8 BELL SYSTEM TECHNICAL JOURNAL 



piece of hair felt but with different degrees of packing. It is thus 

 evident that a felt which has become loosened by handling may have 

 an absorption frequency characteristic quite unlike that of a new piece. 



1.00 



125 250 500 1000 2000 4000 



FREQUENCY-CYCLES PER SECOND 



Fig. 4 — Power absorbed by hair felt 



In Fig. 4 are given the absorption coefficients for various thicknesses 

 of hair felt. These values are in general agreement with those ob- 

 tained by the reverberation method according to published results. 

 Exact agreement is not to be expected, for the values here given apply 

 only to sound waves having a perpendicular incidence on materials 

 solidly backed by a hard surface. When the materials are applied in a 

 room, the support is often more flexible and the absorption is partly 

 due to inelastic bending. However, the agreement between the sets 

 of values is sufficiently good to show that the results obtained by the 

 simpler tube method may be used to get a good approximation to the 

 values of absorption of the materials when applied in rooms for damping 

 purposes. 



Measurements have been made on a large number of porous ma- 

 terials. Although most of these materials are very good absorbers at 

 the higher frequencies, none of them were found to be very efficient in 

 the lower frequency region. Uniform absorption over most of the 

 frequency range was found only in materials which are relatively in- 

 efficient absorbers. High absorption at the lower frequencies was 

 obtained only when the thickness of the material was greatly increased. 

 This fact is typically illustrated by the curves of absorption for hair 

 felt given in Fig. 4. 



When a sound wave of low frequency is reflected from a wall covered 



