Chapter 13: SOUND

Extra Internal Question and Answer:

Question: What is sound and how is it produced?

Answer: Sound is a form of energy that produces the sensation of hearing in our ears. It is produced when an object vibrates. These vibrations travel through a medium like air and reach our ears, where they are heard as sound.
Question: What are the characteristics of sound?

Answer: Sound has three main characteristics: loudness, pitch and quality (timbre).

Loudness depends on the amplitude of vibration, pitch depends on the frequency, and quality helps us distinguish different sounds having the same loudness and pitch.
Question: What are vibrations?

Answer: Vibrations are rapid to and fro (back and forth) movements of a body about its mean position.
Question: Why can sound not travel in vacuum?

Answer: Sound cannot travel in vacuum because it needs a material medium like air, water or solid to travel. Sound is produced by vibrations, and these vibrations are carried from one particle to another. In vacuum, there are no particles to transmit these vibrations, so sound cannot be heard.
Question: A body makes 60 vibrations in 5 seconds. Find its frequency.

Answer: Given, Number of vibrations = 60 and Time = 5 seconds

We have,

Therefore, the frequency of the body is 12 Hz.

Question: What is frequency? What is the SI unit of frequency? How is frequency related to time period?

Answer: Frequency is the number of vibrations or oscillations produced by a vibrating body in one second. The SI unit of frequency is hertz (Hz).
Frequency and time period are reciprocals of each other.
i.e.,

Question: How does frequency affect the pitch of sound?

Answer: Frequency decides the pitch of sound. Higher frequency produces a high pitch sound, while lower frequency produces a low pitch sound. Thus, pitch increases with increase in frequency and decreases when frequency decreases.
Question: What is pitch of sound? On what factor does the pitch of a sound depend? 

Answer: Pitch is the characteristic of sound that helps us distinguish between a shrill (high) sound and a low sound.
The pitch of a sound depends on the frequency of vibration. Higher the frequency, higher is the pitch and lower the frequency, lower is the pitch.

Question: What is meant by loudness of sound? On what factor does the loudness of a sound depend?

Answer: Loudness is the property of sound by which a loud sound can be distinguished from a faint sound. The loudness of sound depends on the amplitude of vibration.

Question: What is audible sound? Give an example. What is the range of frequencies of audible sound?

Answer: Audible sound is the sound that can be heard by the human ear.

Two examples are: (i) sound of a bell  (ii) sound of a drum.

The range of frequencies of audible sound for humans is 20 Hz to 20,000 Hz.

Question: What is inaudible sound? Give an example. What is the range of frequencies of inaudible sound?

What is inaudible sound? Given two example .

Answer:  The sounds which cannot be heard by human ears are called inaudible sounds. Humans cannot detect these sounds because their frequency is either too low or too high.

Two examples: (i) Ultrasound used in medical sonography (ii) Sound produced by bats

The range of frequencies of inaudible sound is below 20 Hz and above 20,000 Hz.

Question: State the difference between audible and inaudible sounds.

Answer: The difference between audible and inaudible sounds is:

Audible Sound	Inaudible Sound
Sounds that can be heard by human ears	Sounds that cannot be heard by human ears
Frequency range is 20 Hz to 20,000 Hz	Frequency is below 20 Hz or above 20,000 Hz
Example: Talking, music	Example: Ultrasonic sound, infrasonic sound

Exercises

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

Answer: (d) solids, liquids and gases.

[ Sound needs a material medium to travel. Since particles exist in solids, liquids, and gases, sound waves can pass through all these states by vibration of particles. ]

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

Answer: (a) Baby girl

[ The voice of a baby girl has the highest pitch, which means the minimum frequency among the given options. ]

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

Answer: (a) Sound cannot travel in a vacuum. (T)

(b) The number of oscillations per second of a vibrating object is called its time period. (F)

(c) If the amplitude of vibration is large, sound is feeble. (F)

(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T)

(e) The lower the frequency of vibration, the higher is the pitch. (F)

(f) Unwanted or unpleasant sound is termed as music. (F)          

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

Answer: (a) Time taken by an object to complete one oscillation is called 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.
5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Answer: Given, number of oscillations = 40 and  time = 4 second
We have,

And

Therefore, the time period of the pendulum is 0.1 seconds and its frequency is 10 Hz.
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?

Answer: Given, Frequency (f) = 500 vibrations per second

We have,

So, the time period of the vibration of the mosquito's wings is 0.002 seconds.
7. Identify the part which vibrates to produce sound in the following instruments.
       (a) Dholak        (b) Sitar         (c) Flute

Answer: (a) Dholak : The stretched membrane (skin) vibrates to produce sound.

(b) Sitar: The stretched strings vibrate to produce sound.

(c) Flute: The air column inside the flute vibrates to produce sound.

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

Answer: The difference between noise and music are :

Noise	Music
It is an unpleasant sound.	It is a pleasant sound.
It is irregular and disordered.	It is regular and well organised.
It causes irritation or discomfort.	It gives pleasure to the ears.
Example: sound of traffic, machines.	Example: sound of guitar, flute

Yes, music can become noise sometimes when it is too loud or played in an improper way, making it uncomfortable to hear.
9. List sources of noise pollution in your surroundings.

Answer: Major causes of noise pollution are

(i) sounds of vehicles, air craft engines ..etc ,

(ii) explosions including bursting of crackers,

(iii) machines, loudspeakers etc.

(iv) Television and transistor radio at high volumes,

(v) Some kitchen appliances,     

(vi) desert coolers and air conditioners .
10. Explain in what way noise pollution is harmful to human.

Answer: Noise pollution is harmful to human health in many ways. It can cause lack of sleep, irritation and headache. Continuous exposure to loud noise may lead to hearing loss. It also increases stress and can affect concentration and working ability. In severe cases, it may even lead to high blood pressure and other health problems.   
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.

Answer: My suggestion is that my parents should buy the house located three lanes away from the roadside. This is because houses near the roadside are exposed to more noise pollution due to heavy traffic. The continuous sound of vehicles, horns, and dust can affect our health and cause disturbance. A house away from the road will be quieter, less polluted, and better for peaceful living and good health.

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

Answer: The larynx is also called the voice box. It is present at the upper end of the windpipe. In humans, sound is produced in the larynx. It has two vocal cords which are stretched across it, leaving a narrow slit for air to pass. When air passes through this slit, the vocal cords vibrate and produce sound.

           
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?

Answer: Lightning and thunder occur at the same time, but we see lightning first and hear thunder later. This is because light travels much faster than sound. So, the light from lightning reaches our eyes quickly, while the sound of thunder takes more time to reach our ears.