HANDBOOK OF ACOUSTICS - online book

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EXAMINATION PAPERS.
265
MUS.BAC. (Cantab.)*
May 21, 1895. 9 till 12. 1. Explain with help of a diagram how a given particle of the air alters its position as a train of sound-waves passes over it. Show how we can determine graphically what happens when two trains of waves pass over the particle simultaneously. Am.                       1                  •
2                        •
3                           •
4                        •
5                     •
6                 •
7             •
8                 •
9                     •
Let the dot on line 1 represent the particle at rest. When the condensed part of the wave reaches it from left, it began to travel to the right, as seen in, line 2. In 3, it has reached its extreme position and begins to return as at 4. In 5, it has reached its orignal position, and is still travelling towards left as at 6. In line 7, it has reached its extreme left hand position, and then returns, as at 8, to its original position at line 9, the wave having now just passed over it. The second part of the question is fully answered at p. 82.
2.    Describe experiments that show the intensity of sound to be connected with the amplitude of vibration of the air, and the pitch with the period of vibration.
Am.—For first part, see p. 52, last eight lines, and p. 53, firat fourteen. For second part, either Wheel Syren, pp. 29 and 30, or the Syren of Cagniard de Latour, pp. 31 and 32, or Savart's Toothed Wheel, p. 6.
3.  (a) Explain how or why a rise of temperature affects the pitch of the wind instruments in an orchestra.
(b) If the velocity of sound is 1,120 ft. per sec. at 60° and 1,140 ft. per sec. at 77°, how much would a trumpet player have to alter the length of the tube of his instrument in order to keep to his original pitch, if the temperature of the concert room rose from 60° to 77° ? (Assume the length of tube in a trumpet to be 5ft.).
Am. (a) See p. 100.
(J) Length of trumpet is 5ft., therefore length of sound-wave at 60° is 10ft., and vibration number of fundamental is 'ff0 = 112.
Hence length of sound-wave at 77° must be VVs0 = W = 10 f^: and length of trumpet must be 5 -^ ft. The player therefore would have to lengthen his instrument by -£$ ft., that is ■§%■ X V4 = tit =
la — 1JL. rn
1 * — XM m*