126 SCIENCE OF COMMON THINGS. 



Effect of surfaces on sound. Construction of halls for speaking. 



797 What must be the conditions of the reflecting surface in order to 

 produce a perfect echo ? 



The surface must be smooth and of some regular 

 form ; for the wave of sound rebounds, according to 

 the same law as a wave of water or an elastic ball, per- 

 pendicularly to the surface if it fall perpendicularly, 

 and if it fall obliquely^ on one side, it departs with an 

 equal degree of ooliquity on the other side. 



798 What must be the effect of an irregular surface ? 



An irregular surface must break the echo ; and if 

 the irregularity be very considerable, there can be no 

 distinct or audible reflection at all. For this reason an 

 echo is much less perfect from the front of a house 

 which has windows and doors, than from the plane 

 end, or any plane wall of the same magnitude. 



799 Why have halls for music plane bare walls f 



Because the hard plane walls reflect the sound regu- 

 larly, and increase the effect of the music. 



800 Why are halls for speaking, theatres, churches, etc., generally 

 ornamented on the wall, and furnished with pittars, curtains, etc. 



Because the ornaments , pillars, curtains, etc., form 

 irregular surfaces, which break up and destroy the 

 echoes and resonances. 



801 Why is a thick curtain often placed behind a pulpit or speaking- 

 desk f 



Because the material absorbs the sound, and by not 

 reflecting it avoids the production of echoes and reso- 

 nances. 



If the room is not rery large, a curtain behind the speaker impedes 

 rather than assists his voice. 



SOS What is a speaking trumpet f 



A speaking trumpet is a hollow tube,, so constructed, 



