1851] WRITINGS OF JOSEPH HENRY. 295 



ON THE LIMIT OP PERCEPTIBILITY OF A DIRECT AND 

 REFLECTED SOUND. 



(Proceedings American Association Adv. of Science, vol. v, pp. 42, 43.) 



May, 6, 1851. 



Professor Henr}'' stated that at the meeting of the Associ- 

 ation at Cambridge* he had made a communication relative 

 to the application of the principles of acoustics to the con- 

 struction of rooms intended for public speaking. In that 

 communication he had stated, as an important proposition, 

 that when two portions of the same sonorous wave reach the 

 ear of an auditor, — one directly from the origin of the sound, 

 and the other indirectly, — after one or more reflections, if 

 the two do not differ in the paths they travel by a difference 

 greater than a given quantity, the two sounds will re-enforce 

 each other, and one louder sound will be perceived. If how- 

 ever the interval is greater than a certain limit, the two 

 sounds will appear distinct, or an echo will be perceived. 



As an illustration, suppose a speaker to stand before a wall 

 at the distance of say ten feet : in this case the audience in 

 front would hear but one sound. The direct and the reflected 

 impulse meet the ear within the limit which he has called 

 the limit of perceptibility. This limit — a knowledge of 

 which is of considerable practical importance — may either 

 be expressed in time or in space. The simplest method of 

 obtaining its amount is that of clapping the hands, while 

 standing before a perpendicular wall ; if the distance of the 

 observer be sufficient, an echo will be heard. If in this case 

 the observer gradually approach the wall and continue to 

 make the sound, at a definite point the echo will cease to 

 be perceived, and the two sounds will appear as one. If the 

 distance from the wall be now measured, twice the distance 

 found will give the limit of perceptibility in space. If the 

 same quantity be divided into the space through which the 



*[This communication, made August 21, 1849, was reported by title 

 only. — Proceed. Am. Assoc, vol. ii, p. 432.] 



