EAR AS AN ORGAN FOR SOUND SENSATIONS. 



391 



tion of an inch at the other. They travel through the air with an 

 average velocity of 11 00 to 1200 feet per second, the exact rate vary- 

 ing with the temperature. When these waves, whatever may be 

 their form, follow each other with regularity — that is, with a definite 

 period or rhythm — a musical sound is perceived provided the 

 rhythm is maintained for a number of vibrations. So that regular- 

 ity or periodicity of the sound waves may be considered as the un- 

 derlying physical cause of musical sounds. Non-musical sounds or 

 noises, which constitute the vast majority of our auditory sensa- 

 tions, are referred, on the contrary, to non-periodical vibrations. 

 Waves of this kind may be due to the nature of the impulse given 

 to the air by the sounding body, — single pulses, for instance, or a 

 series of such pulses or shocks following at a slow or irregular 

 rhythm, or as is more frequently the case, they may result from a 

 mixture of very short and different rhythmical vibrations. As 

 the case of musical sounds is far the simpler, the theory of the 

 action of the cochlea has been based chiefly upon the results 

 obtained from a study of these forms of waves. 



Classification and Properties of Musical Sounds. — Musical 

 sounds exhibit three fundamental properties, each of which may be 

 referred to a difference in the physical stimulus. They vary, in 

 the first place, in pitch, and this difference finds its explanation in 

 the rapidity of vibration of the sounding body and the sound waves 

 produced by it. The more rapid the rate, the shorter will be the 

 waves and the higher will be the pitch of the musical note. Notes 

 of the same pitch may, however, vary in loudness or intensity, and 



Fig. 173. — To illustrate the conception of differences in pitch and in amplitude or intensity: 

 In .4 three pendular or sinus curves of the same period or pitch, but with different amphtudes. 

 In B three pendular or sinus curves of the same amplitude, but with different periods (after 

 Auerbach). 



this difference is referable to the amplitude of the vibrations (see 

 Fig. 173). A given tuning-fork emits always a note of the same 

 pitch, but the loudness of the note may vary according to the amp- 

 litude of the vibrations. The vibrations of the tympanic mem- 

 brane and of the perilymph in the internal ear vary in rate and in- 

 tensity with the sounding body ; so that we may say that the stimu- 

 lation of the hair-cells in the cochlea gives us auditory sensations 



