As we have seen, authors do not agree upon the functions of the different parts of the auditory apparatus in the perception of the intensity, the distance, and the direction of sound. The perception of the intensity of sound seems to depend more on the relative perfection of the whole organ than on any one of its parts. Vibrations are transmitted to every part of the ear, and even to the whole body, in loud noises and sounds. Thus thunder, the report of cannon, the grave notes of an organ or of a double-bass viol, cause a tremor in the whole body; but it is by the vibratory excitement of the auditory nerve that we judge of the intensity of sounds, as the optic nerve enables us to appreciate that of light.
In regard to distance, if it is a sound with which we are familiar, that of the human voice, for example, we judge of it by the greater or less force of the auditory impression. As for noises of which we do not know the intensity at a given distance, as thunder, we estimate it in the same way, but with less certainty according as it is faint or loud.
It is therefore to reasoning, founded on the sensation, that we owe the ability to judge of the distance as well as the intensity of sounds, and it is the same as to their direction. When we hear a sound more distinctly with one ear than with the other, we judge that it comes from the side on which the impression is strongest, and the ability of the organ to seize slight degrees in the intensity enables us to tell in what position of the head the sound is most clearly perceived. We are therefore led to place it in a certain position in regard to the direction, and by this means we acquire an idea of it within certain limits. Hence, if the ears are both in the same situation relative to the sound, as when it is in front of or behind us for example, we find it impossible to distinguish in which direction it is without turning the head.
This uncertainty which we always feel in regard to the exact distance and direction of sounds enables the ventriloquist to produce what are wrongly supposed to be illusions of hearing, but which are simply errors of judgment guided by the imagination. The hollow, feeble voice of the ventriloquist seems to come from a great distance, from above or from a certain depth below us, the sense of the words, the expression of the voice, the varied tones and mimicry of the juggler, do the rest.
Savart has demonstrated that the duration of acoustic impressions is about the tenth of a second. Thus when the vibrations of a body do not exceed nine in a second, the ear perceives a series of distinct impressions, but beyond ten or twelve the sensation becomes continuous.
As the eye may be the seat of luminous impressions produced by other means than light, so sounds and noises may be heard, without the ears having been excited by sonorous waves. Ringing and humming sensations may be produced in or imparted to them, under abnormal conditions into which we shall not inquire; and of which the mechanism is obscure or unknown. A prolonged shock to the auditory nerve by a loud sound or noise will cause a persistent confused sensation, which is felt by everyone after a long journey by railway, or after being near a great waterfall or in a mill for a length of time.