Sound(12th Grade > Physics ) Questions and answers for exam Preparation

12th Grade > Physics

SOUND MCQs

Total Questions : 31 | Page 3 of 4 pages

Question 21. A man standing on a platform hears the sound of frequency 605 Hz coming from 550 Hz train whistle moving towards the platform. If the velocity of sound is 330 m/s. What is the speed of train?

30 m/s
35 m/s
40 m/s
45 m/s

Discuss Question

Answer: Option A. -> 30 m/s
:
A
Apparent Frequency is given by:

n^{'} = n (\frac{v + v_{0}}{v - v_{s}})

605 = 550 (\frac{330 + 0}{330 - v_{s}})

v_{s} = 30 m / s

Question 22. A source of sound S is moving with a velocity 50 m/s towards a stationary observer. He measures the frequency of the source as 1000 Hz. What will be the apparent frequency of the sound when it is moving away from the observer after crossing him? The velocity of the sound in the medium is 350 m/s.

750 Hz
857 Hz
1143 Hz
1333 Hz

Discuss Question

Answer: Option A. -> 750 Hz
:
A
When the source is coming to stationary observer,

n^{'} = (\frac{v}{v - v_{s}}) n

n = (\frac{1000 \times 300}{350}) H z

When the source is moving away from the stationary observer,

n "= (\frac{v}{v + v_{s}}) n = (\frac{350}{350 + 50}) (\frac{1000 \times 300}{350}) = 750 H z

Question 23. A band playing music at frequency f is moving towards a wall at a speed

v_{b}

. A motorist is following the band with a speed

v_{m}

. If v is the speed of sound, the expression for the beat frequency heard by the motorist is

v+vmv+vbf
v+vmv−vbf
2vb(v+vm)v2−v2bf
2vm(v+vb)v2−v2mf

Discuss Question

Answer: Option C. -> 2vb(v+vm)v2−v2bf
:
C
The motorist receives two sound waves: direct one and that reflected from the wall.

f^{'} = \frac{v + v_{m}}{v + v_{b}} f

For reflected sound waves:
Frequency of sound wave reflected from the wall is

f "= \frac{v}{v - v_{b}} \times f

A Band Playing Music At Frequency F Is Moving Towards A Wall...

Frequency of the reflected waves as received by the moving motorist is

f^{'} "= \frac{v + v_{b}}{v - v_{b}} \times f^{'} - \frac{v + v_{m}}{v + v_{b}} f

Therefore, the beat frequency is

f^{'} "= \frac{v + v_{m}}{v - v_{b}} \times f - \frac{v + v_{m}}{v + v_{b}} f

= \frac{2 v_{b} (v + v_{m})}{v^{2} - v_{b}^{2}} f

Question 24. Speed of sound wave is v. If a reflector moves towards a stationary source emitting waves of frequency f with speed u, the wavelength of reflected waves will be

v−uv+uf
v+uvf
v+uv−uf
v−uvf

Discuss Question

Answer: Option C. -> v+uv−uf
:
C
Apparent frequency for reflector (which will act here as an observer) would be

f_{1} = (\frac{v + u}{v}) f

Where f is the actual frequency of source. The reflector will now behave as a source. The apparent frequency will now become

f_{2} = (\frac{v}{v - u}) f_{1}

Substituting the value of

f_{1}

we get

f_{2} = (\frac{v + u}{v - u}) f

Question 25. The sound from a very high burst of fireworks takes 5 s to arrive at the observer. The burst occurs 1662 m above the observer and travels vertically through two stratifier layers of air, the top one of thickness

d_{1}

0^{\circ} C

and the bottom one of thickness

d_{2}

20^{\circ} C

. Then (assume velocity of sound at

0^{\circ} C

is 330 m/s)

d1=342m
d2=1320m
d1=1485m
d2=342m

Discuss Question

Answer: Option D. -> d2=342m
:
D
Time taken is given by

T = t_{1} + t_{2} = \frac{d_{1}}{v_{1}} + \frac{d_{2}}{v_{2}} V_{1} = v_{0}^{\circ} c = 330 m / s v_{2} = (330 + .06 t) = 342 m / s d = 1662 m ∴ T = \frac{d_{1}}{330} + \frac{(d - d_{1})}{342} = 5 s \frac{d_{1} (342 - 330)}{330 \times 342} + \frac{d}{342} = 5 s 12 d_{1} = 5 (342 \times 330) - 330 \times 1662 d_{1} = 1320 m d_{2} = 342 m

Question 26. The frequency changes by 10% as the source approaches a stationary observer with constant speed Vs. What should be the percentage change in frequency as the source recedes from the observer with the same speed? Given that Vs ≪ v(v is the speed of sound in air).

14.3%
20%
16.7%
10%

Discuss Question

Answer: Option D. -> 10%
:
D
When the source approaches the observer,

f_{1} = f (\frac{v}{v - v_{s}}) = f {(1 - \frac{v_{s}}{v})}^{- 1} \approx f (1 + \frac{v_{s}}{v}) o r (\frac{f_{1} - f}{f}) \times 100 = \frac{v_{s}}{v} \times 100 = 10......... (1)

In the second case, when the source recedes from the observer

f_{2} = f (\frac{v}{v + v_{s}}) = f {(1 + \frac{v_{s}}{v})}^{- 1} = f (1 - \frac{v_{s}}{v}) ∴ (\frac{f_{2} - f}{f}) \times 100 = - \frac{v_{s}}{v} \times 100 = - 10

[from Eq.(i)]
In the first case, observed frequency increases by

10 %

while in the second case, observed frequency decreases by

10 %

Question 27. For a sound wave travelling towards +x direction, sinusoidal longitudinal displacement

ξ

at a certain time is given as a function of x(Figure). If bulk modulus of air is

B = 5 \times 10^{5} N / m^{2}

, the variation of pressure excess will be

For A Sound Wave Travelling Towards +x Direction, Sinusoidal...

Discuss Question

Answer: Option D. -> 10%
:
D

ξ = A s i n (k x - ω t) P_{e} x = - B \frac{d ξ}{d x} = - B A k c o s (k x - ω t) A m p l i t u d e o f P_{e x} i s B A k = (5 \times 10^{5}) (10^{- 4}) (\frac{2 π}{0.2}) = 5 π \times 10^{2} P a

Question 28. In the figure shown, a source of sound of frequency 510 Hz moves with constant velocity = 20 m/s in the direction shown. The wind is blowing at a constant velocity = 20 m/s towards an observer who is at rest at point B. Corresponding to the sound emitted by the source at initial position A, the frequency detected by the observer is equal to (speed of sound relative to air is 330 m/s)

510 Hz
500 Hz
525 Hz
550 Hz

Discuss Question

Answer: Option C. -> 525 Hz
:
C
Apparent frequency is given by

n^{'} = n \frac{(u + v_{w})}{(u + v_{w} - v_{s} c o s 60^{\circ})}

= \frac{510 (330 + 20)}{330 + 20 - 20 c o s 60^{\circ}}

= 510 \times \frac{350}{340} = 525 H z

Question 29. A wall is moving with velocity u and a source of sound moves with velocity u/2 in the same direction as shown in the figure. Assuming that the sound travels with velocity 10u, the ratio of incident sound wavelength on the wall to the reflected sound wavelength by the wall is equal to

9:11
11:9
4:5
5:4

Discuss Question

Answer: Option A. -> 9:11
:
A
Wavelength of the incident sound is

λ_{l} = \frac{10 u - \frac{u}{2}}{f} = \frac{19 u}{2 f}

Frequency of the incident sound is

F_{i} = \frac{10 u - u}{10 u - \frac{u}{2}} f = \frac{18}{19} f = f_{r}

When f_r is the frequency of the reflected sound. Wavelength of the reflected sound is

λ_{r} = \frac{10 u + u}{f_{r}} = \frac{11 u}{18 f} \times 19 = \frac{11 \times 19}{18} \frac{u}{f}

∴ \frac{λ_{i}}{λ_{r}} = \frac{19 u}{2 f} \times \frac{18 f}{11 \times 19 u} = \frac{9}{11}

Question 30. A train is moving in an elliptical orbit in anticlockwise sense with a speed of 110 m/s. Guard is also moving in the given direction with same speed as that of train. The ratio of the length of major and minor axes is