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SPEECH, MUSIC, AND HEARING 245
so do these fibres grow shorter. Upon the whole length of this series of fibres there are perched a number of small cells, each of which has what might be likened to short hairs sticking out from it, and these little hairs line the fluid of the spiral canal. The sound which reaches the hair cells of the inner ear sets up in them a nerve current which runs to the brain. It is that current, exciting the hearing cells in the brain, that enables us to hear. One has some notion of the number and delicacy and variety of the cur^ rents that pass along these nerves of hearing by simply considering that when he hears a big orchestra each in-* strument is recognized separately. It may thus be seen that what sound does after entering our ears is to startle certain nerve currents in the terminus of the nerve of hearing.
Human ears differ in their capacity both to hear sound and to recognize pitch. The limits of audibility at the lower end of the scale are given as from 12 to 33 vibrations per second, and at the upper end as between 20,000 and 40,000. The range of musical tones, however, lies between about 30 and about 4,000 — some seven octaves. About 4,000 vibrations per second, sounds are heard as squeals and squeaks and are practically indistinguishable in pitch. The normal range of the human voice is from approximately 60 per second for low bass to 1,300 for high soprano — more than four octaves.
Figure 10 shows the range of pressure and frequencies that the ear can register. Frequencies above about 20,000 cycles or below about 20 do not register as sound. Any frequency is recognized as sound if its pressure is above the lower boundary curve marked "Threshold of Audibility.,, The upper boundary, marked "Threshold of Feeling," indicates the degree of pressure at which feeling begins. Above this line sounds are felt to the degree that they actually cause pain through excessive pressure.