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.

2.  Voice of which of the following is likely to have a minimum frequency?

 (a) Baby girl

 (b) Baby boy

 (c) A man

 (d) A woman 

Answer:  (c) A man

3. In the following statements, tick ‘T’ against true ones and ‘F’ against false ones.

 (a) Sound cannot travel in a 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, the 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 vibration frequency, the higher 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.

(b) False -  The number of oscillations per second of a vibrating object is called its frequency. 

(c)  False - If the amplitude of vibration is small, the sound is feeble. If the amplitude of vibration is large, the sound is loud. 

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

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

(f) False - Unwanted or unpleasant sound is termed noise. On the other hand, musical sound is pleasing to the ear.

(g) True - Noise pollution may cause partial hearing impairment

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. 

Answers:

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 or Hz.

(d) Unwanted sound is called Noise.

(e) The 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: The number of oscillations per second of a vibrating object is called its frequency.

We know that,

Frequency = total number of oscillation / total time taken

= 40/4

= 10 Hz

Now , Time period = 1/ Oscillating frequency

= 1/10

= 0.1s

Therefore, frequency = 10hz and time period = 0.1s

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: Time period is defined as the time to complete one oscillation. It is also the inverse of frequency.

We know that, 

Time period =  1/Oscillation frequency

Oscillation frequency = 500 Hz

Hence, Time period = 1/500 

= 0.002 s

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

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

Answer: (a) In the Dholak, the top covering of the instrument produces sound. The top covering is called the “stretched membrane” and is beaten gently according to the rhythm. 

(b) In the Sitar, the part which produces sound are the “strings”, which vibrate when played using fingers.  

(c) In the Flute, the part which produces sound is the “pipe”, which has holes and vibrates when air is blown through it.

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

Answer: Noise and music are different from each other. The difference between sound and music is that :                                                        

Music is a pleasant sound. For example, sounds from musical instruments like piano, guitar, tabla and soft sounds like singing. 

Whereas noise is a sound that is unpleasant to the ears. They are loud, rough and might irritate ears—for example,  loud sounds from vehicles, factories, machinery, electrical generators, construction sounds etc.

Yes, very loud and rough music can sometimes become noisy and unpleasant to the ears.

9. List sources of noise pollution in your surroundings.

Answer: Various types of sounds can be heard around us. Some sources of noise pollution in our surroundings are:

i) Cars, buses, bikes horns

ii) Electrical generators and motors

iii) Loudspeakers and high volume televisions

iv) Loud noise from mixer-grinders and washing machines

v) Noise from machines in factories

vi) Bursting of crackers

vii) Sounds from living beings like animals and birds

viii) Other electronic appliances such as drill machines etc.

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

Answer: Noise pollution could be potentially harmful to humans. The presence of excessive noise in the surroundings may cause many health-related problems. For example, lack of sleep, hypertension (high blood pressure), anxiety and many other health disorders may be triggered by noise pollution. In addition, a person exposed to loud sounds continuously could get temporary or even permanent hearing impairments.

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: I would suggest my parents buy the house three lanes away from the road. This is because this house would have less noise from the cars and vehicles on the road, less smoke and harmful particles from vehicle exhaust, and less noise from other sources commonly found on streets. In addition, since the home would be away from the road, the intensity of sound from honks of vehicles, traffic jams and people shouting etc., would decrease, creating minor irritation and other health-related issues caused by noise pollution.

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

Answer:

In humans, the sound is produced by the voice box or the larynx. Two vocal cords are stretched across the voice box or larynx to leave a narrow slit between them for the passage of air. When the lungs force air through the slit, the vocal cords vibrate, producing sound. When the vocal cords are tight and thin, the type or quality of voice is different from loose and thick.

13. Lightning and thunder take place in the sky simultaneously and at the same distance from us. Lightning is seen earlier, and thunder is heard later. Can you explain why?

Answer:  Lightning and thunder take place in the sky simultaneously and at the same distance from us. Still, lightning is seen earlier, and thunder is heard later. This is because the speed of light is more than the speed of sound. Due to this phenomenon, the light emitted by lightning reaches our eyes before the sound of thunder reaches our ears, and hence we see lightning before we hear thunder.