. A text-book of human physiology . vibrations, moving toand fro within narrow limits so that the air is alternately condensed andrarefied (wave crest and wave trough). Sound is propagated in the form ofconcentric spherical waves, new particles of air in all directions from thesounding body being successively set in motion. Three qualities of sound are to be distinguished: loudness, pitch andtimbre. A. LOUDNESS The loudness of a sound depends upon the ampJitude of the greater the excursions which, for example, a vibrating piano stringdescri])es, the louder is the sound at a giv
. A text-book of human physiology . vibrations, moving toand fro within narrow limits so that the air is alternately condensed andrarefied (wave crest and wave trough). Sound is propagated in the form ofconcentric spherical waves, new particles of air in all directions from thesounding body being successively set in motion. Three qualities of sound are to be distinguished: loudness, pitch andtimbre. A. LOUDNESS The loudness of a sound depends upon the ampJitude of the greater the excursions which, for example, a vibrating piano stringdescri])es, the louder is the sound at a given distance from its source. Thegreater the distance from the source, the weaker is the sound, the loudnessbeing inversely as the square of the distance. B. PITCH Pitch is determined by the vibration frequency, or in other words by thenumber of vibrations per second, and is independent of the form of the vibra- Lehre von den Tonempfindungen als physiologische Grundlage fiir die Theone derMusik. 30 489 490 HEARING, VOICE AND SPEECH. tion during the period. The more frequent the vibrations in a unit of time—i. e., the shorter the period—the liigher is the tone. For an exposition of the fundamental facts of this subject it is very con-venient to have a special apparatus, like the siren (Fig. 193), which permitsan easy <letermination of the number of atmospheric vibrations producingdifferent tones. A is a thin disk of pasteboard or metal which is provided with holes inseveral concentric rows and at equal distances from each other in the same can be set in rapid rotation by means of the string f which runs over thepulley b. By means of the tube c a blast of air of proper strength is directedtoward one of the rows of holes. Each hole therefore as it passes the mouth ofthe tube lets out a single puff of air and thus when the disk is rotated rapidlyenough a tone is produced whose pitch depends upon the number of holes blo-wn through in a second of number can be
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