Class8.L-8. Sound

EXERCISE

A.Choose the correct option for each of the following option.

1.Sound cannot travel through

a) solid

b) liquid

c) gas

d) vacuum (✔)

2.The audible range of the normal human ear is

a) above 20,000 Hz

c) 20 Hz to 20,000 Hz (✔)

b) below 20 Hz

d) 2,000 Hz to 20,000 Hz

3. Which of the following is the correct sequence of sound propagation through the human ear?

a) Eardrum,inner ear, brain(✔)

b) Inner ear, eardrum, brain

c) Brain, inner ear, eardrum

d) Brain, eardrum, inner ear

4. Loudness of a sound depends on its

a) amplitude (✔)

b) frequency

c) pitch

d) All of these

5. Pitch of a sound is directly proportional to

a) frequency (✔)

b) amplitude

c) time period

d) loudness.

B.State whether the following statements are True (T) or False (F).

1.A bomb exploding inside a vacuum room does not produce sound. (T)

2. Sound produced by ultrasound equipment creates hearing problems in humans. (F)

3. A loud sound produced inside a liquid medium can be heard in air medium. (T)

4. Vocal cords vibrate to help humans to produce sound. (T)

5. Lower is the frequency of vibration, higher is the pitch. (F)

C.Fill in the blanks using the correct words.

1.Loudness of sound depends on- amplitude of the sound waves.(frequency/amplitude)

2.Sounds of frequency above 20,000 Hz are called- ultrasonic

(infrasonic/ultrasonic)

3.Time taken by a particle to complete one oscillation is called- time period sounds.(time period/frequency)

4.The S.l. unit of- frequency  is hertz. (frequency/amplitude)

5.Infrasonic waves have a frequency of- less than 20 Hz. (less than 20 Hz/more than 20,000 Hz)

D.Define the following terms.

1.Vibration : The to-and-fro ,rapid movement of a particle about its fixed point in an object is called vibration.

2.Amplitude : The maximum displacement of a vibrating particle from its mean position to one of the extreme positions Is called amplitude.

3.Frequency : Number of vibrations completed by a particle in one second is called frequency.

4.Pitch : The shrillness of sound experienced by the listener is called the pitch of the sound .

5.Echo : When a sound reflects from a obstacle situating at a distance 17m from the source and if time required by sound  to travel this path is  1/10sec ,then it will produce two distinct  sound .This phenomenon is called echo.

(simple : Reflection of sound to the source at time interval 1/10 sec is called Echo)

6.Infrasonic Sound : The sound of frequency less than 20Hz is called Infrasonic sound.

7.Music : Sounds that give a pleasant  feeling are usually called music or musical sound.

8.Ultrasonic Sound : The sound of frequency more than 20000Hz is called ultrasonic sound .

E. Answer the following questions briefly.

1.Can sound waves produced by a body make another body vibrate?

ANS:Yes . Because sound is a form of energy . When sound is produce by a vibrating body , it transfer its energy to the molecules of  the medium . These are air molecule start vibrating and carry the energy to neighbouring  molecule and can vibrate another body.

2.How does pitch of a sound vary with frequency?

ANS: Pitch directly depends on frequency i.e. as the frequency of sound increases , its pitch also increases .

3.Which are the three types of musical instruments? Give one example for each.

ANS:The three types of musical instruments are :

a)String instrument . e.g- guitar, violin

b)wind instrument . e.g- flute, trumpet

c)Percussion instruments . e.g-drum, tabla

4.Mention any two harmful effects of noise pollution.

ANS:Two harmful effects of noise pollution are :

a) Long  exposure to noise can reduce your concentration and cause restless.

b)Exposure to high noise level may lead to sleep disturbances and discomforts.

5.What do you mean by quality of sound waves?

ANS: Quality or Timbre of sound is a characteristic of sound that help us to distinguish one sound from another of the same pitch and loudness.

F.Answer the following questions in detail.

1.Sound is a form of energy. Substantiate this statement with an

example.

ANS: Sound is a form of energy. For example:To play mouth organ we have to blow air in to it. To blow this air we need some energy. If we blow air with more energy, the sound produce will be louder. That means the energy used to blowing air is converted into sound. Hence we can say that sound is a form of energy. It is a sensation that can be felt by ears.

2.How do sound waves travel in air medium? Explain.

ANS: Sound travels in the form of waves. When sound wave propagate through a medium , the particles of the medium starts vibrating about its mean position. For example we can consider sound wave produce by tuning fork. As the fork vibrate, it compresses the nearby air molecules. The air molecule from this compression region push and transfer the energy to the neighbouring molecules. Hence , the neighbouring region also get compressed . At the same time the air in the previous compression expands i.e. rarefaction. In this manner sound propagate as alternating compression and rarefactions.

3.Suggest one experiment to show that sound requires a medium for propagation.

ANS:

Aim of the experiment : To show that sound requires a medium for propagation.

Materials required: Glass bottle, rubber cork with two holes, vacuum pump, wires, small buzzer and battery .

Procedure:

1. Connect the two wires to the terminals of the buzzer and insert the other ends of the wires through one hole of the rubber cork. Close the hole by applying some quick fix solution.

2. Put the cork into the neck of the bottle so that the buzzer is inside the bottle.

3. Connect the inlet of a vacuum pump to the other hole

of the rubber cork.

4. Now, connect the buzzer to the terminals of a battery.

5. Now, switch on the vacuum pump.

Observations: Sound produced by the buzzer is heard when there was air inside the bottle. When the air is removed using a vacuum pump, the sound produced by the buzzer is not heard.

Conclusion: Sound requires a medium for its propagation.

4.Describe an activity to show that sound travels through liquids.

ANS:

Aim: To show that sound propagates through liquids.

Materials required: Two metal rods, tub or bucket and water .

Procedure:

1.Take water in a bucket.

2.Hold the metal rods in two hands and immerse them in the water.

3.Strike the two metal rods inside the water.

Observations: The sound produced by hitting the metal rods inside the water is heard outside.

Conclusion: Sound propagates through liquids.

5.What is noise pollution? What are the necessary measures to be taken to reduce it?

ANS:It is a type of pollution that produces unwanted , unpleasant ,irritating and loud sound that harm the activity of human or animal life.

Some of the necessary steps to reduce sound pollution are :

a)Avoid very loud music that might hurt your as well as others' ears.

b)Planting more trees helps to reduce noise level. The plants and trees have good ability to absorb the sound.

c)Minimize the use of defective machines and engines that produce unwanted sounds.

d)Avoid unnecessary use of horns of the vehicles while travelling.

e)Wear ear plugs to protect the ears, if working in a noisy atmosphere.

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