Class 8 Science Chapter 11 Question Answers - Sound

Q1: A simple pendulum makes 20 oscillations in 40 seconds. What is the time period and frequency of its oscillation?
Ans: Time period = Time / No. of oscillations = 40/20 = 2 sec.
Frequency = No. of oscillations / Time = 20/40 = 0.5 Hz

Q2: What is frequency?
Ans: Frequency is the number of oscillations per second. It is measured in hertz (Hz).

Q3: What is noise pollution?
Ans: Noise pollution refers to the presence of unwanted and excessive sound in the environment, which can have negative effects on human health and well-being.

Q4: What are the harms of noise pollution?
Ans: Harms of noise pollution include lack of sleep, hypertension, anxiety, and temporary or even permanent impairment of hearing.

Q5: Sonali heard the sound of thunderbolt 5 seconds after she saw the flash of lightning. How far is she from the place where lightning occurs? (speed of sound = 330 m/s)
Ans: Distance of Sonali from the place of lightning = Speed of sound × Time taken
= 330 m/s × 5 s
= 1650 m

Q6: What do you mean by time period?
Ans: The time taken by a vibrating body to complete one oscillation is called the time period.

Q7: What do you mean by oscillatory motion?
Ans: Oscillatory motion refers to the to-and-fro motion of an object from its mean position. It involves regular movement in both directions.

Q8: What do you mean by audible sound?
Ans: Audible sound refers to sound waves that fall within the range of frequencies that the human ear can detect, typically between 20 Hz and 20,000 Hz.

Q9: What are infrasonic sounds?
Ans: Infrasonic sounds are sound waves with frequencies lower than the lower limit of human hearing, which is 20 Hz.

Q10: Two astronauts are floating close to each other in space. Can they talk to each other without using any special device? Give reasons.
Ans: No, they cannot talk to each other without using a special device. In space, there is no atmosphere to transmit sound waves, and sound requires a medium like air to travel. Therefore, they would need a communication device to exchange messages.

Q11: What are the methods to control noise pollution?
Ans: Following are the methods to control noise pollution:

• The noise pollution can be controlled by reducing the respective sources of noise pollution.
• The blowing of horns and speaker should be strictly avoided near schools and hospitals.
• More and more trees should be planted on roadside because trees absorb sound.

Q12: Define frequency.
Ans: The number of oscillations completed by a vibrating body in one second is called frequency. The unit of frequency is hertz. It is denoted by Hz. If a vibrating body makes 20 oscillations in a second we say that its frequency is 20 Hz. The human voice can produce frequency between 60 Hz and 13,000 Hz. It is interesting that a human ear can hear sound of frequency between 20 Hz to 20,000 Hz.

Q13: Differentiate between musical sound and noise.
Ans:
Musical sound

• Musical sound is pleasant, smooth and agreeable to the ear.
• It is produced by periodic vibrations which are regular and continuous.
• There are no sudden changes in loudness and pitch of musical sound.

Noise

• Noise is unpleasant, jarring and disagreeable to the ear.
• It is produced by irregular vibrations which are discontinuous.
• There are sudden changes in loudness and pitch of noise.

Q14: What is the property of vibration which determines the pitch of the sound?
Ans: The frequency is the property which determines the shrillness or pitch of a sound. If the frequency of a sound is higher then we say that the sound has lower pitch.

Q15: State the differences between ultrasonic sound and infrasonic sound?
Ans:
Ultrasonic sound

• Sound of frequency higher than 20,000 Hz is called ultrasonic sound.
• Animals like dog, bat, monkey, deer, etc., can hear ultrasonic sound.

Infrasonic sound

• Sound of frequency lower than 20 Hz is called the infrasonic sound.
• Animals like whale, elephant, hippopotamus, giraffe, etc., can hear infrasonic sound.