. A text book of physics, for the use of students of science and engineering . measuring these positions it is clear that A may be found. Determination of the pitch of a high note by interference. —Ifthe note is a shrill one, it affects a sensitive flame. This affordsa means of detecting the presence of interference. The source ofsound may be a Galtons whistle, A (Fig. 655), which can be blownsteadily by means of air which has been collected in an ordinary gasbag. A is situated at a distance of a metre or so from a verticalplane B, which may be part of the wall of the room, or a largedrawing b
. A text book of physics, for the use of students of science and engineering . measuring these positions it is clear that A may be found. Determination of the pitch of a high note by interference. —Ifthe note is a shrill one, it affects a sensitive flame. This affordsa means of detecting the presence of interference. The source ofsound may be a Galtons whistle, A (Fig. 655), which can be blownsteadily by means of air which has been collected in an ordinary gasbag. A is situated at a distance of a metre or so from a verticalplane B, which may be part of the wall of the room, or a largedrawing board placed vertically. The sound waves from A arereflected at B, and travel backwards as though they came from A,the image of A (p. 698). If the distances AC and AC differ by a whole number of wave-lengths, the direct and reflected waves assist each other, and if 710 SOUND CHAP. placed at this point, the sensitive flame will roar. If the flame benow moved to a point D, such that AD and AD differ by an oddnumber of half wave-lengths, the direct and reflected waves destroy. Fig. 655.—Determination of pitch by sensitive flame each other at D, and the sensitive flame will burn up brightly andsilently. A series of points of alternating quiet and disturbance canbe found in this manner. Since, AC-AC = «A, AD -AD=wA.+ ») 2i and (AD or, AC)+(AC-AD)=^,2CD4CD* Thus, the distance between a point of disturbance and the nearestpoint of silence is a quarter of a wave-length, and therefore the dis-tance between consecutive points of silence is a half the velocity of sound V, and the wave-length A. as deter-mined from the experiment, we can find the frequency from thee(luation V = «A (p. 680). Exit. 153.—Pitch by interference. Arrange the Galtons whistle andsensitive flame near a wall, as shown in Fig. 655. If the flame is fed bygas from an ordinary gas bag, regulate the pressure by placing weights onthe bag, until the Name roars when a whistle or hiss is made in i
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishe, booksubjectphysics
