

Konal institution of (ilr^at %\ 



WEEKLY EVENING MEETIN(^^^N4!».^»^><C>/ 



Friday, January 17, 1902. ^^^^J>^ 



His Grace the Duke of Northumberland, K.G. D.O.L. F.R.S., 

 President, in the Chair. 



The Eight Hon. Lord Rayleigh, M.A. D.C.L. LL.D. Sc.D. 

 F.R.S. M.B.I., Professor of Natural Philosophy, R.I. 



Interference of Sound. 



For the purposes of laboratory or lecture experiments it is con- 

 venient to use a pitch so high that the sounds are nearly or altogether 

 inaudible. The wave lengths (1 to 3 cm,) are then tolerably small, 

 and it becomes possible to imitate many interesting optical pheno- 

 mena. The ear as the percipient is replaced by the high pressure 

 sensitive flame, introduced for this purpose by Tyndall, with the 

 advantage that the effects are visible to a large audience. 



As a source of sound a " bird-call " is usually convenient. A 

 stream of air from a circular hole in a thin plate im23inges centrically 

 upon a similar hole in a j)arallel plate held at a little distance. 

 Bird-calls are very easily made. The first plate, of 1 or 2 cm. in 

 diameter, is cemented, or soldered, to the end of a short supply-tube. 

 The second plate may conveniently be made triangular, the turned 

 down corners being soldered to the first plate. For calls of medium 

 pitch the holes may be made in tin plate. They may be as small as 

 J mm. in diameter, and the distance between them as little as 1 mm. 

 In any case the edges of the holes should be sharp and clean. 

 There is no difficulty in obtaining wave-lengths (complete) as low 

 as 1 cm., and with care wave-lengths of • 6 cm. may be reached, cor- 

 responding to about 50,000 vibrations per second. In experimenting 

 upon minimum wave-lengths, the distance between the call and the 

 flame should not exceed 50 cm., and the flame should be adjusted to 

 the verge of flaring.* As most bird-calls are very dependent upon 

 the precise pressure of the wind, a manometer in immediate connec- 

 tion is practically a necessity. The pressure, originally somewhat 

 in excess, may be controlled by a screw pinch-cock operating on a 

 rubber connecting tube. 



In the experiments with conical horns or trumpets, it is impor- 

 tant that no sound should issue except through these channels. The 

 horns end in short lengths of brass tubing which fit tightly to a 

 short length of tubing (A) soldered air-tight on the face of the front 



* ' Theory of Sound.' "ind etl.. § 371. 

 Vol. XVII. (No. 96.) b 



