54 HANDBOOK OF ACOUSTICS. 

cisely, " The intensity of a sound varies inversely as the square of the distance from its origin. 





Fig. 30.
It should be clearly noted, however, that the conditions under which the above " law of inverse squares," as it is called, is true, rarely or never obtain. The chief disturbing elements in the application of this law are echoes. When a ray of light strikes any reflecting surface at right angles, it is reflected back in the direction whence it came. If a ray of light, A 0, fig. 31, does not fall at right angles upon a reflecting surface P Q, it is reflected along a line C B, which is so situated, that the angle B C H is equal to the angle A C H ; HC being at right angles to P Q. Just so with sound. A person standing at B would hear a sound from A, first as it reaches him in the direction A B, and directly after, along the line C B. If the distances A B and A C B were each only a few yards, the two sounds would be indistinguishable, but if there were any considerable difference between these two 
