18. Sound: Production of Sound
Complete solutions to the Exercises. Maharashtra State Board (Balbharati), Grade Seven General Science.
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Exercises
(a) Sound is generated by the rhythmic vibration of any object.
(b) The frequency of sound is measured in Hertz (Hz).
(c) If amplitude of sound is decreased, its loudness also decreases.
(d) A medium is necessary for transmission (propagation) of sound.
| Group ‘A’ | Group ‘B’ |
|---|---|
| (a) Flute | (3) Vibrations in the air |
| (b) Frequency | (4) Measured in Hz |
| (c) Sound level | (5) Decibel |
| (d) Ultrasonic sound | (2) Frequency more than 20000 Hz |
| (e) Infrasonic sound | (1) Frequency less than 20 Hz |
(a) In earlier times, people used to listen for the arrival of a distant train by putting their ear to the rail.
A moving train produces vibrations in the rails. Sound travels faster through solids than through air. It also travels a longer distance through the rail. So people could hear the sound of the train early by putting their ear to the rail.
(b) The sounds generated by a tabla and a sitar are different.
In a tabla, a stretched skin vibrates. In a sitar, stretched strings vibrate. The vibrating parts are different. They vibrate in different ways. So the sounds of a tabla and a sitar are different.
(c) If you were both on the moon, your friend will not be able to hear you call.
Sound needs a medium to travel. There is no air on the moon. So sound cannot travel there. Hence your friend cannot hear you.
(d) We can hear the movement of a mosquito’s wings but we cannot hear the movement of our hands.
We can hear sounds with a frequency between 20 Hz and 20,000 Hz. A mosquito moves its wings very fast. So its sound is in our hearing range. Our hands move very slowly. The frequency is less than 20 Hz. So we cannot hear it.
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(a) How is sound produced?
Sound is produced by the vibration of an object. When an object vibrates, it moves to and fro again and again. These vibrations travel through the air and reach our ears. Then we hear the sound. When the vibration stops, the sound also stops. Examples: strings of a sitar, skin of a tabla, our vocal cords.
(b) What does the intensity of sound depend upon?
The intensity of sound depends on the amplitude of vibration. Intensity is proportional to the square of the amplitude. If the amplitude is large, the sound is loud. If the amplitude is small, the sound is soft. The sound level is measured in decibels (dB).
(c) Explain how the frequency of oscillation is related to the length of a pendulum and the amplitude of its oscillation.
Length: When the length of a pendulum increases, its frequency decreases. When the length decreases, its frequency increases. So a long pendulum has a low frequency. A short pendulum has a high frequency.
Amplitude: The frequency of a pendulum does not depend on its amplitude. It remains the same even if the amplitude changes.
(d) Explain the two ways by which the pitch of the sound generated by a stretched string can be changed.
- By changing the tension: If the string is tightened, the pitch increases. If it is loosened, the pitch decreases.
- By changing the length: If the vibrating length is shortened, the pitch increases. If it is lengthened, the pitch decreases.
- A bat produces ultrasonic sounds while flying. We cannot hear these sounds.
- The sound waves strike objects in front of it and come back as echoes.
- The bat hears these echoes with its ears.
- From the echoes, it knows where the object is.
So the bat can fly in the dark without hitting anything. It can also catch insects. This method is called echolocation.