Abstract:
An acoustic chamber with low frequency outer wall transmissivity is provided. According to one aspect, an acoustic chamber has an inner wall encompassing an interior of the acoustic chamber and configured to allow acoustic energy to penetrate the inner wall. The acoustic chamber also has an outer wall configured to allow low frequency acoustic energy that penetrates the inner wall to penetrate the outer wall and leave the acoustic chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to and claims priority to U.S. Provisional Patent Application Ser. No. 62/278,143, filed Jan. 13, 2016, entitled “ACOUSTIC CHAMBER WITH LOW FREQUENCY TRANSPARENCY”, the entirety of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an acoustic chamber with low frequency outer wall transmissivity. 
     BACKGROUND 
     Anechoic and hemi-anechoic chambers are designed to mimic an acoustic free field inside of an enclosed environment. An acoustic free field is a region where there are no acoustically reflective surfaces or effects from such surfaces. 
     Current and historical anechoic chambers have all been constructed in similar methods.  FIG. 1  is an acoustic chamber  10  having a solid outer wall  12  constructed to isolate the acoustic environment  14  inside the chamber from that outside the chamber. The outer wall  12  is typically constructed using modular steel panel systems or by conventional techniques such as concrete, cinder blocks, or studs and drywall. The interior of the outer wall  12  is then lined with an acoustic absorber system  16 . The absorber system  16  absorbs nearly all of the acoustic energy within its frequency range of operation creating a free field. This absorber system  16  is typically a wedge or a tuned wedge/wall system where the wedges operate in conjunction with the wall panels. In a tuned wedge/wall system, the outer wall  12  of the chamber  10  is solid steel and the inner wall  20  is usually perforated steel. The inside of the wall enclosure  18  is filled with a variety of different materials depending on what the designer is trying to achieve. 
     In a tuned wedge/wall system, where the inner wall is perforated steel, the outer wall  12  of the acoustic chamber  10  in this case is solid and can be steel or one of the other materials mentioned above. The solid outer wall  12  is reflective and impacts the free field performance of the chamber below the absorber cutoff frequency. At low acoustic frequencies, the acoustic energy is not totally absorbed by the absorber system  16  and some acoustic energy passes through the perforated inner wall  20 . Some of the acoustic energy passing through the inner wall  20  is absorbed by material of the wall enclosure  18 . However, some of the acoustic energy will reflect from the inner surface of the solid outer wall  12  and will propagate back toward the interior  14  of the acoustic chamber  10 . This is undesirable because it degrades the low frequency performance of the acoustic chamber  10 . 
     In order to improve the low frequency performance of an anechoic chamber, conventional solutions call for the chamber size to increase and the depth of the absorber system  16  to increase. This greatly increases cost and size of these chambers and limits the ability of users to obtain a chamber that meets their low frequency requirements. 
     SUMMARY 
     The present embodiments advantageously provide an acoustic chamber with low frequency outer wall transmissivity. According to one aspect, an acoustic chamber has an inner wall encompassing an interior of the acoustic chamber and configured to allow acoustic energy to penetrate the inner wall. The acoustic chamber also has an outer wall configured to allow low frequency acoustic energy that penetrates the inner wall to penetrate the outer wall and leave the acoustic chamber. 
     According to this aspect, in some embodiments, the acoustic chamber further includes an interior absorber system lining the inner wall and configured to absorb acoustic energy above a particular frequency. In some embodiments, the outer wall is made of perforated metal. In some embodiments, the size and density of the perforations determine a frequency response of the outer wall. In some embodiments, the outer wall is made of a porous fabric. In some embodiments, the outer wall is made of a skeletal structure. 
     According to another aspect, a method of constructing an acoustic chamber is provided. The method includes constructing an inner wall configured to encompass an interior region of the acoustic chamber; the inner wall being at least partially acoustically penetrable. The method also includes constructing an outer wall in proximity to the inner wall, the outer wall being at least partially acoustically penetrable. 
     According to this aspect, in some embodiments, the method further includes installing acoustic absorbing material between the inner wall and the outer wall. In some embodiments, the acoustic penetrability of the outer wall is frequency-dependent. In some embodiments, the acoustic penetrability of the inner wall is frequency-dependent. In some embodiments, the method includes the inner wall facing the interior region with an acoustic absorbing material configured to absorb acoustic energy above a particular frequency. In some embodiments, the inner wall and lining material are configured to substantially absorb acoustic energy above a particular frequency. In some embodiments, the outer wall is configured to pass acoustic energy below the particular frequency. In some embodiments, the method further includes installing acoustic absorbing material between the inner wall and the outer wall. In some embodiments, a frequency response associated with the outer wall has a low pass component. 
     According to yet another aspect, a method of constructing a composite wall for an acoustic chamber is provided. The method includes constructing an inner wall having an acoustically penetrable surface. The method also includes constructing an outer wall having an acoustically penetrable surface, the outer wall being positioned in relation to the inner wall to form the composite wall. 
     According to this aspect, in some embodiments, the outer wall is substantially parallel to the inner wall. In some embodiments, the method further includes selecting a thickness of the composite wall to achieve a particular low pass frequency response of the composite wall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present embodiments, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a cross sectional top view of a known acoustic chamber; 
         FIG. 2  is a cross sectional side view of a wall of a known acoustic chamber; 
         FIG. 3  is a cross sectional top view of an acoustic chamber having low frequency transparency; and 
         FIG. 4  is a cross sectional side view of a wall of an acoustic chamber having low frequency transparency. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing in detail exemplary embodiments, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to an acoustic chamber with low frequency transparency. Accordingly, components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. 
     Conventional acoustic chambers have a solid outer surface. For example, the outer surface may be 16 gauge, 11 gauge or 3/16″ thick solid steel. While providing enhanced isolation, the solid outer surface acts like an acoustically reflective surface below the cutoff frequency of the absorber system. This creates a low frequency limit to the performance of the anechoic chamber and influences the overall size of the chamber. 
     By changing the outer surface of the chamber to one that is more acoustically transparent through the use of a material such as perforated steel, the acoustically reflective outer surface is eliminated. Acoustic energy that is not absorbed by the absorber system is allowed to propagate through the absorber and outside of the enclosure. By allowing the energy to escape, the cut off frequency of the acoustic free field is lowered without increasing the size of the chamber or depth of the wedges. 
     Thus, in some embodiments, the outer surface of the acoustic chamber could be made of perforated steel of 16 gauge, 11 gauge or 3/16″ thick. The density and/or size of the perforations can be chosen to achieve a desired frequency response. In alternative embodiments, the outer surface of the acoustic chamber can be fabric or other acoustically transparent material. In yet other embodiments, the outer surface of the acoustic chamber may be skeletal to provide support but leaving large open areas to be substantially acoustically transparent. 
       FIG. 3  is an acoustic chamber  24  constructed according to principles discussed herein. The acoustic chamber  24  has an absorber system  16  in the interior  14  and lining the inner wall  20 , which may be perforated steel, as described above. The acoustic chamber  24  has an outer wall  26  that allows low frequency acoustic energy to escape the acoustic chamber  24 . This can be accomplished by constructing the outer wall  26  from perforated steel or by a flexible or rigid fabric, and/or a skeletal frame with openings. 
       FIG. 4  shows a cross sectional view of the wall formed by the inner wall  20  and outer wall  26  enclosing absorber material  28 . 
     The absorber system  16  absorbs acoustic energy but may have a cutoff frequency below which acoustic energy is not effectively absorbed. Low frequency acoustic energy may propagate through the absorber system  16  and penetrate the inner wall  20  which may be made of perforated steel. Some of the acoustic energy is absorbed by the absorber  28  within the wall of the acoustic chamber. Acoustic energy that is not absorbed by the absorber  28  penetrates the outer wall  26  and propagates into the space surrounding the acoustic chamber  24 . 
     Some embodiment include a method of constructing an acoustic chamber. The method includes constructing an inner wall configured to encompass an interior region of the acoustic chamber; the inner wall being at least partially acoustically penetrable. The method also includes constructing an outer wall in proximity to the inner wall, the outer wall being at least partially acoustically penetrable. 
     In some embodiments, the method further includes installing acoustic absorbing material between the inner wall and the outer wall. In some embodiments, the acoustic penetrability of the outer wall is frequency-dependent. In some embodiments, the acoustic penetrability of the inner wall is frequency-dependent. In some embodiments, the method includes the inner wall facing the interior region with an acoustic absorbing material configured to absorb acoustic energy above a particular frequency. In some embodiments, the inner wall and lining material are configured to substantially absorb acoustic energy above a particular frequency. In some embodiments, the outer wall is configured to pass acoustic energy below the particular frequency. The particular frequency can be adjusted as desired by adjusting the size and density of perforations in the inner wall and by adjusting a thickness and structure of the absorbing material lining the inner wall. In some embodiments, the method further includes installing acoustic absorbing material between the inner wall and the outer wall. In some embodiments, a frequency response associated with the outer wall has a high pass component. 
     Some embodiments include a method of constructing a composite wall for an acoustic chamber. The method includes constructing an inner wall having an acoustically penetrable surface. The method also includes constructing an outer wall having an acoustically penetrable surface, the outer wall being positioned in relation to the inner wall to form the composite wall. In some embodiments, the outer wall is substantially parallel to the inner wall. In some embodiments, the method further includes selecting a thickness of the composite wall to achieve a particular high pass frequency response of the composite wall. Generally, the thicker the composite wall—that is, the greater the distance between the inner and outer wall, when this thickness is filled with absorber—the lower the cutoff frequency of the composite wall, the cutoff frequency being the lowest frequency at which energy is substantially reflected by the wall. 
     It will be appreciated by persons skilled in the art that the present embodiments are not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope of the following claims.