Abstract:
A microphone test fixture. The test fixture includes a test chamber, an acoustic source, a reference microphone, and an acoustic resistor. The acoustic source is configured to produce sound waves in the test chamber. The reference microphone is positioned to receive the sound waves in the test chamber. The acoustic resistor forms a contiguous space with the test chamber, and is sized to prevent resonances and echoes of the sound waves for a fixed high frequency limit.

Description:
RELATED APPLICATION 
     The present patent application claims the benefit of prior filed U.S. Provisional Patent Application No. 61/701,127, filed on Sep. 14, 2012, the entire content of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present invention relates to a microphone test fixture. Specifically, the present invention relates to a microphone test fixture that removes and eliminates resonant and echoed acoustic waves from a test chamber such that the acoustic pressures are consistent throughout the test chamber. 
     Acoustic test chambers only work when the device under test and the reference microphone are exposed to the same acoustic pressure. Acoustic resonances due to standing waves in the cavity prevent uniform pressures in the chamber, the frequency at which these non-uniform pressure fields form are dependent on the dimensions and design of the test fixture. These non-uniform pressure fields prevent accurate and repeatable measurements of the acoustic environment. 
     A typical microphone test chamber has an acoustic source, a test chamber (with all boundaries possessing infinite impedances), and a reference microphone (to determine the pressure that the device under test is experiencing). At least one dimension of the test site, and usually all of the dimensions, is/are longer than the wavelength of sound being measured. 
     At low frequencies, the cavity size is generally very small compared to the wavelength of the acoustic pressure being tested. The wave travels and reflects the off the walls, but the wavelength prevents the perfect cancelation of the reflected wave. 
     This is illustrated in  FIG. 1 . A test chamber  100  includes an acoustic source  105  (e.g., a speaker), a device under test (DUT)  110  (i.e., a microphone), and a reference microphone  115 . The consistent shading inside the chamber  100  indicates the acoustic pressure is equal throughout the chamber  100 . 
     However, at higher frequencies, where the wavelength of the acoustic pressure is smaller than the cavity size, the acoustic pressure throughout the cavity is no longer equal. As shown in  FIG. 2 , acoustic energy leaves the speaker  105  and reflects off the rigid cavity walls, and resonances in the cavity  100  occur. The result is the pressure at the microphone  110  opening is not equal to the source pressure from the speaker  105  or to the pressure at the reference microphone  115 . This is indicated by the inconsistent shading throughout the chamber  100 . 
     In the scenario shown in  FIG. 2 , there is a standing wave mode where there is near zero pressure in the middle of the test chamber  100 , and a maximum pressure at the acoustic source  105  and the reference microphone  115 . At even higher frequencies, additional resonances occur. 
     SUMMARY 
     In one embodiment, the invention provides a microphone test fixture. The test fixture includes a test chamber, an acoustic source, a reference microphone, and an acoustic resistor. The acoustic source is configured to produce sound waves in the test chamber. The reference microphone is positioned to receive the sound waves in the test chamber. The acoustic resistor forms a contiguous space with the test chamber, and is sized to prevent resonances and echoes of the sound waves for a fixed high frequency limit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of low frequency acoustic pressure in a microphone test chamber. 
         FIG. 2  is an illustration of the unequal acoustic pressure that occurs at higher frequencies in the test chamber. 
         FIG. 3  is a cut-away view of a construction of a microphone test chamber. 
         FIG. 4A  is a diagram of a test chamber illustrating the acoustic source reducing the volume in the test chamber. 
         FIG. 4B  is a diagram of a test chamber illustrating the acoustic source increasing the volume in the test chamber. 
         FIG. 5A  is a diagram illustrating the heat produced when the volume in the test chamber is reduced. 
         FIG. 5B  is a diagram illustrating the heat lost when the volume in the test chamber increased. 
         FIG. 5C  is a schematic diagram showing the electrical equivalent of the acoustic compliance in the test chamber. 
         FIG. 6  is a diagram illustrating the ideal acoustic inertance of the test chamber. 
         FIG. 7A  is a diagram illustrating the real acoustic inertance of the test chamber. 
         FIG. 7B  is a schematic diagram showing the electrical equivalent of the real acoustic inertance in the test chamber. 
         FIG. 8A  is a diagram illustrating the real acoustic inertance and the real acoustic compliance of the test chamber. 
         FIG. 8B  is a schematic diagram showing the electrical equivalent of the real acoustic inertance and the real acoustic compliance in the test chamber. 
         FIG. 9  is a schematic diagram where sections of a test chamber have real compliance and real inertance for different frequencies of acoustic pressures. 
         FIG. 10  is a graph showing the inductive component of the real compliance for various frequencies as the diameter of the test chamber is decreased. 
         FIG. 11  is a graph showing the capacitive component of the real inertance for various frequencies as the diameter of the test chamber is decreased. 
         FIG. 12  is a graph showing the resulting change in the relationship of the resistive components to the inductance components as the diameter of the test chamber is decreased. 
         FIG. 13  is a graph showing the resulting change in the relationship of the resistive components to the capacitive components as the diameter of the test chamber is decreased. 
         FIG. 14  illustrates that properly sizing the test chamber results in an acoustic resistor at all desired frequencies. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
       FIG. 3  shows a construction of a microphone test fixture  300  incorporating the invention. The test fixture  300  includes a test chamber (cavity)  305 , an acoustic source  310 , a reference microphone  315 , a device under test (DUT)  320  (e.g., a MEMS microphone), and an acoustic resistor  325 . The test chamber  305  and the acoustic resistor  325  form a contiguous chamber, and are filled with a fluid (e.g., air, nitrogen, helium, etc.). 
     The acoustic source  310  emits sound waves (i.e., an acoustic pressure) which are picked up by the reference microphone  315  and the DUT  320 . The outputs of the reference microphone  315  and the DUT  320  are compared to test the functioning of the DUT  320 . The sound waves emitted by the acoustic source  310  can vary over a range of frequencies (e.g., audible frequencies). 
     For the test of the DUT  320  to be effective, both the DUT  320  and the reference microphone  315  must receive the same sound waves. However, if the test chamber  305  or the acoustic resistor  325  are not sized correctly, echoing of sound waves can result in the reference microphone  315  and the DUT  320  from receiving different sound waves. 
     To improve the performance of the test fixture  305 , the volume of the test chamber  305  is made as small as possible relative to the DUT  320  to make the test chamber  305  smaller than the wave length of the acoustic waves output by the speaker  310  (see  FIG. 1 ). However, this smaller cavity  305  alone does not solve the problem of uneven acoustic pressures existing in the chamber  305 . The small chamber  305  solves the problem for sound waves below a certain frequency but does not solve the problem for higher frequencies (see  FIG. 2 ). In many cases this frequency threshold is still in the audio band where the test remains inaccurate at frequencies where non-uniform pressures are produced in the test chamber  305 . 
     The resonance in the test chamber  305  builds up because the acoustic impedances of all of the cavity walls are infinite. To solve the problem, acoustic impedance (i.e., the acoustic resistor  325 ) is added (e.g., to one of the walls) resulting in the acoustic energy not reflecting back into the chamber  305 , and preventing the resonances. 
     The acoustic resistor  325 , in addition to having acoustic impedance, also has potential energy and kinetic energy storage. To be effective, the acoustic resistor  325  needs to be sized correctly to eliminate the potential energy and kinetic energy storage. 
     Acoustic pressures in pipes are analogous to Voltage in wire. Acoustic volume velocities are analogous to current in a wire. We can use these relationships to describe the propagation of an acoustic wave through a pipe using electrical analogies. In addition potential energy storage (acoustic compliance) is analogous to an electrical capacitor, and kinetic energy storage (acoustic inertance) is analogous to an electrical inductor. 
     Acoustic compliance is determined by the formula: 
             C   =     V     γρ   m             
where:
 
V=active volume [m3]
 
γ=cp/cv ratio of specific heats
 
p m =P o  ambient pressure [Pa]
 
       FIG. 4A  shows the acoustic source  310  reducing the volume in the chamber  305  and  FIG. 4B  shows the acoustic source  310  increasing the volume in the chamber  305  (e.g., the vibration of a speaker cone). When the volume is reduced heat is created and when the volume is increased heat is removed. With ideal conditions, the walls of the chamber  305  do not allow heat to enter or leave the operating fluid. 
     However, in reality, the walls of the chamber  305  do allow heat to enter and leave the fluid. As shown in  FIG. 5A , as the acoustic source  310  reduces the volume in the chamber  305 , heat is produced and some of the heat is absorbed by the walls of the chamber  305 .  FIG. 5B  shows the acoustic source  310  increasing the volume in the chamber  305  causing the fluid to absorb some heat from the walls of the chamber  305 . This transfer of heat, however, is not perfect. The amount of heat absorbed by the walls of the tube and the amount of heat returned to the fluid are not equal. Therefore, instead of ideal acoustic compliance (i.e., a capacitor), the real acoustic compliance includes an impedance R K  as shown in  FIG. 5C . The impedance R K  component is determined by the formula: 
               R   K     =       2   ⁢     γρ   m           ω   ⁡     (     γ   -   1     )       ⁢   ΠΔ   ⁢           ⁢   x   ⁢           ⁢       δ   k     ⁡     (     ω     -   .5       )                 
where:
 
Π=wetted surface area of wall (cross section Area×length)
 
ω=radian frequency
 
δ k (ω −0.5 )=thermal penetration depth
 
K=thermal conductivity of material
 
Cp=specific heat at constant pressure
 
Vm=molar volume
 
     Acoustic inertance ( FIG. 6 ), represented by the analogous inductor, is determined by the formula: 
             L   =       ρΔ   ⁢           ⁢   x     A           
where:
 
ρ=density
 
Δx=effective length
 
A=cross sectional area
 
     However, in reality as shown in  FIG. 7A , the fluid  700  closer to the walls of the chamber  305  “sticks” to the walls and friction between the walls and the fluid, and between fluid  700  close to the walls and fluid  705  in the center of the chamber  305  causes a loss in energy (i.e., an impedance component). Therefore, instead of ideal acoustic inertance (i.e., an inductor), the real acoustic inertance includes an impedance R V  as shown in  FIG. 7B . The impedance R V  component is determined by the formula: 
               R   V     =       μΠΔ   ⁢           ⁢   x         A   2     ⁢       δ   v     ⁡     (     ω     -   .5       )                 
where:
 
μ=bulk viscosity
 
δ ν =√{square root over (2μ/ωρ)}
 
ρ=density
 
       FIG. 8A  shows a chamber  305  having an input terminal P 1  and an output terminal P 2 .  FIG. 8B  represent the chamber  305  using the analogous electrical units. The values of the electrical components are determined by the physical dimensions of the chamber  305 , the operating frequency, and the parameters of the acoustic fluid in the chamber  305  (e.g., air), and happen simultaneously, but separably, in the chamber  305 . 
     Thus, the test chamber  305  of  FIG. 2  can be represented as an anologous electrical circuit shown in  FIG. 9  where sections of the chamber  305  have real compliance and real inertance for different frequencies of acoustic pressures. 
       FIG. 10  shows the inductive component of the real compliance for various frequencies as the diameter of the chamber  305  is decreased.  FIG. 11  shows the capacitive component of the real inertance for various frequencies as the diameter of the chamber  305  is decreased. As can be seen in both graphs, as the diameter of the chamber  305  is decreased, the inductive and capacitive components are reduced.  FIGS. 12 and 13  show the resulting change in the relationship of the resistive components to the inductance and capacitance components as the diameter of the chamber  305  is decreased. As shown in  FIG. 12 , the resistive component R V  has up to five orders of magnitude greater impact than the inductive component (effectively eliminating the inductive component). Similarly, as shown in  FIG. 13 , the resistive component R K  has up to ten orders of magnitude greater impact than the capacitive component (effectively eliminating the capacitive component). 
     Recognizing that resonances only occur when there are components that store energy in the form of potential and kinetic energy, we can reduce the tube dimension based on the highest desired frequency so that the capacitive and inductive components are greatly reduced relative to the resistive components (i.e., effectively leaving only the resistive components). Thus, by properly sizing the chamber  305 , the chamber  305  becomes an acoustic resistor as represented by the analogous circuit shown in  FIG. 14 . The reference microphone  315  can be placed right where the inductor and capacitor would have been in the acoustic circuit. There are no resonating devices left and the pressure along any portion of the acoustic system is dependent on only one frequency independent value. 
     Various features and advantages of the invention are set forth in the following claims.