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VII. § 78.] EXPERIMENTAL STUDY OF BEATS. 161 |
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to exhibit these phenomena to several persons at once, the forks should first be mounted on their resonance-boxes, and, after the pellets have been attached, stroked with the resined bow, care being taken to produce tones as nearly as possible equal in intensity. Slow beats may also be obtained from any instrument capable of producing tones whose vibration-numbers differ by a sufficiently small amount. Thus, if the strings corresponding to a single note of the pianoforte are not strictly in unison, such beats are heard on striking the note. If the tuning is perfect, a wax pellet attached to one of the wires will lower its pitch sufficiently to produce the desired effect. Beats not too fast to be readily counted arise between adjacent low notes on the harmonium, or, still more conspicuously, on large organs. They are also frequently to be heard in the sounds of church bells, or in those emitted by the telegraph wires when vibrating in a strong wind. In order to observe them in the last instance, it is best to press one ear against a telegraph-post and close the other: the beats then come out with remarkable distinctness. It should be noticed that, when we are dealing with two composite sounds, several sets of beats may be heard at the same time, if pairs of partial-tones are in relative positions suited to produce them. Thus, suppose that two t. 11 |
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