greater above, this plane will not remain vertical, but will be tilted over more and more towards the earth; that is, the sound will take a more or less downward direction and keep near the surface. Now consider a wave surface travelling against the wind. The upper part of the wave surface will now lag behind the lower so that the wave surface will be tilted upwards, that is, the sound soon leaves the earth altogether and passing over the observer's head is lost above.
4. (a) Describe an experiment proving the interference of two sounds, and (b) give some familiar examples of the effects of interference.
Ans. (a) See p. 139 or 140. (*) See pp. 141, 147.
5. (a) What is meant by " combination tones P" (b) What is their origin P
(e) What are the frequencies of the combination tones for two notes of frequencies 256 and 384 ? An*, (a) See p. 135. (*) See p. 133. (e) 384—256=128. 384+256=640.
Intermediate Doc. Mus. Examination. 1896. Morning 10 to 1.
1. What evidence can you give to show that periodic compressions and rarefactions are occurring in the air in the neighbourhood of a sounding body ?
Am.—Best by means of a manometric jet (for construction of which see fig. 55 left hand top corner and accompanying description) and a revolving mirror used as explained on p. 5.
2. How does the pitch of the note given out by a stretched string depend (1) on the length of the string, (2) on the weight of unit length of the string, and (3) on the tension of the string ?
Ans.—See p. 87.
3. How could you analyse a complex sound so as to determine whether a note of any particular pitch was present ?
Ans.—See p. 65.
4. Explain the method of production of beats, illustrating your answer by carefully-drawn figures. What effect is produced when two sounds of nearly equal periods, but of somewhat different amplitudes, are sounded together ?
Ans.—See pp. 144, 145, and 146.