. A text book of physics, for the use of students of science and engineering . the separate sounds will be heard until thenumber reaches 25 to 30 per second, when they will cease to makeseparate impressions, and will be heard as a continuous hummingnote. As the frequency of the impulses increases, the pitch of thenote will rise. Evidently the pitch of the note depends upon thefrequency of the impulses. Many examples may be found to illustrate this. Thus, if a longpiece of clock spring be clamped at one end in a vice, and set vibrating,the vibrations, when very slow,do not produce any sensation
. A text book of physics, for the use of students of science and engineering . the separate sounds will be heard until thenumber reaches 25 to 30 per second, when they will cease to makeseparate impressions, and will be heard as a continuous hummingnote. As the frequency of the impulses increases, the pitch of thenote will rise. Evidently the pitch of the note depends upon thefrequency of the impulses. Many examples may be found to illustrate this. Thus, if a longpiece of clock spring be clamped at one end in a vice, and set vibrating,the vibrations, when very slow,do not produce any sensationof sound. On shortening thefree part of the spring, thevibrations obviously becomemore rapid, and soon a lownote will be heard. Withfurther shortening, the fre-quency increases, and a noteof higher and higher pitch willbe heard, until, when the springis something like 1 cm. in length,a very shrill note, that is, one ofvery high pitch, will be heard. That the relation between frequency and pitch is of a quantita-tive nature may be shown by means of the disc siren (Fig. 614).. Fig. 614.—Disc siren. 670 SOUND CHAP. A disc having two circles of holes in it is capable of being rapidlyrotated, and a tube A, leading from a pair of blowpipe bellows, directsa draught of air against the holes. When the rotation is very slow,a puff of air passes through each hole as it comes opposite the tube,and the ear will hear the separate puffs. On increasing the speedthe puffs blend into a note, whose pitch rises with increasing speed. Further, if there are twice as many holes in the outer ring as inthe inner ring, then, for any given speed, there will be an easilyrecognisable relation between the pitches of the notes produced bydirecting the jet of air against each set in turn. The note with twicethe frequency will be the upper octave of the other note. This relationis true whatever the speed of the disc, so that whatever the absolutefrequencies may be, one note is the upper octave of another if
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishe, booksubjectphysics
