fiEFLECTION OF SOUND. 199 



rejfuiar form. An irregular surface must break the echo ; and if the irregu- 

 laricy be very cousiderable, there can be no distinct or audible reflection at 

 all. i or tnia reason an echo is much less perfect from the front of a house 

 wliich has wmdows and aoors, than from the plane end, or any plane wall of 

 the same magnituae. 



How is sound 431 . If the surface upon wliicli the sound- 

 c^ved^ ^6^ waves strike be concave, it may concentrate 

 ^'^^' sound, and reflect all that falls upon it to A 



point at some distance from the surface, called the focus. 



Y^Q IQQ Thus, in Fig. 190, if the sound waves 



proceeding in right hues from the points 

 d, e, /, g, h, strike upon the concave sur- 



^ face, ABC, they will all be reflected to 



f the focus, F, and there concentrated in 



jf such a way as to produce a most powerful 



ll eSect. 



It is upon this principle that whisper- 

 ing galleries are constructed, and domes and vaulted ceUings often exhibit the 

 same curious phenomena. In these instances a whisper uttered at one point 

 is reflected from the curved surface to a focus at a distant point, at which 

 situation it may be distinctly heard, while in aU other positions it will be in- 

 audibla 



._., ^ . All are famihar with the resonance produced by placing a 



Wtiat occasions ^ j r a 



the noise heard sea-shell to the ear — an effect which fancy has likened to the 



in a sea-sheU ? u j-q^j. qJ- ^j,g ^^^y rjj^jg jg caused by the hollow form of the 



shell and its polished surface enabling it to receive and return the beatings 

 of all sounds that chance to be trembhug in the air around the sheU. 



432. Speakmg-tubes and speaking-trumpets depend on the principles of the 

 reflection of sound. 



Fig. 191. 



433. A SpEAKreo-TRUMPET (Fig. 191) is i 

 speaking-Trum. liollow tubc SO coustructed that the rays of 

 ^ sound (proceeding from the mouth when ap- 



plied to it), instead of diverging, and being scattered 

 through the surrounding atmosphere, are reflected from the 

 sides, and conducted forward in straight lines, thus giving 

 great additional strength to the voice. 



