HAND-BOOK OF ACOUSTICS.
., - ^2 */b 41 d
therefore n x — =-------- X ----a?
» / rf /
*•*■ %/« = 2 ^5
or a; = 20
4. An organ pipe, open at both ends, is blown so hard that it sounds only the octave of the fundamental tone, (a) Describe the mode of vibration. (£) Draw a figure indicating the directions of motion in the different parts of the pipe at a given instant, (c) How would you experimentally show the positions of the nodes ?
Am. (a) See p. 103.
<- <-------------- <- -> -------------»
(«) See pp. 103 and 104.
5. Give a brief account of Helmholtz' Theory of Dissonance.
Am.—See Summary, p. 172.
Afternoon. 2 to 5.
1. How could you show that the velocity of sound through air at a constant temperature is independent of the pressure ?
Am.—The pitch of an organ pipe depends upon the velocity. Therefore, by showing that the same pipe, at same temperature gave a note of precisely the same pitch upon two different days when the pressure of the air was distinctly different, the above point would be proved.
2. Explain by the use of diagrams, how the quality of the note emitted by a plucked string depends upon the manner of plucking.
Am.—See pp. 94 and 95.
3. Explain why sound travels badly against the wind.
Am.—The velocity of wind is less near the surface of the earth than above because of friction. Consider in the first place, a wave surface travelling with the wind. For simplicity of explanation, suppose this surface to be plane and perpendicular to the earth's surface; then, as the velocity of the wind is