WO2003002955A1

WO2003002955A1 - Method and system for modification of an acoustic environment

Info

Publication number: WO2003002955A1
Application number: PCT/DK2002/000440
Authority: WO
Inventors: Preben Kaeseler
Original assignee: Kkdk A/S
Priority date: 2001-06-28
Filing date: 2002-06-28
Publication date: 2003-01-09

Abstract

Method for determining the modification of an acoustic environment, where the actual acoustic response is measured after which a model calculation is performed to reveal the amount of damping modules to be placed in the environment in order to reduce the Reverberation Time at certain frequencies in the environment.

Description

Method and system for modification of an acoustic environment

FIELD OF THE INVENTION

The present invention relates to a method and a system for modification of an acoustic environment.

BACKGROUND OF THE INVENTION

In contrast to open space, where the energy from an emanating sound proceeds out- ward, the distribution of sound emanating from a source in a closed room is altered by confining the energy to the boundaries of the enclosure. Objects within a room as well as the geometry and structure of the walls of the room alter the diffusion of sound by scattering the directions of the sound waves. Furthermore, in dependence of the physical properties of the objects and the walls, the frequency spectrum of the sound is changed due to absorption of energy of the sound waves. As a rule of thumb, porous materials, for example fibres, damp the higher frequencies, that is to say from a few hundreds of Hz and upwards, because the sound waves are converted into heat by moving through the interstitial spaces of the material and by the vibration of the small fibres. On the other hand, while so called hard and heavy absorbents, for example pan- els, have a higher absorption at low frequencies, because a panel usually vibrates with lower frequencies.

In addition, the propagation of sound in a room is affected by resonance phenomena in the room. Resonance phenomena in a room are dependent on the size of the room and the damping properties of the walls and interior. The period required for a sound level to decrease with a certain amount, in particular 60 dB, after sound source has stopped producing sound is defined as the Reverberation Time. A long Reverberation Time at frequencies corresponding to speech is disturbing in an office environment, why these corresponding frequencies are aimed to be damped as much as possible. On the other hand, Gregorian music is mostly enjoyable in rooms with long Reverberation Times, for example churches. For Rock concerts, a short Reverberation Time is desirable for the lower frequencies. Typical desired Reverberation Times are for class rooms and office environments 0.6 seconds, for Rock music 0.7 - 1 second, for classical concerts 2 seconds.

In order to alter the resonance phenomena in a room in dependence of the desired sound experience, different kind of damping equipment is known.

From US patent no. 4 362 222, a system is known with sound absorbents in the form of plates or mats that are arranged at an angle in a corner area formed by the walls and ceiling of the room. In the corner area behind the absorbent, an air volume is trapped so that the absorbent due to the sound influence has a membrane effect.

From US patent no. 5 418 340 a system is known for acoustical partition for a stereo or listening room which functions to balance the frequencies from the sound output by absorbing standing base wavelengths and providing multi-wavelength absorption and reflection capabilities for mid- wavelengths and high- wavelengths. This is achieved by an acoustical partition comprising a wall section defining an enclosed space, a frame structure contained within the wall section, and an insulation layer contained within the frame structure such that an air cavity is created at the most central area within the wall section.

These systems have a certain characteristic that does not allow to vary the damping properties in a multiple way, which would be desirable if a room should be used for different purposes, for example a classical concert one day and a presented talk on another day. In connection with these systems no advice is given, how these systems should be applied to achieve a certain predetermined acoustic characteristic of the room.

The latter problem has been solved by a system as disclosed in International Patent Application WO90/04071. In this system for modifying an acoustic environment modular absorption and/or diffusion elements are provided, which can be located at the walls or ceiling of the space constituting the acoustic environment. Each modular element may be one of three types; the first type providing variable absorption, the second type providing variable absorption in a mutually exclusive frequency range to the first type, and the third type providing variable absorption or alternatively diffusion over a broad frequency range. Each modular element contains a local control means to effect the variation, and an external actuation sensor for manual control over each modular element. There are also one or more transducers and a noise source located within the space which provides a determination of the actual acoustic environment.

Though this latter system has a broad range of application, is suffers from the disadvantage of being very expensive. Furthermore, no advice is given, where the elements should be placed from the beginning. The customer thus needs a number of these ex- pensive elements and has to determine the right position and response by on-line measurements in the room. This difficult task requires an expert at the location for optimal adjustment.

DESCRIPTION/SUMMARY OF THE INVENTION

It is the purpose of the invention to provide an improved method and system for modification of an acoustic environment, which preferably is cheap for the customer to establish.

This purpose is achieved with a method for determining the modification of an acoustic environment comprising

- measuring at predetermined places in the environment the actual acoustic response of the environment to an emitted sound profile,

- registering response data indicative of the actual acoustic response, - registering environment data indicative of the enclosure of the environment,

- identifying an intended acoustic response profile for the environment,

- computing from the response data and the environment data the locations of acoustic modules in the environment for achieving the intended response profile in the environment, wherein the acoustic modules comprises acoustic dampers and acoustic dif- fusers.

The method comprises measuring at predetermined places in the environment the actual acoustic response of the environment to an emitted sound profile. The environ- ment may for example be concert hall, a classroom, an office environment, a listening room, or a restaurant. Even for outdoor environments may the method of the invention find application.

In this environment, for simplicity in the following called room without loss of generality, a sound with a certain profile is emitted and the response is measured, preferably with a microphone with known characteristics, and recorded. Such a sound should usually have a profile which reveals the response of the room for a broad range of frequencies. However, the method works as well for sound profiles limited to a narrow frequency range. Broad noise profiles are achieved with white noise, pink noise, and sound from an explosion of a gas balloon. For example, a gas balloon may be brought to explosion at certain locations inside the actual room, after which the acoustic response of the room is measured and recorded, typically as a set of response data, preferably digital data. The acoustic response of the room may be different depending on the location of the explosion and recording different sets of data for different locations would result in a more complete response characterisation of the room. However, in many cases, the response data for a single explosion may be sufficient.

The acoustic response of the room may be different from the intended acoustic re- sponse profile that has been identified as the optimum. For example, for a certain room to be used as a classroom, the Reverberation Time may be very long, while the intention may be a short Reverberation time, especially at higher frequencies. In this case, the actual acoustic response has to be changed in order to obtain a satisfactory environment.

For completeness of the later calculation, environment data indicative of the enclosure of the environment are registered as well. The enclosure means primarily the form of the room with walls, plateaus and the like. For the environment data, also the properties and locations of acoustically influencing objects in the enclosure may be taken into account. Certain objects, as for example heavy armchairs are known to have damping properties as well, and the location of these objects give an indication of the altering of the response profile in dependence of these objects. From the response data and the environment data, a model is constructed, preferably in a computer program, where virtual acoustic modules, which are acoustic dampers and/or diffusers with certain characteristics, are placed in the model room at different locations for achieving the intended response profile in the environment. Thus, the computer routine performs a calculation of the change of the acoustic response profile in dependence of placement of the number, type, and location of the modules. In order to obtain changes of the acoustic response for certain frequencies, the computing implies decomposition of the response data into subsets of data corresponding to discrete frequency intervals and determining for the subsets of data the corresponding Rever- beration Time. This Reverberation Time may be dependent on the location in the room, which during computation can be taken into account if several sets of response data are available.

This computer routine may be performed with an operator, but may as well be an it- erative process done automatically by the computer. In the iterative process, virtual modules are placed in the model room in the computer program at certain start positions, for example dampers may be placed in corners, if the damping should be mostly at low frequencies, and on walls or ceilings, if the damping primarily is for higher frequencies. The influence of these modules in the model room is calculated with the actual response data as a starting data set. The modules are then replaced and a new response profile is calculated. Changing the location of the modules and adding or subtracting these virtual modules after relatively few iterations results in a response profile which approximately is equal to the intended response profile.

The final data set reveals the number of necessary modules and their location inside the room. The modules may then be purchased for the real environment and placed in the room in accordance with the model. To ensure that the acoustic adjustment is easily applicable for the user of the environment and also affordable, the modules are passive modules, that is to say, preferably no electronics with active function is incor- porated in these modules. However, if the electronic response of the modules is known, the method according to the invention may also be used with this kind of modules. Preferably, the measuring, storing and identifying is performed by a user of the environment, while the computing is performed by an agent. The term agent covers a person performing the calculation with a computer program, but covers also an automated computer program performing the calculation without human interference.

For example, the user may after having performed the measurements send the response data, the environment data and the identification of the intended response profile to the agent. This transmitting from the user to the agent is done via a data transmission network, preferably the Internet. After computing by the agent, the calculated results with the proposed data for the location of the dampers and/or diffusers in the room are transmitted back to the user.

This embodiment allows the user at low cost and with high efficiency to change the acoustic environment of a room in a direction desired by the user. As the Internet is available at most places on the earth, the analysis of the room with the proposal of dampers and diffusers is not geographically limited. The user has to purchase the dampers and eventual diffusers afterwards, eventually by post order through the Internet, and install them at the prescribed locations. However, the user needs not buy more modules than proposed by the agent, which is an advantage. Without the calculation, the user might have been tempted to buy a higher number of modules in order to have sufficient degrees of freedom for by trial and error methods to achieve an acceptable result. The method according to the invention is therefore a very efficient way of optimising acoustic environments.

Advantageously, the modules may be categorised in at least two categories, where the frequency of the maximum damping efficiency of the modules are dependent on the category. In case the user desires to change the acoustic environment for higher frequencies, he may be proposed to buy the corresponding modules, dampers and/or diffusers for this frequency range. The advantage is, that costs for the user and weight are kept at a minimum, in contrast to for example certain damping modules according to prior art, where a broader range of frequencies is damped and where the modules are larger and heavier, and therefore more expensive. The modules of the system that the user may install are in principle different for different categories depending on the frequency range and whether the module is a damper or a diffuser. However, the modules may have a visual appearance independent of the category. This is aesthetically advantageous, because the modules can be designed with a more anonymous appearance when installed in a room. For example, the modules may be designed as boxes with a thin fabric in front which has negligible influence on the damping or diffusing properties. However, even if the front cover would have influence on the damping or diffusion properties of the module, the method of the invention allows for incorporation of the effect in the calculation proc- ess, and the intended acoustic response may be reached nevertheless.

Especially important is that the dimensions of the different modules are equal. In case that one module is placed on top of another module, the lower module may be exchanged with a module from another category without changing the location of the upper module. This would not be possible if the lower module would be exchanged with a larger one. This is important for the calculation, because this way, one virtual module can be exchanged with another to investigate the effect of this change without having to take into account the eventual displacement of the remaining virtual modules. Also for the placement in the environment in reality, this leaves more degrees of freedom for the actual placement of the modules, than if modules in different categories would have different sizes. Also, equal dimensions of different modules in the system are advantageous for the aesthetic appearance of the final configuration in the environment.

Preferably, the modules, which the user may be proposed to purchase and install, are categorised in three categories, where the first category implies intended damping in the low frequency range between 20 Hz and 230 Hz, preferably between 50 Hz and 160 Hz, where the second category implies intended damping in the mid frequency range between 80 Hz and 1600 Hz, preferably between 100 Hz and 1 kHz, and the third category implies intended damping in the high frequency range between 100 Hz and 20 kHz, preferably between 125 Hz and 10 kHz. Low frequency dampers according to the invention for the purpose as described may have a damping membrane with an area weight of between 2 and 4 kg/m , preferably between 2.5 and 4 kg/m², and most preferably between 3.12 and 4 kg/m².

Mid frequency dampers according to the invention may have an absorber front equipped with openings, wherein these openings have a diameter of between 1 and 10 mm and an opening depth of between 4 to 6 mm, and wherein the depth of the modules in the second category preferably is between 0.18 and 0.20 m.

For the higher frequency damping, the dampers have porous and/or a fibrous material inside.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to the drawings, where

FIG. 1 shows a room, wherein the acoustic profile is to be measured, FIG. 2 is a flow diagram for the method according to the invention, FIG. 3 shows a room with arranged modules, FIG. 4 shows a damper in detail, FIG. 5 shows the frequency plate for a mid frequency damper,

FIG. 6 shows a diffuser, FIG. 7 shows the resonance frequency of a low frequency absorber in dependence of the module depth and the membrane weight, FIG. 8 shows absorption profiles in dependence of frequency for three different dampers,

FIG. 9 shows resonance frequencies for a mid range absorber with 296 holes in dependence of hole radius for different ratios between hole radius and module depth, FIG. 10 shows resonance frequencies for a mid range absorber with 517 holes in de- pendence of hole radius for different ratios between hole radius and module depth. DETAILED DESCRIPTION / PREFERRED EMBODIMENT

In FIG. 1, part of a room 1 is shown, wherein the acoustic profile is to be measured. A sound source 2, for example a loudspeaker or a gun, is placed in a corner 3 of the room. A different sound source 2 may be a gas balloon. The emitted sound from the exploding balloon 2 is measured and recorded with a microphone 4. Preferably, the characteristic of the microphone is known, which is an advantage for the calculation at a later stage.

As an alternative to the balloon explosion, other noise may be emitted, for example white or pink noise from a sound generator. This can be a specific generator with a known output characteristic, but also the users own stereo system may be used for this purpose. The performance of the loudspeakers may influence the frequency spectrum of the noise, however, as only the Reverberation Time is taken into account in the cal- culations, the computed result is independent of the loudspeaker performance.

FIG. 2 shows in a flow diagram, the method according to the invention. In a first step, a sound is emitted 201 from a certain location of the room. The actual acoustic response is measured 202 with a microphone and recorded. This procedure may be re- peated 203 for different locations in order to achieve a more complete data set for the acoustic performance of the room.

This data set may be recorded and translated into a digital format 204 that without loss can be transmitted 205 to the location for the computing process 208. The computing process may be performed at the location of the room, but may also be at a distant location to which the data are sent 205 via the Internet.

Also environment data 206 indicative of the enclosure of the room, that is to say data describing the walls and object in the room, are created and sent 205 to the location of the computing process. Typically the environment data and the intended acoustic profile 207 are sent together with the data set for the measured acoustic response. The intended acoustic profile may contain information about the intended Reverberation Time for different frequencies at certain locations in the room, for example the centre of the room.

From the environment data, a model for the room is created 209, and according to predetermined algorithms in the computer program, a first set of virtual dampers and/or diffusers are introduced 210 into the model and a certain location is found 211. Typically, low frequency absorbers are placed in corners of the room and mid frequency and high frequency absorbers are placed on walls and ceilings. From the actual acous- tic response profile, a new profile is calculated 212 on the basis of the virtual dampers, eventual virtual diffusers and their location in the model. From the amount of dampers, the efficiency of damping can be calculated for different frequencies and the corresponding Reverberation Times can be calculated. The calculated profile is then compared 213 with the intended acoustic profile.

If there is approximate agreement within predetermined acceptable uncertainties, the location of the modules and the identifications of the modules is sent 124 to the user as a prescription for modifying the acoustic characteristics of the room. The user has to purchase the determined modules and place those modules at the right location in congruence with the computed data. The computer program takes into account that as few modules as possible should be used to find the appropriate acoustic profile.

If there on the other hand is no sufficient agreement between the computed acoustic response and the intended acoustic response, new locations of dampers and eventual diffusers are found 212. Eventually, new virtual dampers and/or diffusers are introduced 212 into the model. This iterative process is fast and may be computed automatically by a computer without the requirement of an operator.

After having received the information of which modules to purchase and where to place those modules, the user may modify the room to achieve the intended acoustic response. Examples for typical arrangements of modules are shown in FIG. 3a, 3b, and

3c. The situation as shown in FIG. 3a and 3c is typical, if a short Reverberation Time is desired for high frequencies, while the situation as shown in FIG. 3b is characteristic for decrease of damping low frequencies.

The user may chose to receive more than one computational result for different in- tended profiles. This may be of interest, if the room is used for different sound experiences, for example a classical concert and also rock concerts, where in the latter situation the Reverberation Time should be much shorter for preferred sound experiences. In this case, the computation may take into account the intention of the user to relocate as few as possible modules for changing between the different acoustic response pro- files. This may be important for the user in case that a concert hall has to be modified acoustically within a very short time, for example from one day to another.

In FIG. 4, a typical constructional scheme is shown for a damper according to the invention. Inside a box with side walls 401, and back wall 402, a damping material 403, typical fibrous or porous material, for example paper, is located in case that the damper is used for high frequency damping. A front cover 404 is used to achieve an outer appearance independent of the type of damper.

For a mid frequency damper, a frequency plate 405 is placed in front of the damping material 403. The frequency plate 405 is shown in more detail in FIG. 5. It comprises an arrangement of holes 501 in the plate, where the diameter of the holes, the depth of the holes depending on the thickness of the plate, and the number and location of the holes influence the performance of the plate.

For a low frequency damper, the frequency plate 405 is substituted by a damping membrane 406. The damping membrane, for example made of plastic or bitumen, is elastic but has a relatively high weight in order to absorb energy from the low frequency wave.

A diffuser 600 is shown in FIG. 6. The diffuser 600 comprises a number of diffusing elements 601 which have to be oriented perpendicular to the desired diffusing direction. The ability to diffuse efficiently in a certain frequency range depends on the dimensions of the diffusing elements 601. In case that a broad frequency range is desired to be diffused, it is possible in accordance with the invention to equip a module with more than one type of diffuser. In FIG. 6, two diffusers 600, 600' with different types of diffusing elements 601, 602 rare shown. The two diffusers 600, 600' can be combined to one module according to the invention. Inside such a module, more than two diffusers can be arranged as well if this should be desirable.

Typically, diffusers have been fabricated with plates or sticks of wood, which, however, results in very heavy diffusers. Diffusers according to the invention are preferably made of a light weight material, for example rigid polymer foam. The reason for the use of a diffuser in an acoustic environment is to counteract certain acoustic reflections which are experienced as disturbing for the listener. This may be achieved with dampers as well, but it might be desirable to avoid these disturbing reflections without influencing the Reverberation Time. In the latter case, a diffuser is preferred to a damper.

The different types of modules are covered with the same front cover 404, which may be a fabric that does not influence the propagation of sound through it. However, if the front cover 404 influences the sound propagation through it, this may easily be taken in to account in the computing process.

For a bass damper, typical resonance frequencies are shown in FIG. 7 in dependence of the thickness of the module. The lowest curve corresponds to a membrane weight of 4.0 kg/m", while for the other curves, the membrane has a weight of 3.12, 2.5 and 2.0 kg/m² respectively. It is seen that for a module thickness of 0.20 m, the resonance fre- quency of approximately 77 Hz if the membrane weight is 3.12 kg/m². The higher the resonance frequency, the higher are the frequencies that are absorbed efficiently.

FIG. 8 shows the characteristic absorption curves 801, 802, 803, for a low, mid and high frequency damper, respectively.

The dampers are 0.6m x 0.6 m with a depth of 0.20 m. The low frequency damper has a membrane with a weight of 3.12 kg/m². As can be seen from the curve 801, the damping is efficient a very broad region of 20 to 230 Hz, typically more than one octave, and highly efficient in the region of 50 Hz to 160 Hz.

The shown absorption curve 802 corresponds to the performance of a mid frequency damper with 296 holes, hole depth of 8 mm, hole diameter of 5 mm, and a module depth of 180 mm.

In FIG. 9, the resonance frequency of a mid frequency damper is shown in dependence of the hole radius in mm. From the lowest to the most upper solid curve, the ratio of hole depth to the module depth is 10/200, 8/200, 6/200, and 4/200, respectively, and for the lowest to the upper most stippled curve, the ratio of hole depth to the module depth is 10/180, 8/180, 6/180 and 4/180, respectively. It is seen that decreasing the hole depth, decreasing the depth of the module, and decreasing the hole diameter increases the resonance frequency.

In FIG. 10, the resonance frequencies for a mid range frequency absorber is shown with 517 holes. It is seen in comparison with FIG. 9 that an increase of the number of holes implies an increase of the resonance frequency, especially for larger hole diameters.

Thus, for each certain frequency range, a damper can be found which is especially efficient for this frequency range.

Claims

1. Method for determining the modification of an acoustic environment comprising

- measuring at predetermined places in said environment the actual acoustic response of said environment to an emitted sound profile,

- registering response data indicative of said actual acoustic response,

- registering environment data indicative of the enclosure of said environment,

- identifying an intended acoustic response profile for the environment,

- computing from said response data and said environment data the locations of acoustic modules in the environment for achieving said intended response profile in said environment, wherein said acoustic modules comprises acoustic dampers and acoustic diffusers.

2. Method according to claim 1, wherein said environment data also are indicative for the properties and locations of acoustically influencing objects in said enclosure.

3. Method according to claim 1 or 2, wherein said computing implies decomposition of said response data into subsets of data corresponding to discrete frequency intervals and determining for said subsets of data the corresponding Reverberation Time.

4. Method according to any single one of the claims 1 - 3, wherein said computing implies calculation of an altered acoustic response profile for the environment in dependence of said acoustic modules at determined fictive locations in said environment and changing said determined fictive locations of said applied modules in an iterative process for achieving approximate agreement between said intended response profile and said altered response profile.

5. Method according to any single one of claims 1 - 4, wherein said acoustic modules are passive modules placeable at different locations in said environment.

6. Method according to any single one of claims 1 - 5, wherein - said measuring, registering and identifying is performed by a user of said environment,

- said computing is performed by an agent,

- said method further comprises, prior to computing by said agent, transmitting from said user to said agent via a transmission network said response data, said environment data and said identification of said intended response profile, and, after computing by the agent, transmitting to the user data indicative of the finally calculated fictive locations for the modules in said environment.

7. Method according to claim 6, wherein said transmission network comprises at least one from the group consisting of the Internet and a telephone network.

8. Method according to any single one of the claims 1 - 7, wherein said modules are categorised in at least two categories, where the frequency of the maximum damping efficiency of the modules are dependent on the category.

9. Method according to any single one of the claims 1 - 8, wherein said emitted sound profile is known and at least one from the group consisting of white noise, pink noise, and sound from an explosion of a gas balloon.

10. Use of a method according to any single one of the preceding claims for acoustically modifying a room, where said room is at least one from the group consisting of a concert hall, a classroom, an office environment, a listening room, and a restaurant.

11. Modular system of passive acoustic modules usable for any single one of the methods in claims 1 - 9, wherein said modules comprises acoustic dampers and acoustic diffusers and are categorised in at least two categories, where the frequency of the maximum damping and/or diffusion efficiency of the damping modules are dependent on the category.

12. Modular system according to claim 11, wherein said modules have a visual appearance independent of the category.

13. Modular system according to claim 11 or 12, wherein said modules are categorised in three categories, where the first category implies intended damping in the low frequency range between 20 Hz and 230 Hz, preferably between 50 Hz and 160 Hz, where the second category implies intended damping in the mid frequency range be- tween 80 Hz and 1600 Hz, preferably between 100 Hz and 1 kHz, and the third category implies intended damping in the high frequency range between 100 Hz and 20 kHz, preferably between 125 Hz and 10 kHz.

14. Modular system according to claim 13, wherein modules in said first category are dampers with a damping membrane having an area weight of between 2 and 4 kg/m², preferably between 2.5 and 4 kg/m², and most preferably between 3.12 and 4 kg/m².

15. Modular system according to claim 13, wherein said modules in said second category are dampers with an absorber front equipped with openings, wherein these openings have a diameter of between 1 and 10 mm and an opening depth of between 4 to 6 mm, and wherein the depth of said modules in said second category is between 0.18 and 0.20 m.

16. Modular system according to claim 13, wherein said modules in said third cate- gory are dampers with a porous and/or a fibrous material.

17. Modular system according to claim 11 - 16, wherein each of said modules has solid sidewalls and a solid back wall and an acoustically transparent front cover forming a box with a depth of between 0.18 m and 0.25 m, where the width and the height of the box is between 0.5 and 0.7 meters, preferably 0.6 m.