Patent Publication Number: US-8995674-B2

Title: Multiple superimposed audio frequency test system and sound chamber with attenuated echo properties

Description:
This application is a divisional application of U.S. utility patent application Ser. No. 12/391,227, filed Feb. 23, 2009, which claims priority to U.S. provisional application 61/151,442, filed Feb. 10, 2009, which are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     An echo, or acoustic reflection, occurs when an acoustic wave encounters an object such as an enclosure wall. When a reflection occurs, the reflected wave interacts with the wave that was originally directed towards the object causing the reflection. The waves are often labeled as the incident and reflected waves. At low amplitudes the two waves interact in simple superposition, adding to produce a sound pressure pattern in space. In a typical system, the acoustic wave/reflection result occurs in three dimensions. In an environment with walls that reflect most of the wave directed at them, points can be seen where the resultant sound pressure decreases to 10 percent or less of the amplitude of the initial incident wave. 
     The addition of incident and reflected waves produce a sound pressure pattern that is typically quite complicated. This pattern is also dependent on the frequencies of the waves. A complex waveform containing many frequencies will have a set of reflection patterns, each dependent on an individual frequency. The result is that it is very difficult to know the sound pressure at any point in a 3 dimensional space that contains reflective surfaces. 
     A device to be tested, be it a sound emission device like a speaker, a sound reception device like a microphone, or a combination device like a hearing aid, has apparent acoustic properties affected by the environment in which it is tested. If the environment contains surfaces that reflect acoustic waves, the properties of the device under test are subject to reflection artifacts. Unfortunately, surfaces and objects reflect acoustic waves. The best that can be done is to provide a surface, or combination of surfaces, that have small acoustic reflections that do not significantly affect the measurement of the device under test. 
     Some acoustic devices are constructed to have directional properties. For these devices it is important to measure device characteristics in an acoustic environment with few reflections. Often a chamber known as an “anechoic chamber” is used for such testing. As noted above, there is no such thing as a chamber that has no reflections. However, chambers have been constructed that have sufficient attenuation of reflections to allow reasonable testing of these directional devices. Typically, these chambers are large. Current technology uses sound absorbing wedges that are a substantial percentage of a wavelength deep. For low frequency operation, the chamber must be large in order so that the walls formed by the wedges are thick enough to absorb the sound waves. 
     The wedges are typically constructed using a wire form that is stuffed with fiberglass. The wire itself reflects a certain amount of acoustic energy, as does the fiberglass. If the wedges have relatively sharp edges, only very high frequencies will be reflected off of the wedge edges, and only a small percentage of the waves will be reflected back toward the generator of the incident wave. 
     The wedges are also constructed with relatively sharp angles. Waves that encounter a wedge side surface will reflect off the surface. The sharp angles of the wedge sides cause the wave reflection to move inward toward a surface of another adjacent wedge. The adjacent wedge then reflects the wave back toward a deeper portion of the first wedge. Thus, the acoustic wave works its way towards the wedge base and hopefully is mostly absorbed by the time the wave reaches the wedge base. Of course, the wedges hold fiberglass, which is a good absorber of sound. Therefore most of the signal that hits the side of the wedge is absorbed in the fiberglass material and only a small percentage is reflected. 
     The reflection behavior of a wave from a surface is dependent on the dimensions of the surface and the wavelength. If a sound chamber is small compared with the wavelength, then reflections may be ignored and the enclosure may be thought of as a pressure box. Relatively small anechoic chambers are therefore not effective for low frequencies with wavelengths that exceed the dimensions of the chamber. The damping action of the wedges in a sound chamber is also reduced when the dimensions of the wedges are an appreciable percentage of a wavelength. 
     In recent years, certain types of open cell foams have been available for acoustic damping of surfaces in chambers and rooms. Some of these foams have desirable properties that reduce sound transmission through the foam and also attenuate reflections of waves directed at the surface of the foam. The foams come in a variety of densities and construction. 
     As with fiberglass, sound incident on a foam surface is partially reflected as well as attenuated upon entering the material. A portion of a sound wave hitting a simple surface covered with a thickness of foam will be reflected from the surface of the foam and a portion will travel into the foam. If the thickness of the foam is increased, sound will be attenuated as it proceeds through the foam. When the sound travels completely through the foam thickness, it will eventually encounter the underlying surface. For example, a concrete or wood wall surface that supports the foam. Most of the sound encountering this surface will be reflected back into the foam material and undergo further attenuation before emerging from its outer surface. 
     Thus an incident sound wave encountering a simple plane damping surface will split. Some will be reflected and the rest will travel into the damping material and eventually emerge attenuated in amplitude. This returning attenuated sound will add to the initially reflected sound from the front surface of the damping material. The portion of the incident sound that is initially reflected from the front surface appears to be unaffected by an increase in the thickness of the damping material. 
     Acoustic devices of all types, including receivers (microphones) and generators (speakers), have a pattern to the way they operate. The sound that they receive or generate typically has a 3 dimensional directional component. For speakers, the sound emanating from the device is typically directed in one particular direction more than other directions. The same is sometimes true for microphones. Sometimes microphones or devices that employ microphones are constructed in a way that enhances the directional capability of the device. The directional characteristic of the acoustic device is also typically dependent on the acoustic frequency. Because of the wavelength nature of a sound wave, devices handle different frequencies in different ways. 
     From an engineering and manufacturing perspective, it is desirable to know the pattern that the acoustic device exhibits at each frequency. Tests are typically run on the device in areas that are as free of reflected sound as possible, such as in an anechoic chamber or in a chamber free of echo. Sounds from speakers can then be tested for their directional pattern. Microphones can be located at different points in the sound generation path of the speaker to collect this information. Or the microphone can be kept in one spot and the speaker moved to different orientations for the test. 
     Directional microphones can be tested in similar ways. The microphone can be held in a constant position and the sound source moved to make a test, or the microphone orientation can be changed, holding a fixed sound source location. 
     The typical system will test the speaker or microphone directional pattern characteristic one frequency at a time. The data is often displayed in a graphical format called a polar plot. The plot exhibits the directional performance of the device for that frequency in a particular plane of operation and is labeled as amplitude vs. angular position within that plane. 
     Another possible display of the information is in the form of a series of overlaid frequency response curves. Each curve has a different positional angle from a reference angle. Sometimes this information will be confined to the angle at which the greatest sensitivity or efficiency is demonstrated and the angle at which the sound is at the lowest amplitude. There are a number of ways in which the information may be displayed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a sound chamber with improved sound dampening. 
         FIG. 2  is a partial top sectional view of the sound chamber shown in  FIG. 1 . 
         FIG. 3  is a partial side sectional view of the sound chamber shown in  FIG. 1 . 
         FIG. 4  is a block diagram of a multi-frequency testing system. 
         FIG. 5  is a flow diagram showing in more detail how the testing system in  FIG. 4  generates a composite acoustic signal. 
         FIG. 6  is a flow diagram showing in more detail how the testing system in  FIG. 4  identifies frequency characteristics for a device tested using the composite acoustic signal. 
         FIG. 7  is a polar plot generated from frequency characteristics identified in  FIG. 6   
     
    
    
     DETAILED DESCRIPTION 
     Sound Chamber with Attenuated Echo Properties 
     It is desirable in the testing of small acoustic devices like microphones and hearing aids to build small chambers with desirable nearly anechoic properties. It is also known that traditional anechoic techniques require large chambers or rooms to achieve a desired reduction in reflection from chamber surfaces. Therefore a different technique is needed when constructing a small chamber with desired anechoic properties. Because of the surface reflection problems noted above, there is a limit to the amount of reflection reduction that can be achieved with the use of simple plane foam damping materials placed on the surfaces of a sound chamber. 
       FIG. 1  shows a new composite dampening structure  14  that reduces reflections of acoustic energy in a relatively small sound chamber  12 . The sound chamber  12  includes an exterior wooden box  15  having a bottom portion  15 A that contains a speaker  20  and a device under test (DUT)  18 . An upper portion  15 B of the box  15  rotates downward and covers a lower open section of bottom portion  15 A. The DUT  18  can be any type of audio device that requires acoustic testing. For example, the DUT  18  may be a directional microphone, hearing aid, transducer, speaker, or any other type of audio transmitter or receiver. 
     The relatively small sound chamber  12  uses the composite damping structure  14  to substantially reduce the reflection of audio signals. The composite damping structure  14  includes a layer of wedges  26  made of a first damping material and a second base layer  16  made of another damping material. In one embodiment, the wedges  26  and base layer  16  are both constructed of a foam material. However, in some embodiments the wedges  26  and base layer  16  are made of different types of foam materials. 
     The composite dampening structure  14  forms an inner cavity  22  where the speaker  20  and DUT  18  are located. A support column  24  suspends the DUT  18  in the middle of the cavity  22  and the speaker  20  is located on the back end of the lower box portion  15 A. The composite damping structure  14  surrounds the periphery of the speaker  20  and extends around the sides, top, and bottom of the entire cavity  22 . 
       FIG. 2  is an isolated top sectional plan view of the sound chamber  12  and  FIG. 3  is an isolated side sectional view of the sound chamber  12 . The wedges  26 A are shown in a vertically aligned orientation in  FIG. 2  for illustrative purposes but could alternatively be aligned horizontally as shown in  FIGS. 1 and 3 . Similarly, the side wedges  26 B and  26 C could be aligned in horizontal orientations as shown in  FIG. 1  or in vertical orientations as shown in  FIG. 2 . 
     A controller  30  generates electronic signals  34  that are output as audio waves  36  by speaker  20 . The receiver  18  detects the audio waves  36  and generates an electronic test signal  38 . The controller  30  controls what acoustic frequencies are output from speaker  20 . The controller  30  can also change the orientation  40  of the DUT  18  either horizontally or vertically with respect to the speaker  20  according to control signals  42 . In one embodiment, a slight rotation of the DUT  18  is allowed for improving response, but there is no vertical orientation adjustment, and only rotation of the DUT in the horizontal plane is provided. Of course other rotation and orientation configurations are also possible. 
     In one embodiment, the wedges  26  have a height  52  of about 2.5 inches and a base width of around 1.0 inches. The base layer  16  has a thickness  50  of around 1.5 inches and extends around the entire inside surface of wooden box  15 . The cavity  22  is around 4 inches in width, length, and 8 inches in height. The box  15  is around 12 inches in height and width, and around 16 inches in depth. 
     In one embodiment, the wedges  26  are made from a felt open cell foam, such as a permanently compressed reticulated foam (SIF) with a grade of 900 with 90 pores per lineal inch. The foam used for wedges  26  is made by Scotfoam Corporation of Eddystone, Pa. In one embodiment, the form used in the base layer  16  is reconstituted carpet foam with a 5 pound (lb) rebond. 
     In one embodiment, the wedges  26  have a stiffer structure than the base layer  16 . The shape of the wedges  26  allows a stiffer material to be used without significant acoustic reflections. The base layer  16  has a relatively flat shape that is substantially perpendicular to the direction of wave travel. Therefore, the base layer  16  is made of a softer material to improve sound absorbsion and further reduce sound reflections. These are just examples of the possible combination of dimensions and stiffness for the composite damping structure  14  used in sound chamber  12 . Other material shapes, sizes, and stiffness could also be used. 
     The wedges  26  provide two functions. At high frequencies, the wedges  26  act like the wedges in traditional anechoic sound chambers. The wedges  26  have sharp sides that reflect smaller acoustic waves  60   n  ( FIG. 2 ) inward toward the base of the wedges  26 . At lower audio frequencies  60 A ( FIG. 3 ), the wedges  26  act as transition elements, providing a progressively greater and greater density of damping foam material as relatively large acoustic waves  60 A propagate inward toward the base layer  16 . Thus the initial energy that would have normally been reflected because of the abrupt transition from air to foam is reduced significantly by wedges  26 . 
     Thus, the composite damping structure  14  comprising the foam wedges  26  with relatively sharp edges in combination with the relatively thick base foam layer  16  provides improved sound dampening. As a result, the wedges  26  do not have to be as tall or large to dampen a larger range of audio frequencies. This allows the sound chamber  12  to have a relatively smaller size than conventional anechoic chambers. The overall reduction of acoustic reflections provided by the composite damping structure  14  allows devices like directional microphones and hearing aids to be tested in a relatively small space. 
     Simultaneous Testing of Multiple Audio Frequencies 
     While it is possible to make directional tests one frequency at a time for each rotation of a device under test, it is desirable to collect and measure directional pattern information by collecting the patterns of several frequencies with only one rotation of the device under test. It is possible to present several pure tone test signals sequentially, one after another, at each rotational position. However, it is faster for all of the test frequencies to be presented, and results measured, simultaneously. 
     A multi-frequency acoustic test system uses linear superposition to combine multiple different pure tone components together into a single composite test signal. The composite test signal is then applied to a device under test so the device can be tested with multiple different frequencies at the same time. This allows complete multi-frequency testing of the device in one rotation. 
     Composite Signal Generation 
       FIG. 4  shows an audio testing system  58  that includes controller  30 , speaker  20 , and sound chamber  12 .  FIG. 5  is a flow diagram further explaining how a composite audio signal  74  is generated. The controller  30  in  FIG. 4  includes a processor  72  and a memory  70 . It should be understood that some of the individual functions shown in  FIG. 4  may be performed by the processor  72 . For example, a Discrete Fourier Transform (DFT)  86  and window function  87  may be performed by the processor  72  in response to software instructions. However these functions are shown as separate boxes in  FIG. 4  for explanation purposes. 
     The memory  70  stores a composite frequency set  71  that contains samples from multiple different audio signals  60  with different frequencies. The different audio signals  60  are shown in separate analog form in  FIG. 4  for illustration purposes. However, the memory  70  actually contains digital values in composite frequency set  71  that represent different samples for each of the different audio signals  60 . In one embodiment, the memory  70  contains one set of digital samples  71  for all of the different audio frequency signals  60 A- 60 N. 
     Any number of different audio signals  60 A- 60 N can be used to create the composite frequency set  71 . However, in one embodiment, the composite frequency set  71  contains samples for around 80 different audio frequencies. The period of a base frequency  60 A is set by the width of a time window and generates the lowest frequency in the composite set  71 . Each additional frequency  60 B- 60 N in the composite set  71  is an integer multiple of the base frequency  60 A. In operation  100  of  FIG. 5  sample sets are generated for different audio frequencies. 
     The width of the time window used for obtaining samples of signals  60 A- 60 N is adjusted to be exactly the same as a rectangular window  87  used for filtering test data received back from the DUT  18  prior to performing Discrete Fourier Transform (DFT) frequency analysis. For a base frequency  60 A of 100 Hz, a time window 10 milliseconds (mSec) wide is used for collecting the needed samples. If 256 samples are collected in this 10 mSec time period, audio frequencies up to a maximum of 12.8 kHz (the Nyquist frequency) can be analyzed. Of course, different numbers of samples and different widow sizes could also be used. 
     Time delays related to the generation of the composite signal, the transmission of the resulting composite analog signal  74  from the speaker  20  to the DUT  18 , and the device under test are also taken into account. It is typically necessary to generate and hold the composite signal  74  constant for a period of time longer than the width of a single time window. This gives the system enough time to receive and test a full 10 mSec period of the composite analog signal  74 . 
     The phases of the individual frequencies  60 A- 60 N are typically skewed or offset in operation  102  to arrive at a desirable signal crest factor. Crest factor is equal to the peak amplitude divided by the RMS amplitude of the signal. When a series of sinusoidal signals that are integer multiples of each other are all added together with no difference in their individual phases, the result is a composite signal with a very high crest factor. Therefore, in constructing a composite signal the phases of the individual frequencies  60 A- 60 N are typically skewed or offset in operation  102  to arrive at a desirable signal crest factor. The phase shift added to each frequency may be changed from one system to another to arrive at different desired properties. 
     If the DUT  18  is a directional microphone, it may be desirable to first individually equalize the amplitudes for each of the different audio frequencies  60 A- 60 N in operation  103  so that the amplitude of each frequency component is of a desired value. This can be done by using a reference microphone instead of DUT  18  for first recording the frequency response of the transducer in speaker  20 . The amplitude of each frequency component of the composite signal can then be adjusted to arrive at a desired measured amplitude. The actual DUT  18  is then placed in the same position previously occupied by the reference microphone. 
     The samples of the different audio frequencies  60 A- 60 N are combined together into a single composite frequency set  71  in operation  104  using linear superposition. The digital composite frequency set  71  is converted into an analog signal by a digital to analog (D/A) converter  80  in  FIG. 4 . The output of D/A  80  is selectively attenuated by attenuator  82 . An amplifier  84  amplifies the composite signal prior to being output from speaker  20  as composite analog signal  74  in operation  106 . 
     The DUT  18  receives the composite analog signal  74  and generates a test signal  38 . The test signal  38  is then processed by the controller  30  in operation  108 . The controller  30  in operation  110  may then send control signals  42  to the motor  43  ( FIG. 3 ) that rotates the DUT  18  into a different horizontal and/or vertical position. The controller  30  then outputs another composite analog signal  74  in operation  106  for testing the DUT  18  again in the new position. This process repeats until the DUT  18  is tested with the composite analog signal  74  at each desired position in operation  112 . In one example, the DUT  18  is rotated and tested in different positions around a 360 degree circle. 
     Data Collection 
     Referring now to  FIGS. 4 and 6 , with the source and collection systems synchronized, a complete determination of the amplitudes of multiple different frequency components can be determined with the collection of only one composite set of samples  71 . The DUT  18  generates a test signal  38  in response to the composite analog signal  74  in operation  120 . A pre-amplifier  92  amplifies the test signal  38  and an attenuator  90  attenuates the amplitude of the analog test signal according to a signal generated by the controller  30 . 
     The different responses of the DUT  18  to the multiple different audio frequencies  60  superimposed into the composite signal  74  are all contained in the test signal  38 . It is therefore necessary to unravel and extract these different frequency responses from test signal  38 . It is possible to extract the individual frequency responses one at a time using analog filters, with the filter outputs measured by conventional means. 
     However, in the embodiment shown in  FIG. 4 , the different frequency responses are obtained by first digitally sampling the composite test signal  38  with A/D  88  in operation  122 . A rectangular window  87  is then applied in the digital samples in operation  124  that coincides with the 10 mSec window of 256 samples used for generating the composite frequency set  71 . 
     A mathematical filter  86  is applied in operation  126  to generate the different frequency components contained in the test signal  38 . In one embodiment, the filter  86  is a Discrete Fourier Transform (DFT) or a Fast Fourier Transform (FFT). The amplitudes of the different frequency components are extracted from the transformed test signal in operation  127  and stored in a table located in memory  70  in operation  128 . The controller  30  then may change the position of the DUT  18  in operation  130  as explained above in  FIGS. 2 and 3 . The controller  30  then outputs the same composite analog signal  74  as explained above in  FIG. 5 . The controller  30  goes back to operation  120  and again generates another test signal  38  associated with the new position of the DUT  18 . The controller  30  repeats operations  122 - 130  until all of the different DUT positions have been tested with the composite signal  74  in operation  132 . 
     The controller  30  may then further process and display the test results. The controller  30  may display different frequency responses for the DUT  18  on a graphical user interface (GUI). For example, a user may select a particular frequency for displaying or printing out by the controller  30 . The controller  30  may then display the response of the DUT  18  for the selected frequency at each of the different DUI positions. Alternatively, a user may direct the controller  30  to display multiple frequency responses for one particular DUT position. The controller  30  accordingly, obtains the amplitude data from memory  70  for all of the multiple frequencies at that particular DUT position and displays or prints out the identified data on a GUI (not shown). It is also possible to display the results of the measuring function before the complete 360 degree rotation of the DUT and before the complete polar plot is derived. 
       FIG. 7  shows a polar plot  149  that can be generated by the controller  30  from the test signal  38  described above. Each smaller circle  160  in polar plot  149  represents a drop of ten decibels (dbs). Each line  162  extending radially outward from the center of polar plot  149  represents a different orientation of the DUT  18  with respect to the speaker  20 . For example, at zero degrees, the front of the DUT  18  may be pointed directly at the speaker  20 . 
     As explained above the DUT  18  can be rotated to different positions in a 360 degree horizontal plane as well as being rotated into different positions in a vertical plane. For each of the different rotational positions of the DUT  18 , the controller  30  determines the gain values for the amplitude components for each of the different frequencies contained in the test signal  38  ( FIG. 4 ). The controller  30  then builds a table in memory  70  that contains each of the different gain values for each of the different frequencies and associated DUT positions. The data in the table is then used to generate polar plot  149 . 
     The polar plot  149  includes a plot  150  showing the signal gain for a frequency of 500 Hz, a plot  152  showing the gain for a frequency of 1000 Hz, a plot  154  showing the signal gain for a frequency of 2000 Hz, and a plot  156  showing the signal gain for a frequency of 4000 Hz. Of course the gain for other frequencies can also be plotted by the controller  30 . 
     Because all of the multiple different frequency components are contained within the same test signal  38 , the DUT  18  only has to be rotated once 360 degrees inside of the sound chamber  12  in order to generate all of the plots  150 - 156 . Thus, the audio test system  58  requires less time to test audio devices and allows polar plots to be generated with a single 360 rotation of the DUT  18 . 
     The system described above can use dedicated processor systems, micro controllers, programmable logic devices, or microprocessors that perform some or all of the operations. Some of the operations described above may be implemented in software and other operations may be implemented in hardware. 
     For the sake of convenience, the operations are described as various interconnected functional blocks or distinct software modules. This is not necessary, however, and there may be cases where these functional blocks or modules are equivalently aggregated into a single logic device, program or operation with unclear boundaries. In any event, the functional blocks and software modules or features of the flexible interface can be implemented by themselves, or in combination with other operations in either hardware or software. 
     Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention may be modified in arrangement and detail without departing from such principles. I/we claim all modifications and variation coming within the spirit and scope of the following claims.