# A Sound Source, Fixed at the Origin, is Continuously Emitting Sound at a Frequency of 660 Hz. the Sound Travels in Air at a Speed of 330 M S−1. - Physics

Sum

A sound source, fixed at the origin, is continuously emitting sound at a frequency of 660 Hz. The sound travels in air at a speed of 330 m s−1. A listener is moving along the lien x= 336 m at a constant speed of 26 m s−1. Find the frequency of the sound as observed by the listener when he is (a) at y = − 140 m, (b) at y = 0 and (c) at y = 140 m.

#### Solution

Given:
Frequency of sound emitted by the source $n_0$= 660 Hz
Velocity of sound in air v = 330 \text { ms}^\(-)1
Velocity of observer $v_0$= 26 ms−1
Frequency of sound heard by observer n = ?
(a) At y = 140 m:
Frequency of sound heard by the listener, when the source is fixed but the listener is moving towards the source:

$n = \frac{v + v_0}{v} n_0$

Here ,

$v_0 = v_0 \cos\theta$

On substituting the values, we get:

$n = \frac{v + v_0 \cos\theta}{v} n_0$

$= \frac{330 + 26 \times \frac{140}{364}}{330} \times 660$

$= 340 \times 2 = 680 \text{ Hz }$

(b) When the observer is at y = 0, the velocity of the observer with respect to the source is zero.
Therefore, he will hear at a frequency of 660 Hz.

(c) When the observer is at y = 140 m:

$n = \frac{v - v_0}{v} \times n_0$

Here,

$v_0 = v_0 \cos\theta$

On substituting the values, we get:

$n = \frac{330 - \frac{26 \times 140}{364}}{330} \times 660$

$n = \frac{330 - 10}{330} \times 660 = 640 \text { Hz }$

Concept: Wave Motion
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#### APPEARS IN

HC Verma Class 11, 12 Concepts of Physics 1
Chapter 16 Sound Waves
Q 79 | Page 357