1897.] The Bight Hon. Lord Eayleigh on Limits of Audition. 417 



WEEKLY EVENING MEETING, 



Friday, April 9, 1897. 



Sir Frederick Bramwell, Bart. D.C.L. LL.D. F.R.S. Honorary 

 Secretary and Vice-President, in the Chair. 



The Eight Hon. Lord Rayletgh, M.A. D.C.L. LL.D. F.R.S. M.B.L 

 Professor of Natural Philosophy, B.I. 



The Limits of Audition. 



(Abstract.) 



In order to be audible, sounds must be restricted to a certain range 

 of pitch. Thus a sound from a hydrogen flame vibrating in a large 

 resonator was inaudible, as being too low in pitch. On the other 

 side, a bird-call, giving about 20,000 vibrations per second, was 

 inaudible, although a sensitive flame readily gave evidence of the 

 vibrations and permitted the wave-length to be measured. Near 

 the limit of hearing the ear is very rapidly fatigued ; a sound in the 

 first instance loud enough to be disagreeable, disappearing after a 

 few seconds. A momentary intermission, due, for example, to a rapid 

 passage of the hand past the ear, again allows the sound to be heard. 

 The magnitude of vibration necessary for audition at a favourable 

 pitch is an important subject for investigation. The earliest estimate 

 is that of Boltzmann. An easy road to a superior limit is to find 

 the amount of energy required to blow a whistle and the distance to 

 which the sound can be heard (e.g. one-half a mile). Experiments 

 upon this plan gave for the amplitude 8 X 10~^ cm., a distance 

 which would need to be multiplied 100 times in order to make it 

 visible in any possible microscope. Better results may be obtained 

 by using a vibrating fork as a source of sound. The energy resident 

 in the fork at any time may be deduced from the amplitude as ob- 

 served under a microscope. From this the rate at which energy is 

 emitted follows when we know the rate at which the vibrations of 

 the fork die down (say to one-half). In this way the distance 

 of audibility may be reduced to 30 metres, and the results are less 

 liable to be disturbed by atmospheric irregularities. If s be the 

 proportional condensation in the waves which are just capable of 

 exciting audition, the results may be expressed: — 



frequency == 256 j s = 6-0 x 10 



„ = 384 p = 4-6 X 10 



=r 512 ' s = 4-6 X 10 



showing that the ear is capable of recognising vibrations which 

 involve far less changes of pressure than the total pressure out- 

 standing in our highest vacua. 



