114 



SCIENCE PROGRESS 



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C, C/2 C3 C4 C^ Cq C- 



Fig. 4. — The absorbing power of 

 45 -cm. thick brick wall. 



metre of complete absorption. In the case of the upper curve 



the ordinates are thus ordinary coefficients of absorption. It 



will be noted that there is almost 

 complete absorption for high notes. 

 The effect on absorption of 

 painting a brick wall is shown by 

 Fig. 4. Here the upper curve re- 

 presents the absorbing power of 

 the unpainted surface of hard but 

 unglazed bricks set in cement. 

 The lower curve gives the new 

 absorbing power of the surface 

 after the application of two coats 

 of oil-paint. The difference repre- 

 sents the absorption due to the 

 porosity of the bricks, and this is 

 most marked, as would be expected, 

 in the case of sounds of high pitch. 

 The ordinates are absorption co- 

 efficients. 



The absorbing power of cushions 

 of different types is illustrated by 

 Fig. 5. Curve i is for cushions of 



vegetable fibre, covered with canvas ticking and thin cloth. 



Curve 2 is for cushions of long 



hair covered with similar ticking 



and plush. The cushions of curve 



3 are hair cushions covered with 



ticking and a thin leatherette, and 



those of curve 4 are of elastic cotton 



covered with ticking and short nap 



plush. All the curves show maxima 



due to resonance within an octave. 



In addition three of the curves have 



humps in a second octave. This 



may be due to secondary resonance. 

 A special set of experiments was 



carried out to determine the nature 



of the absorption by felt of different 



thicknesses, and the results of these 



are given in Figs. 6 and 7. 



In Fig. 6 the absorption co- 

 efficient is plotted against the pitch. 



The felt was i*i cm. thick, and 



curve I is for a single thickness, 



curve 2 for two thicknesses placed one on top of the other, and 



so on. It will be noted that the single thickness absorbs 



ro 



o 



Fig. 5. 



'-'J '-'4 *^() ^--Q ^B 



-The absorbing power 

 of cushions. 



