8.2 Problems
355
(iii) When the train is moving away from you at 15 m/s>
[University of Aberystwyth, Wales 2007]
8.2.6 Shock Wave
8.77 (a) What is a shock wave?
(b) What is the Mach number when a plane travels with a speed twice the
speed of the sound?
(c) Calculate the angle of Mach cone in (b).
8.2.7 Reverberation
8.78 Calculate the reverberation time of a room, 10 m wide by 18 m long by 4 m
high. The ceiling is acoustic, the walls are plastered, the floor is made of concrete and there are 50 persons in the room. Sound absorption coefficients are
acoustic ceiling 0.60, plaster 0.03, concrete 0.02, the absorbing power per
person is 0.5.
8.2.8 Echo
8.79 A man standing in front of mountain at a certain distance beats a drum at
regular intervals. The drumming rate is gradually increased and he finds the
echo is not heard distinctly when the rate becomes 40/min. He then moves
nearer to the mountain by 90 m and finds that the echo is again not heard
when the drumming rate becomes 60/min. Calculate (a) the distance between
the mountain and the initial position of the man and the mountain and (b) the
velocity of sound.
[Indian Institute of Technology 1974]
8.80 A rifle shot is fired in a valley formed between two parallel mountains. The
echo from one mountain is heard 2 s after the first one.
(a) What is the width of the valley?
(b) Is it possible to hear the subsequent echoes from the two mountains simultaneously, at the same point? If so, after what time, given sound velocity
= 360 m/s.
[Indian Institute of Technology 1973]
8.2.9 Beat Frequency
8.81 Two whistles are sounded with frequencies of 548 and 552 cycles/s, respectively. A man directly in the line between them walks towards the lower
355
(iii) When the train is moving away from you at 15 m/s>
[University of Aberystwyth, Wales 2007]
8.2.6 Shock Wave
8.77 (a) What is a shock wave?
(b) What is the Mach number when a plane travels with a speed twice the
speed of the sound?
(c) Calculate the angle of Mach cone in (b).
8.2.7 Reverberation
8.78 Calculate the reverberation time of a room, 10 m wide by 18 m long by 4 m
high. The ceiling is acoustic, the walls are plastered, the floor is made of concrete and there are 50 persons in the room. Sound absorption coefficients are
acoustic ceiling 0.60, plaster 0.03, concrete 0.02, the absorbing power per
person is 0.5.
8.2.8 Echo
8.79 A man standing in front of mountain at a certain distance beats a drum at
regular intervals. The drumming rate is gradually increased and he finds the
echo is not heard distinctly when the rate becomes 40/min. He then moves
nearer to the mountain by 90 m and finds that the echo is again not heard
when the drumming rate becomes 60/min. Calculate (a) the distance between
the mountain and the initial position of the man and the mountain and (b) the
velocity of sound.
[Indian Institute of Technology 1974]
8.80 A rifle shot is fired in a valley formed between two parallel mountains. The
echo from one mountain is heard 2 s after the first one.
(a) What is the width of the valley?
(b) Is it possible to hear the subsequent echoes from the two mountains simultaneously, at the same point? If so, after what time, given sound velocity
= 360 m/s.
[Indian Institute of Technology 1973]
8.2.9 Beat Frequency
8.81 Two whistles are sounded with frequencies of 548 and 552 cycles/s, respectively. A man directly in the line between them walks towards the lower