Q. 87

# A source of sound emitting a 1200 Hz note travels along a straight line at a speed of 170 m s^{-1}. A detector is placed at a distance of 200 m from the line of motion of the source. (a) Find the frequency of sound received by the detector at the instant when the source gets closest to it. (b) Find the distance between the source and the detector at the instant it detects the frequency 1200 Hz. Velocity of sound in air = 340 m s-1.

Answer :

(a) Let us assume a point A from which sound was emitted and till it reaches the listener till the source reaches the closest point.

If v_{s} be the source speed and v be the speed of the sound wave,

And,

the angle between the path of the sound and the line of motion will be,

So, the frequency will become

(b)

To find 1200 Hz we need to consider the minimum distance which is 200m.

Now,

time to travel 200 m = time to travel x distance

Hence the distance between the source and the listener will be,

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