Question 1
Question: What is meant by echo? What are the conditions necessary for its formation?
Answer: The sound heard after reflection from a rigid obstacle (a cliff, hill or wall) is called an echo.
Echo is heard if the sound reaches our ear 0.1 s after the original sound dies. To hear echo distinctively the distance between the reflecting surface and the listener should be approximately 17 m.
Question 2
Question: Why do echoes produced in an empty auditorium usually decrease when it is full of audience?
Answer: When the hall is empty there is no obstacles in between to reflect the sound other than the walls. When the hall is full of audiences the sound produced undergoes multiple reflection from the people and so it overlaps with the sound produced. Hence the listener is not able to distinguish between the original sound and the echo.
Question 3
Question: A boy fires a gun and hears the echo 2 seconds later. If he is 480 m away from a wall, calculate the velocity of sound in air.
Answer: v = 
= 
= 480 m s-1
Question 4
Question: A girl claps and hears the echo after reflection from cliff which is 660 m away. If the velocity of sound is 330 m s-1, calculate the time taken for hearing the echo.
Answer: v x t = 2 d
= 
= 4 s
Question 5
Question: A rifle shot is fired in a valley between two parallel walls. The echo from one wall is heard 3 s later and the echo from the other wall is heard 8.3 s later. If the velocity of sound at 0oC is 330 m s-1 and the temperature in the valley is 10oC. For every 1 oC. Calculate the width of the valley. For every 1 oC rise in temperature the velocity of sound increases by 0.61 m s-1.
Answer: Velocity of sound in valley at 10 oC = 330 + 10 0.61
= 330 + 6.1
= 336.1 m s-1
Distance of the nearest wall,
d = 
= 
= 504.15 m
Distance of the farthest wall,
d = 
= 
= 1008.3 m
The total distance = 504.15 + 1008.3
= 1512.45 m
Question 6
Question: Explain how is the principle of echo used by
(a) the bat during its flight at night.
(b) the dolphin to locate small fish as its prey.
Answer: (a) Bats during the flight send a series of twittering sound waves which strike against the object in their path of light and send back echoes to the bat's ear. Hearing the echoes the bats know how they must turn to avoid colliding with the objects at night.
(b) Dolphins are aquatic animals which send out ultrasonic sound to communicate with each other. They have a sound sensing system which enables them to find animals under water with great accuracy due to the echo of the ultrasonic sound produced by them.
Question 7
Question: Define the terms velocity and wavelength applied to sound wave and state the relation between velocity, frequency and wavelength.
Answer: The distance traveled by a wave in one second is its velocity.
The distance traveled by the wave when it completes one oscillation. It is the distance between two consecutive particles executing the motion of the wave in same phase.
v = f
where v is the velocity, 
is the wave length and f is the frequency.
Question 8
Question: What is the wavelength of sound waves produced in air by a vibrating tuning fork whose frequency is 256 Hz when the velocity of sound in air is
330 m s-1?
Answer: v = f
330 = 256 
= 
= 1.3 m
Question 9
Question: State the characteristics of a musical note on which its pitch, loudness and quality depend.
Answer: The pitch of a musical sound depends on the frequency of the wave. The higher the frequency higher the pitch.
The loudness of a musical note depends on its amplitude, distance of the observer from the source.
Quality or timbre of a musical note depends on the wave form produced by it.
Question 10
Question: Figure shows a snap shot of a wave form of frequency 50 Hz in a string. The number in the string represent distance in centimeter. For this wave motion, find:
(a) wavelength
(b) amplitude, and
(c) velocity
Answer: (a) 20 cm
(b) 4 cm
(c) v = f
= 50 20
= 1000 cm s-1
