61.

A person carrying a whistle emitting continuously a note of 272 Hz is running towards a reflecting surface with a speed of 18 km/h. The speed of sound in air is 345 ms^-1. The number of beats heard by him is

• 4

• 6

• 8

• 3

The equation of a transverse wave travelling along positive X-axis with amplitude 0.2 m, velocity 360 m/sec and wavelength 60 m can be written as

• $\mathrm{y} = 0.2 \sin \mathrm{\pi } \left[6\mathrm{t} + \frac{\mathrm{x}}{60}\right]$

• $\mathrm{y} = 0.2 \sin \mathrm{\pi } \left[6\mathrm{t} - \frac{\mathrm{x}}{60}\right]$

• $\mathrm{y} = 0.2 \sin 2\mathrm{\pi } \left[6\mathrm{t} - \frac{\mathrm{x}}{60}\right]$

• $\mathrm{y} = 0.2 \sin 2\mathrm{\pi } \left[6\mathrm{t} + \frac{\mathrm{x}}{60}\right]$

If v_m is the velocity of sound in moist air, v_d is the velocity of sound in dry air, under identical conditions of pressure and temperature.

• v_m < v_d

• v_m > v_d

• v_mv_d = 1

• v_m = v_d

If T is the reverberation time of an auditorium ofvolume V, then

• T ∝ V²

• T ∝ V

• $\mathrm{T} \propto \frac{1}{\mathrm{V}}$

• $\mathrm{T} \propto \frac{1}{{\mathrm{V}}^2}$

Two wires are fixed in a sonometer. Their tensions are in the ratio 8 : 1. The lengths are in the ratio 36 :35. The diameters are in the ratio 4 : 1. Densities of the materials are in the ratio 1 :2. If the higher frequency in the setting is 360 Hz, the beat frequency when the two wires are sounded together, is

• 8

• 5

• 10

• 6

A sound source is moving towards stationary listener with $\frac{1}{10}$th of the speed of sound. The ratio of apparent to real frequency is

• ${\left(\frac{9}{10}\right)}^2$

• $\frac{10}{9}$

• $\frac{11}{10}$

• ${\left(\frac{11}{10}\right)}^2$

If v is the speed of sound in air, then the shortest length of the closed pipe which resonates to a frequency n, is

• $\frac{\mathrm{v}}{2\mathrm{n}}$

• $\frac{\mathrm{v}}{4\mathrm{n}}$

• $\frac{4\mathrm{n}}{\mathrm{v}}$

• $\frac{2\mathrm{n}}{\mathrm{v}}$

Cavitation is a special application property exhibited only by

• ultrasonics

• electromagnetic waves

• audible sound

• infrasonics

Waves that cannot be polarised are

• electromagnetic waves

• light waves

• longitudinal waves

• transverse waves

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