JEE Main · 2021 · 26 Aug 2021 · Shift 1 · Waves & Sound

A source and a detector move away from each other in absence of wind with a speed of

20 m / s

with respect to the ground. If the detector detects a frequency of 1800 Hz of the sound coming from the source, then the original frequency of source considering speed of sound in air

340 m / s

will be ______ Hz.

Answer: 2025

Solution

The detector recedes from the source and the source recedes from the detector, so both motions lower the detected frequency:

f^{'} = f_{0} \frac{v - v _{o}}{v + v _{s}}

Substituting

v = 340

m/s,

v_{o} = v_{s} = 20

m/s and

f^{'} = 1800

Hz,

1800 = f_{0} \frac{340 - 20}{340 + 20} = \frac{8}{9} f_{0}

f_{0} = 1800 \times \frac{9}{8} = 2025 Hz

JEE Main · 2021 · 26 Aug 2021 · Shift 1 · Integer answer

Practise more: all JEE Main PYQs on Waves & Sound →

More JEE Main questions on Waves & Sound

Two loudspeakers ( $L_{1}$ and $L_{2}$ ) are placed with a separation of 10 m , as shown in figure. Both speakers are fed with an audio input signal of same frequency with constant volume. A voice recorder, initially at point $A$ , at equidistance to both loud speakers, is m …
JEE Main 2026View solution →
In an open organ pipe $v_{3}$ and $v_{6}$ are $3^{rd}$ and $6^{th}$ harmonic frequencies, respectively. If $v_{6} - v_{3} = 2200$ Hz then length of the pipe is $____$ mm. (Take velocity of sound in air is $330$ m/s.)
JEE Main 2026View solution →
The velocity of sound in air is doubled when the temperature is raised from $0^{\circ} C$ to $α^{\circ} C$ . The value of $α$ is ______.
JEE Main 2026View solution →
The fifth harmonic of a closed organ pipe is found to be in unison with the first harmonic of an open pipe. The ratio of lengths of closed pipe to that of the open pipe is 5/x. The value of x is ____.
JEE Main 2026View solution →
A point source is kept at the center of a spherically enclosed detector. If the volume of the detector increased by 8 times, the intensity will
JEE Main 2026View solution →
The speed of a longitudinal wave in a metallic bar is $400 m / s$ . If the density and Young's modulus of the bar material are increased by $0.5%$ and $1%$ respectively then the speed of the wave is changed approximately to ______ …
JEE Main 2026View solution →