NCERT Solutions: Sound

# NCERT Solutions for Class 8 Science Chapter 11 - Sound

Sound can travel through
(a) Gases only
(b) Solids only
(c) Liquids only
(d) Solids, Liquids, and Gases.
Ans: (d) Solids, Liquids, and Gases.
Sound speed in various medium

Q2. Which of the following voices is likely to have a minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman
Ans: (c) A man

Q3. In the following statements, tick ‘T’ against those which are true, and ‘F’ against those which are false.
(a) Sound cannot travel in vacuum. (T/F)
Ans: True

(b) The number of oscillations per second of a vibrating object is called its time period. (T/F)
Ans: False

(c) If the amplitude of vibration is large, sound is feeble. (T/F)
Ans: False

(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
Ans: True

(e) The lower the frequency of vibration, the higher is the pitch. (T/F)
Ans: False

(f) Unwanted or unpleasant sound is termed as music. (T/F)
Ans: False

(g) Noise pollution may cause partial hearing impairment. (T/F)
Ans: True

Q4. Fill in the blanks with suitable words.
(a) Time taken by an object to complete one oscillation is called _____________
Ans: Time period

(b) Loudness is determined by the _____________ of vibration.
Ans: Amplitude

(c) The unit of frequency is _____________
Ans: Hertz

(d) Unwanted sound is called _____________
Ans: Noise

(e) Shrillness of a sound is determined by the _____________ of vibration.
Ans: Frequency and Amplitude

Q5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Ans: Frequency is defined as the number of oscillations per second.

Since the time period is the time taken to complete one oscillation. It is the inverse of the frequency.

Q6. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?
Ans: Since the time period is the time taken to complete one oscillation.

Q7. Identify the part which vibrates to produce sound in the following instruments:
(a) Dholak
(b) Sitar
(c) FluteMusical InstrumentsAns: (a) In Dholak, there is a stretched membrane at the top known as the head. When the head is beaten, the stretched membrane vibrates and thus the sound is produced.
(b) In Sitar, there are many stretched strings. When these strings are plucked, the string vibrates and thus the sound is produced.
(c) In Flute, there is a hollow pipe. When air is blown into it, the air inside it starts vibrating, and thus the sound is produced.

Q8. What is the difference between noise and music? Can music become noise sometimes?
Ans:

• The unwanted sound that is unpleasant to the ear is called noise. A pleasant sound is called music.

• Music can become noise in many instances. When someone tries to enjoy very loud music, it can be noise for someone else. When loud music is played during religious celebrations or marriages, it can be annoying for many people.

Q9. List sources of noise pollution in your surroundings.
Ans: Following is the list of sources of noise pollution in my surroundings:

• Horns of vehicles.
• Bursting of crackers.
• Machines
• Loudspeakers
• Television and radio at high volumes.
• Kitchen appliances like mixer, etc.
• Air conditioners

Q10. Explain in what way noise pollution is harmful to humans.
Ans: There are many harmful effects of noise pollution on humans like:

• Lack of sleep.
• High blood pressure (Hypertension).
• Temporary or permanent impairment of hearing.
• Stress

Ans: I will suggest my parent buy a house which is away from the roadside because the house at the roadside would be noisy because of running vehicles. On the other hand, the house which is three lanes away from the roadside would be quieter.

Q12. Sketch the larynx and explain its function in your own words.
Ans:

• The larynx is the part of the throat that is responsible for producing sound.
• Inside the larynx, there are two vocal cords, which are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air.
• When we speak, the air is forced into this small gap by the lungs, which prompt vocal cords to vibrate.
• When the lungs force air through the slit, the vocal cords vibrate, producing sound.

Q13. Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain?
Ans: The speed of sound is 330 m/s while that of light is 3 x 108 m/s.
Thus, the speed of light is much higher than the speed of sound.
This is the reason light reaches us before sound during lightning.

The document NCERT Solutions for Class 8 Science Chapter 11 - Sound is a part of the Class 8 Course Science Class 8.
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## Science Class 8

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## FAQs on NCERT Solutions for Class 8 Science Chapter 11 - Sound

 1. What is sound and how is it produced?
Ans. Sound is a form of energy that produces a sensation of hearing in our ears. It is produced by the vibration of an object, which creates a disturbance in the surrounding medium, such as air. This disturbance travels through the medium in the form of a wave, which we perceive as sound.
 2. How does the frequency of sound affect its pitch?
Ans. The frequency of sound refers to the number of vibrations per second of the sound wave. The higher the frequency, the higher the pitch of the sound. For example, a sound wave with a frequency of 440 Hz is perceived as the musical note A, while a sound wave with a frequency of 220 Hz is perceived as the musical note A one octave lower.
 3. What is the difference between loudness and intensity of sound?
Ans. Loudness and intensity are both measures of the strength of a sound wave. Loudness refers to the subjective perception of the sound by the listener, while intensity refers to the physical measure of the sound wave's power per unit area. In other words, loudness is how loud a sound seems to the human ear, while intensity is the actual amount of energy in the sound wave.
 4. How do we measure the speed of sound in air?
Ans. The speed of sound in air can be measured using a variety of methods, but one common method is to use an echo. This involves measuring the time it takes for a sound wave to travel from its source to a reflective surface, and then back to the source. By dividing the distance traveled by the time taken, we can calculate the speed of sound in air.
 5. How do different materials affect the speed of sound?
Ans. The speed of sound varies depending on the material through which it is traveling. In general, sound travels faster through denser materials, such as solids, than through less dense materials, such as gases. For example, sound travels at a speed of approximately 330 m/s in air, but at a speed of 5000 m/s in steel. This is because the particles in denser materials are closer together, which allows sound waves to travel more quickly.

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