frequencies in the room and an actuator adapted to change the physical dimension.

The responsive dynamic acoustic resonator may further comprise any of the features of the dynamic acoustic resonator.

Preferably, the acoustic resonator may be operable in a one or more of a plurality of operating modes.

Preferably, the modes are selectable by a user. The user-selectable modes may comprise one or more of:

1. steady, wherein the dynamic acoustic resonator maintains a set geometry and therefore attenuates the same frequencies over time;

2. preset, wherein the acoustic resonator varies its geometry according to a preset or pre-determined pattern;

3. random, wherein the dynamic acoustic resonator varies its geometry at random, optionally wherein user may set the speed and/or magnitude of the variations of geometry, eg. by setting end-points

4. responsive, wherein the dynamic acoustic resonator detects the dominant frequencies and attenuates them by a corresponding change of geometry

According to another aspect of the invention there is provided a space or building having installed an acoustic resonator as herein described.

Further features of the invention are characterised by the dependent claims, where appended.

The invention also provides a computer program and a computer program product for carrying out any of the methods described herein, and/or for embodying any of the apparatus features described herein, and a computer readable medium having stored thereon a program for carrying out any of the methods described herein and/or for embodying any of the apparatus features described herein.

The invention also provides a signal embodying a computer program for carrying out any of the methods described herein, and/or for embodying any of the apparatus features described herein, a method of transmitting such a signal, and a computer product having an operating system which supports a computer program for carrying out the methods described herein and/or for embodying any of the apparatus features described herein.

The invention extends to methods and/or apparatus substantially as herein described with reference to the accompanying drawings.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied apparatus aspects, and vice versa.

Equally, the invention may comprise any feature as described, whether singly or in any appropriate combination.

Furthermore, features implemented in hardware may generally be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly.

As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure, such as a suitably programmed processor and associated memory, for example.

The invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows an example of a) typical frequency response and b) typical reverberation time of different frequencies in a single-occupancy office;

Figures 2 show (a) a simple Helmholtz resonator and (b) the absorption co-efficient of a Helmholtz resonator in the frequency domain;

Figures 3 show external and internal views of a dynamic acoustic resonator panel;

Figures 4 show further examples of dynamic acoustic resonator panels;

Figures 5 show examples of dynamic acoustic resonator panels in cross-section;

Figure 6 shows a dynamic responsive acoustic resonator panel;

Figure 7 shows examples of different operating modes of the dynamic responsive acoustic resonator; and

Figures 8 show further examples of dynamic acoustic resonator panels in crosssection.

Overview

Dynamic acoustic resonator

Figures 3 show (a) external and (b) internal views of a dynamic acoustic resonator panel 10 comprising a perforated front panel 20 of thickness L, perforations or apertures 25 of area S, and a solid rear panel 30 separated by a plurality of props 40 and side walls 50. Each perforation defines a volume V of a simple Helmholtz resonator between the panels 20 and 30. The dimensions L, S and V therefore correspond to those of the simple Helmholtz resonator described previously.

An actuator mechanism provides means for the dynamic changing of any of main elements or dimensions which determine its resonant frequency (f₀), ie. by changing one or more of the three elements V, L or S. The actuator may be hydraulic or electro-mechanical. Examples of suitable actuators include those provided by Inmoco and Festo (www.festo.com/cat/en-us_us/products).

Preferably, the actuator is damped so as to reduce the risk of introducing noise disturbance during operation.

The perforations 25 may be partially obscured by a cover (not shown), which could be actuated so as to vary how much of each perforation is covered, allowing S to be varied.

The front panel 20 may comprise a flexible material or membrane which may be stretched by actuators provided at the edge of the panel 20, so as to vary the area S of the apertures.

The thickness L of the front panel 20 may be varied for example by means of actuated telescopic struts contained within the front panel 20 or by providing the front panel 20 with flexible side walls, allowing the front panel 20 to be inflated to various thicknesses L.

The props 40 may be arranged to be telescopic, and may be actuated so as to increase or decrease in height, so as to vary volume V. Alternatively, the side walls 50 may comprise one or more inflatable chambers, allowing the volume V to be varied.

Figures 4 show further examples of dynamic acoustic resonator panels.

Figures 4(a) and (b) correspond to Figures 3(a) and (b) except in this case front panel 20 have perforations of two different sizes, Si and S₂ to enable selective acoustic absorption at two frequencies.

Evidently, panels may be made with a plurality of different sized perforations to enable selective acoustic absorption at a plurality of frequencies.

Figure 4(c) shows an embodiment in which the front panel 20 and rear panel 30 are fixed and the volume of the resonator cavity is changed by movement of an internal moving surface 45.

Figures 5 show examples of dynamic acoustic resonator panels in cross-section.

Figure 5(a) shows an embodiment with a fixed perforated front panel 20 and fixed and rear panel 30 and an internal moving surface driven by a piston mechanism which changes the volume of the cavity.

Figure 5(b) shows another embodiment with a fixed perforated front panel 20 and fixed and rear panel 30 wherein the front panel is hidden behind a membrane.

In some embodiments rear panel 30 may be porous or absorbent.

It will be appreciated that a variety of methods could be used to vary the elements V, L, or S.

For randomly changing resonance frequencies the transitions need not be rapid and a slow-moving piston moving slowly to random positions, increasing and decreasing the volume (V) of the panel, may be sufficient.

The result is a resonator panel 10 with a dynamically-changing resonant frequency, or a set of resonant frequencies.

When dynamic acoustic resonator panel 10 is placed in an enclosed space it changes the acoustic identity of the enclosed space over time, continually changing the peaks and troughs of the frequency response of the space. This may reduce the acoustic sharpness (proportion of high frequency sound) of the space, resulting in a more comfortable sonic environment, minimising acoustic fatigue.

The panel can be fitted on the area of a ceiling, the wall or can be adapted to fit the shape of - or incorporated into - a table, chair or any piece of furniture.

Typically, the panel 10 further comprises a mesh cover (not shown), usually for aesthetic reasons .

Power to the panel may be supplied for example via a light fitting, direct connection or batteries. In some embodiments power is supplied wirelessly.

In some embodiments the panel is of a modular design, with the base of the panel meshing with other panels and power being routed between the panels.

Dynamic responsive acoustic resonator

Figure 6 shows a dynamic responsive acoustic resonator panel 100. The dynamic acoustic resonator panel 100 comprises a perforated front panel 120, of thickness L and comprising perforations 125 of area S, and a solid rear panel 130 separated by a plurality of props 140 (not shown) and side walls 150. As such, the dynamic acoustic resonator panel 100 may have the same underlying structure as the previously described panel 10.

One or more miniature microphones 110 are attached to the dynamic acoustic resonator panel 100, in order to detect the sound-field.

Typically, one microphone attached on the panel is sufficient, although more may be used. Ideally, the microphone is located at the (typical) position of the listener(s) in order to determine the dominant frequency that the listener(s) experience and also to avoid the high-level reflections which would result if it were mounted on the panel.

The microphone 110 is connected to a controller system (provided in or external to the panel 100) which detects the dominant frequencies of the sound field (more specifically, by first calculating the average values in the various frequency bands and determining the average dominant frequency). Typically this is in response to the instantaneous sound field, as predicting the dominant frequencies is difficult.

The controller then calculates the geometric parameters of the panel (V,L and/or S) that need to be modified in order to absorb the acoustic energy at these dominant frequencies.

The geometry of the panel is then changed accordingly by an actuator mechanism, as previously described.

Altering the opening surfaces may be accomplished with a layer moving across the panel (behind the openings) and covering parts of their opening surfaces.

This is therefore an adaptive or responsive acoustic resonator system, which detects the interior sound field and alters its geometry thereby changing its resonant frequency (or frequencies) in order to provide a non-intrusive soundscape by minimising the acoustic energy at the dominant frequency or frequencies, which is what mainly affects the sharpness of sound.

Operating modes

Figure 7 shows examples of different operating modes of the dynamic (responsive) acoustic resonator. Generally, the dynamic (responsive) acoustic resonator is operable in different modes, preferably selectable by the user, including one or more of:

a) steady, wherein the dynamic acoustic resonator maintains a set geometry and therefore attenuates the same frequencies over time;

b) random, wherein the dynamic acoustic resonator varies its geometry at random, optionally wherein user may set the speed and/or magnitude of the variations of geometry, eg. by setting end-points;

c) responsive, wherein the dynamic acoustic resonator detects the dominant frequencies and attenuates them by a corresponding change of geometry

Alternatively, or in addition, the dynamic acoustic resonator may vary its geometry according to a pre-set or pre-determined pattern.

Examples of suitable patterns of response include those recited in the following:

• C. L. S. Gilford, Helmholtz resonators in the acoustic treatment of broadcasting studios, British Journal of Applied Physics, 1951.

• K.U. Ingard, On the theory and design of acoustic resonators, JASA 25, 1953.

Figure 8(a) shows an embodiment wherein the panel 10 further comprises a diaphragm 145 located between the front and back panels 120, 130 and being arranged to extend (preferably, between predetermined boundaries) between the side walls 150, in this case ‘horizontally’. In this embodiment, the volume V is defined between the front panel 120 and the diaphragm 145. The diaphragm may be arranged to be movable by an actuator to alter the volume V. The actuator may comprise an ‘artificial muscle’ made of an electroactive polymer.

Figure 8(b) shows an embodiment comprising two separate diaphragms or internal moving surfaces, and which may move dependently or interdependently, in this case ‘vertically’. Preferably a separator is present between the two moving surfaces, effectively dividing the internal volume of the resonator in two, V1 and V2.

Other embodiments may have multiple separate internal moving surfaces and/or separators dividing the internal volume of the resonator into yet more parts.

Yet other embodiments may comprise a mix of types of diaphragms and/or separators of the types described above.

Also provided is a hybrid embodiment comprises dynamic and dynamic-responsive acoustic resonators in combination, either as separate units or as an acoustic resonator having the features of both types. For example, more than one dynamic acoustic resonator or panel is installed in the same room. The panels may operate in dependence on one another or independently. Preferably, a panel operating in random mode is arranged to operate independently of a panel operating in responsive mode.

In summary, dynamic resonators are described which dynamically change the resonator geometry in order to constantly and/or repeatedly change the frequency response of the room, optionally attenuating the dominant frequency.

It will be understood that the invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Reference numerals appearing in any claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

Claims

1. An acoustic resonator, comprising:

a cavity; and a plurality of apertures, the resonant frequency of the resonator dependent on the volume of the cavity and the properties of the apertures; and an actuator adapted to change, in dependence on time, the volume of the cavity and/or properties of the apertures and therefore the resonant frequency of the resonator.

2. An acoustic resonator according to claim 1, wherein the actuator is adapted to change the volume of the cavity and/or properties of the apertures by predetermined amounts at predetermined times.

3. An acoustic resonator according to claim 2, wherein the amounts and/or times are continuous or discontinuous.

4. An acoustic resonator according to any preceding claim, wherein the actuator is adapted to change the volume of the cavity and/or properties of the apertures by random amounts and/or at random times.

5. An acoustic resonator according to any preceding claim, wherein the plurality of apertures are of different sizes.

6. An acoustic resonator according to claim 5, wherein the size of at least some apertures is variable.

7. An acoustic resonator according to claim 5 to 6, wherein at least some apertures are partially or fully covered.

8. An acoustic resonator according to any preceding claim, further comprising a panel, wherein at least some apertures comprise perforations in the panel.

9. An acoustic resonator according to any of claims 6 to 8, further comprising a membrane, wherein at least some apertures comprise perforations in the membrane, and the size of the apertures is variable due to stretching of the membrane by an actuator.

10. An acoustic resonator according to any preceding claim, comprising a surface or diaphragm moved by the actuator to change the volume of the cavity.

11. An acoustic resonator according to claim 10, comprising a plurality of diaphragms, adapted to move dependently or interdependently.

12. An acoustic resonator according to claim 11, further comprising a plurality of separators arranged between the plurality of diaphragms, thereby defining separate cavity volumes within the resonator.

13. An acoustic resonator according to any preceding claim, further comprising:

a receiver adapted to receive a signal representative of a sound; wherein the actuator is adapted to change, in dependence on the received signal, the volume of the cavity and/or properties of the apertures and therefore the resonant frequency of the resonator.

14. An acoustic resonator according to claim 13, wherein the receiver is adapted to detect the dominant frequency or frequencies in the space in which the acoustic resonator is located.

15. An acoustic resonator according to any preceding claim, operable in a one or more of a plurality of operating modes.

16. An acoustic resonator according to claim 15, wherein the modes are selectable by a user.

17. An acoustic resonator according to claim 15 or 16, the modes comprising one or more of:

a) steady, wherein the acoustic resonator maintains a set geometry and therefore attenuates the same frequencies over time;

b) preset, wherein the acoustic resonator varies its geometry according to a pre-set or pre-determined pattern;

c) random, wherein the acoustic resonator varies its geometry at random, optionally wherein user may set the speed and/or magnitude of the variations of geometry, eg. by setting end-points; and

d) responsive, wherein the acoustic resonator detects the dominant frequencies and attenuates them by a corresponding change of geometry.

18. A space or building having installed an acoustic resonator according to any preceding claim.