Page:Elementary Text-book of Physics (Anthony, 1897).djvu/180

166 The number of vibrations of a sounding body may sometimes be determined by attaching to it a light stylus which is made to trace a curve upon a smoked glass or cylinder. Instead of attaching a stylus to the sounding body directly, which is practicable only in a few cases, it may be attached to a membrane which is caused to vibrate by the sound-waves which the body generates. A membrane reproduces very faithfully all the characteristics of the sound-waves, and the curve traced by the stylus attached to it gives information, therefore, not only in regard to the number of vibrations, but to some extent in regard to their amplitude and form.

139. Concord and Discord.—When two or more tones are sounded together, if the effect be pleasing there is said to be concord; if harsh, discord. To understand the cause of discord, suppose two tones of nearly the same pitch to be sounded together. The resultant curve, constructed as in §132, is like those in Fig. 53, which represent the resultants when the periods of the components have the ratio 81:80 and when they have the ratio 16:15. The figure indicates, what experiment verifies, that the resultant sound suffers periodic variations in intensity. When these variations occur at such intervals as to be readily distinguished, they are called beats. These beats occur more and more frequently as the numbers expressing the ratio of the