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71.
The difference between the apparent frequency of a source of sound as perceived by the observer during its approach and recession is 2% of the frequency of the source. If the speed of sound in air is 300 ms^-1, the velocity of the source is

1.5 ms^-1

12 ms^-1

6 ms^-1

3 ms^-1

3 ms^-1

When source approaches the observer, the apparent frequency heard by observer is

$n' = n (\frac{v}{v - v_{s}}) . . . . . (i)$

v_s = speed of source of sound.

During its recession, apparent frequency

$n " = n (\frac{v}{v + v_{s}}) . . . . . (ii)$

Accordingly, $n' - n " = \frac{2}{100} n (given)$

$∴ n (\frac{v}{v - v_{s}}) - n (\frac{v}{v + v_{s}}) = \frac{2}{100} n or v [\frac{v + v_{s} - v + v_{s}}{(v - v_{s}) (v + v_{s})}] = \frac{2}{100} or \frac{2 {vv}_{s}}{(v - v_{s}) (v + v_{s})} = \frac{2}{100} or 100 {vv}_{s} = v^{2} - {v_{s}}^{2}$

But speed of sound in air, v = 300 m/s

$∴ 30000 v_{s} = (300)^{2} - {v_{s}}^{2} \Rightarrow {v_{s}}^{2} + 3000 v_{s} - 90000 = 0 ∴ v_{s} = \frac{- 30000 \pm \sqrt{{(30000)}^{2} + 4 \times 90000}}{2} = \frac{30000 \pm 30006}{2} = \frac{6}{2} = 3 {ms}^{- 1} 9 (taking + ve sign only)$

72.

The loudness and pitch of a sound note depend on

intensity and velocity
frequency and velocity
intensity and frequency
frequency and number of harmonics

73.

In Melde's experiment in the transverse mode, the frequency of the tuning fork and the frequency of the waves in the string are in the ratio

2 : 1
4 : 1
1 : 1
1 : 2

74.

If two waves of the same frequency and amplitude respectively on superposition produce a resultant disturbance ofthe same amplitude, the waves differ in phase by

$π$
zero
$π / 3$
$2 π / 3$

75.

A man, standing between two cliffs, claps his hands and starts hearing a series of echoes at intervals of one second. If the speed of sound in air is 340 ms^-1, the distance between the cliffs is

680 m
1700 m
340 m
1620 m

76.

Sound waves in air are always longitudinal because

of the inherent characteristics of sound waves in air
air does not have a modulus of rigidity
air is a mixture of several gases
density of air is very small

77.

A particle on the trough of a wave at any instant will come to the mean position after a time (T = time period)

78.

The disc of a siren containing 60 holes rotates at a constant speed of 360 rpm. The emitted sound is in unison with a tuning fork of frequency

10 Hz
360 Hz
216 Hz
60 Hz

79.

The ratio of velocity of sound in hydrogen and oxygen at STP is

16 : 1
8 : 1
4 : 1
2 : 1

80.

In an experiment with sonometer a tuning fork of frequency 256 Hz resonates with a length of 25 cm and another tuning fork resonates with a length of 16 cm. Tension of the string remaining constant the frequency of the second tuning fork is