Chapter 13 : Sound - Ncert Solutions for Class 8 Science CBSE

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Chapter 13 - Sound Excercise 168

Solution 1

(d) Sound can travel through solids, liquids, and gases.

Concept insight: As sound travels by vibrations of particles, so it needs a medium to travel.

Solution 2

(c) A man

The voice of an adult man is of lower pitch in comparison to the voices of a baby boy, a baby girl and a woman. Since frequency of a sound is directly proportional to its pitch, man's voice is likely to have minimum frequency due to its lowest pitch.

Concept insight: Frequency of sound is directly proportional to pitch.

Chapter 13 - Sound Excercise 169

Solution 1

(a) True

Concept insight: Sound cannot travel in vacuum because vacuum has no molecules which can vibrate and carry sound waves.

(b) False

Concept insight: The number of oscillations per second of a vibrating object is known as its frequency.

(c) False

Concept insight: Loudness of a sound is proportional to the amplitude of its vibrations. When the amplitude of vibrations of a sound is large, the sound is loud. The sound is feeble for small amplitude.

(d) True

Concept insight: Humans can hear a sound whose frequency falls in the range of 20 Hz to 20,000 Hz. The sound having frequency out of this range is inaudible to humans.

(e) False

Concept insight: The pitch of a sound is directly proportional to its frequency.

(f) False

Concept insight: Unwanted or unpleasant sound is known as noise. Sounds that is melodious and pleasing to ears is known as music.

(g) True

Concept insight: If one is subjected to loud unpleasant sound continuously for a long time, then it may cause temporary hearing impairment.

Solution 2

(a) time period

(b) amplitude

(c) hertz (Hz)

(d) noise

(e) frequency

Solution 3

The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency.

Number of oscillation = 40

Total time taken = 4 seconds

Time period = time taken in one oscillation

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Frequency of oscillations is defined as the number of oscillations of a vibrating body per second. It is given by

Solution 4

The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency.

Frequency of oscillations = 500 Hz

Solution 5

(a) Dholak is a musical instrument. Sound in dholak is produced by the vibrations of stretched membrane.

 (b) Sitar is a musical instrument. It consists of stretched strings. When a string is plucked, it sets into vibrations and produces sound.

(c) Flute is a hollow pipe. When air is pumped into it, the column of air inside the pipe is set into vibrations. As a result, a pleasant sound is produced.

Solution 6

The sound that is pleasing to the ears is called music. For example, the sound produced by violins, pianos, flutes, pungs, etc.

The sound that is unpleasing to the ears is called noise.

Some examples of noise are as follows:

(i) Sound produced by horns of buses and trucks.

(ii) Sound of electrical generators.

(iii) Sound of a gun shot.

(iv) Sound produced by jackhammers.

Yes. Music can become noise when played at high volumes.

Solution 7

Some sources of noise pollution are as follows:

(i) Televisions and transistors running at high volumes.

(ii) Loudspeakers and crackers.

(iii) Horns of buses, cars and trucks.

(iv) Home appliances such as mixer, desert cooler, etc.

Solution 8

Noise pollution can lead to a number of health-related problems. Some of them are as follows:

(i) Hearing loss

(ii) Insomnia; inability to sleep

(iii) Hypertension

(iv) Severe headache

(v) Stress

Solution 9

I would suggest my parents to buy the house which is three lanes away from the busy road. This is because being away from the busy road will reduce noise pollution caused by heavy traffic on the road.

Solution 10

Larynx is a part of the throat. It is responsible for production of sound. A sketch of a human larynx is shown in the following figure.

Larynx moves when we swallow something. Inside the larynx, there are two vocal cords. There is a small gap between them. This small gap allows air to pass through. When we speak, air is forced into this small gap by the lungs. This prompts vocal cords to vibrate. Since vibrating objects produce sound, sound is produced due to the vibration of vocal cords.

Concept insight: Vibration of vocal cords produces sound.

Solution 11

Speed of sound in air = 334 m/s

Speed of light in air = 300000 km/s

As the speed of light in air is more than speed of sound in air, so lightning appears first and sound is heard later.

Concept insight: Light travels faster than sound.


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