268 HAND-BOOK OF ACOUSTICS.
Am. (a) 8uppose a point Q, revolves A
round the circle A Q B with uniform speed, and also let a point P be vibrating to and fro, along the line A D, then if the imaginary straight line connecting P and Q is always parallel to P Q, then point P is said to be executing simple harmonic or pendular vibrations. See p. 81. In figure 30, p. 54, in which the vibrating body is supposed to be at A; the spherical surface at D is a wave front, that is, a continuous surface, at every B
point of which the air is in the same stage of vibration at the same moment. B and G are also wave fronts.
Suppose two series of equal waves pass over the same particles of air, if one series is half a wave length before the other, they are said to be in opposite phase ; if one is a quarter wave length behind the other, the difference of phase is one quarter; and so on. See pp. 136, 137, and 144. For wave length see p. 22.
(b) See either p. 12 or 14.
(e) It is shown on p. 13 that each particle makes one complete vibration in same time as wave takes to travel its own length. Now if » be vibration number of the sound, J is the time it takes to travel a wave length—that is, the time of vibration of each particle. Now it is shown on p. 39 that wave length = velocity of sound (v) divided by the vibation number (») or
wave length = -
(i.e.) „ = v X i
„ = velocity of wave X time of vibration of one of the
2. (a) Describe some accurate method of finding the velocity of sound in air.
(b) How would the velocity be affected if the height of the barometer changed, or
(c) If the temperature changed ?
Ans. (a) See pp. 19 and 20. The air between the two stations should be at rest. (b) Not at all. (e) Velocity is increased by a rise of temperature. See pp. 20 and 100.
3. (a) A string attached to two rigid supports gives out very little sound when made to vibrate. If attached to a sound board the sound is much louder. Explain this and give instances of similar effects.