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SPEECH, MUSIC, AND HEARING 251
therefore, the fundamental and the first three or four overtones are necessary in order to distinguish the tones of various instruments. When overtones higher than these are eliminated the tones lose a characteristic best described by the terms "brilliance" or "definition"; they seem to lose life and to become dull. The prominence of these characteristics varies with the type of instrument, the composition of the music, and the personality of the musician.
The notes which are used most in music are contained in the octaves below and above middle C, or from 128 to 512 cycles. Since the fourth overtone of 512 cycles has a vibration frequency of 8,192 cycles, tones of this frequency, and below, occur commonly in music. A trained ear could no doubt detect the elimination of frequencies above this range from the ordinary run of music, but the average individual would have difficulty in detecting the elimination of frequencies above even 6,000 or 7,000 cycles unless he gave particularly close attention to the percussion instruments.
Another phenomenon of hearing which enters into the sensation of sound is called "masking." The tones of low pitch in a given sound deafen the auditor to the higher tones. The deafening or masking effect becomes quite marked when the sound pressures of the lower tones are greater than 20 sensation units. The optimum loudness for the interpretation of speech corresponds to a sound pressure between o and 20 sensation units. If the sound pressure is less than this the fainter sounds are inaudible. If it is greater, the masking effect impairs the interpretation. When sounds are increased in loudness the lower registers are accentuated because of the masking effect; so that, for most faithful reproduction, sounds should be reproduced with about the same loudness as the original.
Practical tests of the hearing of 500 persons have dis