1892.] On the Audibility of single Sound Waves, $c. o21 



vibration ought not to take place if the audition depended upon 

 adventitious vibrations set up in bodies in contact with the fork. But, 

 if each vibration transmits a distinct movement to the ossicles of the 

 ear, both these effects should be expected, and are easily explained. 

 And the fact that, with the low rates of vibration, the waves in air 

 ;are so long and the changes of pressure so gradual that the sound 

 'ceases to be appreciated through the air, while still distinctly audible 

 through the bones of the head, also becomes clear. We may then con- 

 clude that when the rate of vibration is reduced to that at which the 

 individual waves are no longer heard, the fork has become absolutely 

 inaudible ; while the power of differentiating tone is lost long before 

 this limit has been reached. 



It might be alleged that, although the fork gives rise to a series of 

 pure pendular vibrations, these set up secondary vibrations of higher 

 frequency in the membrane of the tympanum, or some medium through 

 which the waves pass. 



To test this point, we constructed a kind of phonautograph in 

 imitation of the tympanic membrane. It consisted of a circular metal 

 frame, over which a thin india-rubber membrane was stretched with 

 very slight and adjustable tension. An extremely light lever, armed 

 with a fine writing point, and poised on jewelled bearings, similar to 

 the escapement of a watch, was brought into connexion with the 

 centre of this membrane, so as to record its slightest movement on a 

 smooth, lightly- smoked, moving surface. With this apparatus we 

 were able to record the vibration of forks at the rate of 25 and 30 

 Y.D. per second, when the fork was held about 2 or 3 cm. from 

 the membrane, and the vibrations thus transmitted through the 

 medium of the air. The tracing was made up of a series of regular 

 undulations, entirely free from any indication either of over- tones or 

 "self-tone" of the membrane. It appears, then, exceedingly im- 

 probable, if such a comparatively rough mechanism can transmit 

 these slow toneless waves without exhibiting any tendency to per- 

 sistent vibration of its own, that this should occur with the membrana 

 tympani, the structure of which is adaptively modified so as to check, 

 in particular, any such effect, and which is capable of differentiating 

 such a wide range of tones with equal precision. 



From the foregoing it would appear that not only is there no 

 difficulty in understanding how these waves, which are too slow to 

 produce a tone, are carried to the nerve, but also that the pure 

 pendular vibrations of the tuning fork furnish conclusive evidence of 

 the audibility of each single wave when reduced to a rate at which 

 tone can no longer be discriminated. 



In order to obtain single vibrations of the more rapid rate of higher 

 notes (i.e., vibrations of short durations isolated from the series 

 causing the note) we employed a disk siren in which any number of 



