Sound Class 9 Worksheet Science Chapter 11

Q.1. Fill in the blanks

(i) Quality of sound depends on Waveform .
(ii) The speed of sound in a solid is greater than the speed of sound in air.
(iii) Sound is caused by Vibrations .
(iv) Pitch of sound depends on frequency .
(v) Sound cannot travel through Vacuum .

Q.2. If 25 sound waves are produced per second, what is the frequency in hertz?
(a) 25 hertz
(b) 50 hertz
(c) 75 hertz
(d) 1/25 hertz

Correct Answer is Option (a)

The frequency in hertz is equal to the number of waves produced per second.
Since 25 waves are being produced per second, so the frequency of the sound waves is 25 hertz (which is also written as 25 Hz).

Q.3. Which of the following vibrates when a musical note is produced by the cymbals in an orchestra?
(a) Stretched strings
(b) Stretched membranes
(c) Metal plates
(d) Air columns

Correct Answer is Option (c) 

Q.4. Inner Ear is called as
(a) Cochlea
(b) Pinna
(c) Hammer
(d) Anvil

Correct Answer is Option (a) 

Q.5. Frequency of ultrasonic sound wave is
(a) Greater than 20 HZ
(b) Greater than 20,000 HZ
(c) Greater than 2 HZ
(d) Greater than 2 MHZ

Correct Answer is Option (b) 

Q.6. Why are the ceilings of concert halls curved?

The ceilings of concert halls are curved because such architecture helps the sound to reach all the corners and places of concert hall.

Q.7. What happens when sound travels in air?

When sound wave travels through air then the pressure and density of air changes. As the disturbance propagates through a medium, then alternate regions of pressure variations are created.

The region where particle come closer to each other (high density) and pressure of air is high is called as compression. The region where particles far apart from each other (low density) and pressure of air is less is called as rarefaction compression and rarefactions always occur together.

Q.8. What is an echo? Name two areas of its application?

When sound waves strike a surface, they are reflected with the same velocity and the reflected sound wave are heard as echo. It is used in SONAR and detecting flaws in metal objects.

Q.9. What is SONAR? Write its working?

SONAR stands for sound Navigation and Ranging. It uses ultrasonic waves. It consists of a transmitter which produces and transmits ultrasonic waves. These waves travel though water and after striking the object on the sea bed gets reflected back and are sensed by detector. The waves are then converted to electrical signals by detector. The time taken by wave to reach detector is recorded. Now, distance of the object from the ship is calculated by wring formula

Q.10. How can ultrasound be used to detect the defect in metal block?

Ultrasound are those waves which have frequency greater than 20 KHZ. Now, metal blocks are subjected to ultrasound at one end and at the other end, detectors are placed. If the metal block does not contain any defect then ultrasound travel through and are detected by detectors. If the metal block has any defect, than from that region ultrasounds are not detected and gets reflected back indicating the presence of defect in the block.

Q.11. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

There is no air on moon hence there is no medium for sound propagation on moon. As a result, me and my friend will not be able to hear any sound produced by my friend.

Q.12. Sound requires a medium to travel? Justify experimentally.

Sound requires medium for propagation and it can be proved by following experiment:

• Take a bell jar and suspend an electric bell in it,
• The bell jar is connected to a vacuum pump. Till the air is in the bell jar, the sound of the electric bell is louder.
• Now, with the help of vacuum pump, pump out the air gradually
• Now as air is pumped out, the sound of the bell gets fainter and fainter.
• Now, when the bell jar is completely vacuumed no sound is heard.
• This shows that air is required for propagation of sound.

Q.13. How does the sound produced by a vibrating object in a medium reach your ear?

As we speak, the particles of air near our mouth are pushed forward so they get compressed. Then they compress the other particles of air. As the compression proceeds the particles of air near our mouth expand again and thus rarefaction occurs. This process is repeated further and as a result sound wave propagates in the form of compressions and rarefactions to the listener’s ear.

Q.14. Why are sound waves called as mechanical waves?

Mechanical waves are those which requires a medium for their propagation, since sound also requires medium for its Propagation, hence is a mechanical wave.

Q.15. Explain how sound is produced by your school bell.

When the peon strikes the school bell with a hammer, the particles of bell metal start vibrating and those vibrations produce sound.