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XI— ON MODES OF DEALING WITH ECHOES IN EOOMS. Bt 

 GEOEGE JOHNSTONE STONET, D. Sc, E. U.S., a Vice- 

 President of the Society. 



[Read, December 18, 1882.] 



In almost all large apartments echoes are troublesome. If the 

 room is used for music, echoes destroy the purity of the sound ; if 

 for oratory, they render the utterance of the speaker indistinct. 

 Any mode of mitigating these bad effects is useful, and if it goes 

 the length of practically annulling them, it will be of signal ser- 

 vice. This induces me to lay before this section of the Society the 

 result of some experiments on the subject which I had occasion to 

 make several months ago. 



Echoes in rooms arise from sound reaching the ear, not only 

 directly but also after reflection, and sometimes after more than 

 one reflection, from the ceiling, the four walls, and the floor. If a 

 large audience be present, the floor ceases to act as a reflecting sur- 

 face, and there will remain the walls and ceiling. In all eases, the 

 effect which results largely depends on the circumstance that the 

 sound-waves, especially those of low pitch, are of considerable size 

 compared with the extent of the wall, so that reflection is not of 

 that simple and definite kind which occurs when light is reflected 

 from a mirror, b]it. partakes largely of the diffuse character which 

 we see when light falls on surfaces which are not polished. The 

 reflection of sound-waves is intermediate between the two ways in 

 which light is reflected, and tends more towards true reflection in 

 the case of the short waves which belong to sounds of high pitch, 

 while the reflection becomes more and more an irregular scattering 

 of sound from the reflecting surface, in the case of long waves of 

 low pitch. This difference of effect in the cases of long and short 

 waves is well known to the mathematicians who have studied the 

 interference of waves, and it is unnecessary to give the proof here 

 as it may be found in all text-books on the subject. But it is well 

 to point out that it contributes largely to that marked indistinct- 

 ness which echo in a room ffives to the utterance of an orator. The 



