584 



REPORT — 1897. 



3. A New Instrument for Measuring the Intensity of Sound. 

 By A. G. Webster and B. F. Sharpe. 



The instrument consists of a spherical resonator, to which is attached a thin- 

 glass diaphragm, the excursions of which are measui'ed by the displacements of 

 interference fringes in a Michelson interferometer. The diaphragm carries at its 

 centre a small plane mirror about 4 mm. square, which is made the movable plane 

 of the interferometer, there being besides two fixed glasses and one movable in a 

 slide by a slow-motion screw. The apparatus is solidly fastened to a bronze base, 

 and is completely enclosed by a felt-covered box, leaving exposed only the resonator 

 with a hole opposite the diaphragm. The apparatus is adjusted so that the fringes 

 are parallel and vertical, using first monochromatic and then white light. When 

 a sound is made, the fringes become blurred, so as to disappear, and must accord- 

 ingly be observed stroboscopically. Accordingly for the source of sound is chosen 

 a tuning-fork electrically maintained, while the fringes, reduced by a horizontal 

 slit to a line of points moving horizontally, are observed by a small telescope 

 whose objective is carried by the prong of a second independently maintained 

 tuning-fork vibrating synchronously with the source of sound, the lens moving, 

 vertically. The fringes are accordingly seen as inclined lines, the inclination of 

 which is measured by a graduated circle and rotating cross-hair in the eyepiece. 

 The excursion is proportional to the tangent of the angular displacement. This 

 was found more convenient than counting the number of fringes displaced. 



The chief difficulty after that of securing absolute freedom froui extraneous 

 noises is in maintaining the constancy of the source of sound. This was finally 

 accomplished by making the break of the fork which interrupted the circuit for the 

 source proper a large mercury surface, the controlling fork being placed upon a 

 solid pier, and boxed in, so as to emit no sound. The source proper was a fork 

 mechanically connected to a diaphragm mounted upon a spherical resonator, all 

 being boxed in except a circular orifice in the resonator, so that the sound pro- 

 ceeded from a definite point. This could be moved about the room without the 

 intensity changing. 



Observations were made in the middle of the night. The following data wili 

 give an idea of the constancy of the conditions : — 



w = width of one fringe in micrometer divisions. 

 A = vertical height of stroboscopic image. 

 a = angle of fringes with vertical. 

 I = intensity of sound. 

 t = time of observation. 



In these observations the source was a fork of 256 complete vibrations, sound- 

 ing as if rather gently bowed. Observations of the displacement for a certain 

 steady pressure were made, and from observations on the inertia of parts of the 

 apparatus it is intended to reduce them to absolute measure. 



