Goa Board Class 8 Solutions for Science – Sound

Goa Board Class 8 Solutions for Science – Sound (English Medium)

Exercise 168:

Question 1:
Choose the correct answer.
Sound can travel through
(a) gases only
(b) solids only
(c) liquids only
(d) solids, liquids and gases
Solution :
(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.

Question 2:
Voice of which of the following is likely to have minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman
Solution :
(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.

Question 3:
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)
(b) The number of oscillations per second of a vibrating object is called its time period. (T / F)
(c) If the amplitude of vibration is large, sound is feeble. (T / F)
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T / F)
(e) The lower the frequency of vibration, the higher is the pitch. (T / F)
(f) Unwanted or unpleasant sound is termed as music. (T / F)
Solution :
(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.

Question 4:
Fill in the blanks with suitable words.
(a) Time taken by an object to complete one oscillation is called _________.
(b) Loudness is determined by the ________ of vibration.
(c) The unit of frequency is _________.
(d) Unwanted sound is called _________.
(e) Shrillness of a sound is determined by the _________ of vibration.
Solution :
(a) time period
(b) amplitude
(c) hertz (Hz)
(d) noise
(e) frequency

Question 5:
A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Solution :
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

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

Question 6:
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?
Solution :
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

Question 7:
Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak  (b) Sitar  (c) Flute
Solution :
(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.

Question 8:
What is the difference between noise and music? Can music become noise sometimes?
Solution :
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:

1. Sound produced by horns of buses and trucks.
2. Sound of electrical generators.
3. Sound of a gun shot.
4. Sound produced by jackhammers.

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

Question 9:
List sources of noise pollution in your surroundings.
Solution :
Some sources of noise pollution are as follows:

1. Televisions and transistors running at high volumes.
2. Loudspeakers and crackers.
3. Horns of buses, cars and trucks.
4. Home appliances such as mixer, desert cooler, etc.

Question 10:
Explain in what way noise pollution is harmful to humans.
Solution :
Noise pollution can lead to a number of health-related problems. Some of them are as follows:

1. Hearing loss
2. Insomnia; inability to sleep
3. Hypertension
4. Severe headache
5. Stress

Question 11:
Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.
Solution :
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.

Question 12:
Sketch larynx and explain its function in your own words.
Solution :
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.

Question 13:
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 why?
Solution :
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.