CBSE Class 8 - Science - CH13 - Sound

Sound

loud music can seriously
damage your ears
image credits:openclipart

(NCERT Chapter Answers)

Q1. Choose the correct answer :
       Sound can travel through
        (a) gases only
        (b) solids only
        (c) liquids only
        (d) solids, liquids and gases.

Answer: (d) Sound can travel through solids, liquids, and gases as it requires a medium to travel through. Solid, liquid and gas provide the medium for sound to travel. Sound cannot travel in vacuum.



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

Answer: (c) A man

Frequency of a sound is directly proportional to its pitch. 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. The voice of a baby girl has highest pitch among this group.

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

Answer:
(a) True ✓: Sound needs a medium to travel through. Since vacuum do not provide any medium, sound cannot travel through it.

(b) False ✗: The number of oscillations per second of a vibrating object is known as its frequency.

(c) False ✗: When the amplitude of vibration of a sound is large, the sound is very loud. The sound is feeble when the amplitude is small. Loudness of a sound is proportional to the square of the amplitude of its vibration.

(d) True ✓: Human beings can hear sounds only within the range of 20- 20,000 Hz. We cannot hear the sound having frequency out of this range.

(e) False ✗: Pitch of sound increases when the frequency of vibration increases and vice-versa. The pitch of a sound is directly proportional to its frequency.

(f) False ✗: Unwanted or unpleasant sounds are termed noise. Sounds which are melodious and pleasing to ears are called music.

(g) True ✓: Unwanted or unpleasant sounds are called noise. If one is subjected to noise continuously for a long time, then one may suffer from partial hearing impairment.


Q4. 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.

Answer:

(a) Time taken by an object to complete one oscillation is called __time period__.
(b) Loudness is determined by the __amplitude__ of vibration.
(c) The unit of frequency is __hertz (Hz)__.
(d) Unwanted sound is called __noise__.
(e) Shrillness of a sound is determined by the __frequency__ of vibration.

Q5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Answer: Frequency is the number of oscillations per second of a vibrating body.
It is denoted as:
Frequency of oscillations is defined as the number of oscillations of a vibrating body in one second. It is given by
              Frequency = number of oscillations/total time taken
                                =  40/4 Hz
                                = 10 Hz
The time taken to complete one oscillation (to and fro) is known as time period.
Time period= 1/frequency of oscillation
   = 1/10 sec
   = 0.1 sec                                   (...Answer)

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?

Answer: The time taken to complete one oscillation (to and fro) is known as time period.
Time period= 1/frequency of oscillation
=1/500 sec
=0.002 sec

Q 7. Identify the part which vibrates to produce sound in the following instruments:

(a) Dholak (b) Sitar (c) Flute

Answer:

(a) Dholak: Dholak has stretched membranes which vibrate (to and fro) to produce sound.

(b) Sitar: A sitar is a musical instrument which has stretched strings. When these strings vibrate, they produce sound.

(c) Flute: A flute is a wind or reed musical instrument which uses air column to produce sound.

Q 8. What is the difference between noise and music? Can music become noise sometimes?

Answer: Music is a sound which produces a pleasing sensation while noise is an unwanted and unpleasant sound. Music is produced by chirping of birds, by experts of musical instruments etc. Noise is produced by horns of vehicles, by machines etc.
Yes, music can become noise sometimes. When we play it at very high volume it becomes a noise.


Q 9. List sources of noise pollution in your surroundings.

Answer: Some sources of noise pollution are:

• loud-speakers
• honking of horns
• sound of machines
• loud music
• loud volume of television
• sound of shooting of gun
• sound of fire-crackers

Q10. Explain in what way noise pollution is harmful to humans.

Answer: : Noise pollution is a cause of many health problems, such as:

• Lack of sleep
• High blood pressure
• Anxiety
• Complete or partial hearing loss(when exposed to loud sound continuously over a long period of time)
• Stress

Larynx
credits:wpclipart

Q11. 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.

Answer: I would suggest a house which is three lanes away from the roadside because the pollution level (both air and noise pollution) is somewhat less there. With increase in distance, the intensity of sound decreases. So, there will be less noise pollution.

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

Larynx Top View
credits:wpclipart

 Answer: In humans, the sound is produced by the voice box or the larynx. A hard bump, at the upper end of the windpipe, that seems to move when we swallow is known as the voice box. Two vocal cords, 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 the lungs force air through the slit, the vocal cords vibrate and produce 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?

Answer: Speed of sound is 330 m/s and speed of light is 300,000,000 m/s. Therefore light travels much faster than sound and is seen earlier.

Q14: What is the unit used to measure the loudness of sound?

Answer:  Decibel (db).

Q15: What is the audible sound range for human beings?

Answer: 20 Hz to 20,000 Hz.

Q16: What are ultrasonic sounds?

Answer: An sound wave having frequency higher than 20,000Hz us called ultrasonic sound.