Chapter: Sound
Class: 7
Subjective Questions
1. What is sound? Mention the conditions for sound to be heard.
Sound is a form of energy produced by vibrating objects. Conditions required to hear sound are: • A vibrating source of sound • A material medium (solid, liquid or gas) • A receiver such as the human ear
2. Describe the principle on which SONAR works.
SONAR works on the principle of reflection of sound. It sends ultrasonic waves into water, which get reflected from underwater objects and return as echoes. The distance of the object is calculated using the time taken by the echo.
3. Mention the conditions required for an echo to be heard.
Conditions required for hearing an echo are: • The reflected sound should reach the ear at least 0.1 s after the original sound • The reflecting surface should be large and hard • Distance between source and reflector should be at least 17 m
4. What is a soundproof box? What are its applications?
A soundproof box is a structure designed to prevent sound from entering or leaving it. Applications: • Recording studios • Sound laboratories • Music rooms • Industrial noise control
5. Name the four categories of musical instruments.
The four categories of musical instruments are: • String instruments • Wind instruments • Membrane instruments • Solid instruments
Numericals
1. The interval between a flash of lightning and the accompanying thunder is 12 s. The height of the cloud is 3960 m. Find the speed of sound in air.
Speed = Distance ÷ Time Speed = 3960 ÷ 12 Speed = 330 m/s
2. A motorboat sends ultrasound towards the seabed and receives an echo after 4 s. Over a shipwreck, the echo is received after 3.75 s. Find the height of the shipwreck. (Speed of sound in water = 1450 m/s)
Distance to seabed = (1450 × 4) ÷ 2 = 2900 m Distance to shipwreck = (1450 × 3.75) ÷ 2 = 2718.75 m Height of shipwreck = 2900 − 2718.75 = 181.25 m
Exemplar Problems
1. A metallic ruler is struck by an astronaut on the surface of the Moon. a. Will the ruler vibrate? b. Will the astronaut hear the sound?
a. Yes, the ruler will vibrate. b. No, the astronaut will not hear the sound because sound needs a medium and there is no air on the Moon.
2. A girl reduced the volume of music she was listening to. Which property of sound changed and how?
The loudness of sound changed. Reducing volume decreases the amplitude of sound waves, making the sound softer.
Draw and Learn
Draw a well-labelled diagram to show the formation of compression and rarefaction by a vibrating tuning fork.
The diagram should show a vibrating tuning fork producing alternate regions of compression (high pressure) and rarefaction (low pressure) in air.
