The general terms—pitch, loudness, quality, and duration, also embrace all the distinctions heard in ordinary sounds. These sounds differ from the musical in the pitch constantly varying throughout their duration, as the human voice in speaking, and the voice of quadrupeds. Acoustically such sounds are composed of an unequal number of impulses or noises produced in equal tones. And from this circumstance pitch, in the strictly musical sense, is not a property of ordinary sound.
The general terms—loudness and quality, embrace all the distinctions heard in a noise, as in the collision of two unelastic sticks. Pitch and duration can scarcely be considered as belonging to common noise. Thus we have—(1) noise whose audible distinctions are comprehended under the general terms loudness and quality; (2) common sound, whose audible distinctions are comprehended under the general terms—loudness, quality, duration, and every varying pitch; (3) musical sound, whose audible distinctions are comprehended under the general terms—loudness, quality, duration, and uniform pitch.
Phonation, or the production of voice, is a result of actions taking place under two distinct classes of laws—namely, the ordinary mechanical laws of acoustics, and the physiological laws of muscular movement. The adjustment of the vocal mechanism to be brought into operation by the current of air, is made by actions under the latter laws; and phonation is the result of the reaction of the mechanism on the current of air, by mechanical movements under the former laws. Now, the pitch of the voice essentially depends on the tension of the vocal ligaments; the loudness or the extent of the excursion of these ligaments in their vibration; the duration on the continuance of the vocalizing causes; the quality on the organization of the larynx, and also on the form and size of the vocal tube. The form and size of this tube can be altered in various ways—for instance, by dilating or contracting the pharynx; by dilating or contracting the mouth; by contracting the communication between the pharynx and mouth, so as to constitute them distinct chambers, or by dilating the opening so as to throw them into one, which is chiefly attained by movements of the soft palate; and by altering the form of the mouth’s cavity, which is effected by varying the position of the tongue. Each of these modifications of the vocal tube conveys a peculiarity of quality to the voice,—all, however, being local or laryngeal sounds. Moreover, sounds can be produced in the vocal tube, apart from the larynx. These, strictly speaking, are not vocal sounds, though some of them may be of a definite and uniform pitch, while others are mere noises—as rustling, whispering, gurgling, whistling, snoring, and the like. Now, as everything audible comes under the classes of noise, sound, or musical sound, and as each variety originates in the vocal apparatus of man, it is obvious that an ordinary vocal apparatus is all that is required for the achievement of the feats of ventriloquism.
A person having an ear acutely perceptive to the nice distinctions of sounds, may, by a little practice, imitate many sounds with accuracy. Those persons, however, who are highly endowed with the mental requisites, which consist of an intense desire to mimic, coupled with the ability to originate mimetic ideas, are able to imitate sounds at first hearing.
We next proceed to treat of those illusions, where the voice so perfectly counterfeits the reality intended, that it appears not to issue from the mimic, but from an appropriate source, in whatever direction, and at whatever distance the source may be. We do not hear the distance which a sound has travelled from its source, but we judge the distance from our former experience, by comparing the loudness which we hear with the known distance and known loudness of similar sounds heard on former occasions. Common experience will prove that we oftener err in estimating the distance of uncommon than of familiar sounds. In apology for such an error, the ordinary language is, “It seemed too loud to come so far,” or “It seemed too near to be so faint a sound,” as the case may be,—both of which are apologies for an erroneous judgment, and not for faulty hearing. Near sounds are louder than distant ones. Now, by preserving the same pitch, quality, and duration, but with an accurately graduated reduction of loudness, a series forming a perspective of sounds may be created, which, falling in succession on the ear, will suggest to the mind a constantly increasing distance of the sound’s source. The estimate, then, which is formed of the distance which a sound has travelled before reaching the ear is a judgment of the mind formed by comparing a present perception (by hearing) with the remembrance of a former loudness in connection with its known distance. With regard to direction, it is observed, “The direction whence a sound comes seems to be judged of by the right of left ear receiving the stronger impression, which, however, can only take place when the sound’s source is in a plane, or nearly so, with a line passing through both cars. It is familiarly known that a person in a house cannot by the noise of an approaching carriage judge with certainty whether it is coming from the right or left. He accurately judges it to be approaching, passing, or receding, as the case may be, by the gradations of loudness, but is unable to decide with certainty whether its approach or recession is from up or down the street. Enough has been stated to show that we do not hear, but that we judge the direction a sound has travelled from its source on reaching the ear.” The ventriloquist indicates, either directly or indirectly, the direction from which he wishes his audience to believe the sound is coming. Thus he directly indicates it by words, such as—“Are you up there?” “He is up the chimney,” “He is in the cellar,” “Are you down there?” &c., as illustrated in the various examples. He indirectly indicates it by some suggestive circumstance, as an action or gesture, which is so skilfully unobtrusive and natural as to effect its object without being discovered. Thus, when the ventriloquist looks or listens in any direction, or even simply turns towards any point, as if he expected sound to come thence, the attention of an audience is by that means instantly directed also to the same place. Thus, before a sound is produced, the audience expect it to come in the suggested direction; and the ventriloquist has merely, by his adjustment of vocal loudness, to indicate the necessary distance, when a misjudgment of the audience will complete the illusion which he has begun.
The effect which is produced on sound by its travelling from a distance, is observed to be:—
(1) That its loudness is reduced in proportion to its distance.
(2) That its pitch remains unaltered.
(3) That its quality or tone is somewhat altered.
(4) That its duration remains unaltered.