MINIMUM AUDIBLE INTENSITY OF SOUND. 435 



to flow through the coils of the fork F and set the latter in vibration. 

 The amplitude of this vibration was governed by a rheostat Q in the 

 driving circuit. 



During the course of an experiment, the observer sat within a box 

 placed against the wall of the room, so constructed that only his head 

 was exposed. The aggregate of absorbing material in the room was 

 thus diminished, the residual sound prolonged, and the accuracy of the 

 observations increased. On the other hand, it created a certain 

 difficulty. In an enclosed room the sound establishes an interference 

 system which must be reckoned with in any acoustical investigation. 

 On account of this interference pattern, the observer, just as the sound 

 ceases to be audible, may be either at a point of maximum or of mini- 

 mum disturbance, or between the two. It is therefore desirable to 

 take a number of observations at different points relative to the inter- 

 ference system in the room, which is obviously impossible if the ob- 

 server is to be confined within the box. The difficulty was overcome, 

 however, in a manner much more effective than by moving the ob- 

 server. The interference system itself was made to shift by a pair of 

 large reflectors reaching more than half way across the room, suspended 

 from a horizontal bar, and revolved by clock work about a vertical 

 axis. This served also to eliminate the effect of the reaction of the 

 sound in the room on the source. 



The time of decay was measured by a stop-watch so arranged that it 

 began to record at the instant the source of sound was stopped by the 

 key K, and continued to record until stopped by the observer when the 

 residual sound in the room reached the limit of audibility. 



Much difficulty was experienced, at first, owing to the continued 

 vibration of the sounding-fork after the current had been broken. An 

 effective method of damping was secured by causing the relay R to 

 close a 110 volt direct current through the coil of the fork F when the 

 driving current was broken. This pulled the prongs of the fork 

 violently together and stopped the vibration instantly. The amount 

 of current was so great, however, as to burn out the coil of the fork in a 

 very short time. The ideal to be sought, evidently, was some means 

 of permitting a momentary flow of a current sufficiently great to damp 

 the fork instantly, followed by an immediate reduction in the current- 

 strength to an amount which could be safely maintained through the 

 coil. Not only this, but the current must be reduced gradually in 

 strength in order to allow the tynes of the fork to resume their normal 

 position slowly and not produce further sound, as would be the case if 

 they were allowed to snap back into place. A way of solving these 



