ARCHITECTURAL ACOUSTICS 



115 



1-0 



•2 =-^-f- 



C, Ca Cg C4 Cg Qq Cy 



Fig. 6. — The absorbing power of 

 felt of different thicknesses. 



scarcely any sound for Cj, C^, and C3, that the absorption in- 

 creases rapidly for C^ and C5, and is then practically constant. 



The absorption by greater thickness 



is greater in proportion for low 



notes than for high. All the curves 



show a maximum, each succeeding 



maximum corresponding to a slightly 



lower note. For six thicknesses the 



maximum is very near C4. 



Absorption may be ascribed to 



three causes : Porosity, resonance 



(due to vibration), and surface 



friction. The maxima represent 



resonance. 



Further information is afforded 



by Fig. 7, in which the same results 



are plotted in a different way. The 



ordinates are absorption coefficients 



and the abscissae thicknesses of felt. 



There is one curve for each of the 



notes Cj, Cg, etc. 



For Cj the absorption is very 



nearly proportional to the thickness 



over the range tested. The curves for increasing pitch show 



increasing values for the coefficients of absorption. They all 



show that if the thickness of felt were 

 sufficient a limit would be reached, 

 which is, of course, obvious. For Cg, 

 Cg, and Q this limit was reached. 



The curves of Cj,, Cg, C3, C^, if pro- 

 duced, would pass through O. For 

 these zero thickness would thus produce 

 zero effect. Hence for these surface 

 friction as an absorption agent is un- 

 important, so that roughening walls 

 has little effect. The curves C5, C^, C, 

 do not give evidence on this point. 



Absolute absorption coefficients in 

 Jager's sense have also been determined 

 for many substances by a special 

 method by H. O. Taylor (9), and yield 

 results which are in close agreement 

 with those of Sabine. A new point 

 which these experiments have brought 



out is the dependence of absorbing power on the distance of 



the absorbing material from the wall. This is natural, since 



the air near the wall is normally almost at rest and damping 



J ;^ 3 4 5 6 



Fig. 7. — The absorbing power 

 of felt of different thicknesses. 



