372 THE SPECIAL SENSES. 



musical sounds within the range of hearing the length of the waves 

 may vary from forty to seventy feet, at the one extreme, to a frac- 

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

 average velocity of 1100 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 

 this difference is referable to the amplitude of the vibrations. A 

 given tuning fork emits always a note of the same pitch, but the 

 loudness of the note may vary according to the amplitude of the 

 vibrations. The vibrations of the tympanic membrane and of 

 the perilymph in the internal ear vary in rate and intensity with 

 the sounding body; so that we may say that the stimulation of 

 the hair cells in the cochlea gives us auditory sensations that vary 

 in pitch with the rate of excitation and in intensity with the ampli- 

 tude of the vibratory movement. A third property of musical 

 sounds is their variations in quality or timbre. The same note of 

 the same amplitude when given by different musical instruments 

 varies in quality, so that we have no difficulty in recognizing the 

 note of a piano from the same note when given by a violin or the 

 human voice. The underlying physical cause of variations in timbre 

 is found in the form of the sound waves produced, and immediately, 

 therefore, in the form of vibratory movement communicated to 

 the perilymph. Examination of the forms of sound waves produced 



