Question

A source emits sound with a frequency of 860 Hz. It is moving at 20.0 m/s toward a stationary reflecting wall. If the speed of sound is 343 m/s, what frequency does an observer riding with the source hear

Answer 1

Answer:

Explanation:

f = 860 Hz

velocity of  source, vs = 20 m/s

velocity of sound, v = 343 m/s

use the formula of doppler's effect

the frequency reflected from the wall is

$f' = \frac{v}{v+ v_{s}}\times f$

$f' = \frac{343}{343+20}\times 860$

f' = 812.62 Hz

Frequency heard by the observer

$f'' = \frac{343}{343-20}\times 812.62$

f'' = 862.93 Hz

Answer 2

Answer:

$f_o=860\ Hz$

Explanation:

Given:

• original frequency of sound wave, $f=860\ Hz$

• speed of the sound source, $v_s=20\ m.s^{-1}$

• original speed of sound wave from the source, $s=343\ m.s^{-1}$

According to the Doppler's effect:

$\frac{f_s}{f_o} =\frac{s+v_s}{s-v_o}$

The sound is reflected from the wall and the source is moving towards the wall and observer is also riding the same source.

The velocity of the observer, $v_o=-20\ m.s^{-1}$

$\frac{860}{f_o} =\frac{s+20}{s-(-20)}$

$f_o=860\ Hz$