10 BELL SYSTEM TECHNICAL JOURNAL 



with a porous material, the velocity of the air particles near the 

 reflecting surface is small and hence there can be but little absorption. 

 We may look at the phenomenon of reflection in still another way. 

 In order to have a small coefficient of reflection the mechanical im- 

 pedance of the wall per unit area should, as nearly as possible, be 

 equal to the acoustic impedance of the air per unit area. The reason 

 for the high reflection at low frequencies by a rigid wall covered with a 

 porous material lies in its high stiffness reactance. At a given fre- 

 quency this reactance can be compensated by loading the air near the 

 reflecting surface. This may be accomplished in various ways. One 

 of these ways is to place at a short distance from the wall a second wall 

 which is porous or perforated. This arrangement has the eff'ect of 

 covering the wall with a multiplicity of resonators, which may be given 

 any desired resonance frequency by properly proportioning the size, 

 length and number of perforations and the spacing of the walls. The 

 surface of the walls forming the air space should be absorbing or else 

 the space should be provided with absorbing material. 



To get a wider absorption band two or more perforated walls with 

 proper spacing may be used, as this arrangement is equiv^alent to an 

 aggregate of multiple resonators. The values of absorption coeffi- 

 cients of a number of structures of this type are given in the accom- 

 panying table. The measurements refer to sound which is incident 

 from right to left as the structures are given in the table. The 

 building board referred to in the table is a commercial type of insu- 

 lating-board one inch thick with 400 1/4 inch by 3/4 inch holes per 

 square foot. The felt in all cases is one-inch hair felt. These values 

 show that relatively high absorption may be obtained at low as well 

 as at high frequencies without an excessive amount of absorbing 

 material. The use of combinations of absorbing materials, such as 

 are given in the table, offers the advantage that more uniform damping 

 at all frequencies can be obtained, and the degree of damping can 

 be readily controlled by covering the proper area of surface. These 

 two factors have become increasingly important in studio and audi- 

 torium design, with improved technique in recording and reproducing 

 speech and music. 



