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1 - frequency
2 - noise
3 - hertz
4 - infrasonic

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In hide and seek game, the blind folded person guesses the person closest to him by recognising the direction of loud sounds.

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I would suggest my parents to buy the house which is three lanes away from the roadside, because there we will be less noise pollution. The house in front of the road may have noises due to vehicles and hustle and bustle; and noise pollution can cause severe physiological and psychological problems.

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(c) vacuum

Sound needs a material medium to propagate. It cannot travel through vacuum.

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(c) boy's voice
A boy's voice has the highest frequency because of his thin and small vocal chords, which can vibrate with a greater frequency.

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(b) unbearable and shrill

Frequency of sound above 80 dB is unbearable and shrill.

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(c) flute

A flute is a wind instrument, while the others are stringed instruments.​

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(d) dholak

Dholak is not a wind instrument.

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1.The unit of frequency is hertz.
2. Frequencies less then 20 Hz are called infrasonic.
3. Only vibrating bodies produce sound.
4. The unit of loudness of sound is decibel.

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 Column A Column B 1. Unit of frequency (c) Hertz 2. Instrument producing sound of single frequency. (e) Turning fork 3. Maximum displacement of an oscillating object. (b) Amplitude 4. Sound with frequency more then 20000 Hz (d) Ultrasonics 5. Low frequency sound which we cannot hear. (a) Infrasonics

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1. True.
2. True.
3. False. Humans can hear sound frequencies between 20 Hz to 20000 Hz.
4. False. Material medium is necessary for the propagation of sound.
5. False. The louder the sound, the larger will be the amplitude of the vibrating body.

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Plants help in reducing noise pollution by absorbing the sounds from the surroundings.

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The vibrations from the surroundings are collected by the ear pinna, which send these vibrations to ear drum. The ear drum vibrates as it receives these collected sound and transmits these vibrations to the internal ear. From the internal ear, these vibrations are transmitted to the brain in the form of electro-chemical signals. In the absence of ear drum, the vibrations of the sound will not reach the brain and we will not be able to hear.

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Sound requires a material medium for its propagation. We cannot hear any sound on the moon, because it does not have an atmosphere and sound cannot propagate through vacuum.

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The voices of men, women and children differ from each other due to the following factors:

(1) The pitch is one of the most essential factors to determine the voice of an individual. For example, men have lower-pitched voices, while women have higher-pitched voices.
(2) The second factor which causes the difference in voices is the length and thickness of vocal cords. Men have broader vocal chords as compared with women and children. In children, the vocal chords are the narrowest and they have very high pitched voices.

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Hertz (Hz) is standard international unit of frequency.

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A sound which is unpleasant to our ear is called noise.

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Galton's whistle is used to give instructions while training dogs.

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If a body makes 1 oscillation in a second, then the frequency of the body is said to be 1 Hertz. Hertz (Hz) is the unit of frequency.

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Decibel (dB) is the unit of loudness.

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Vibration is the back and forth movement of a body about its mean position.

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Sound is a form of energy which is produced by vibrating objects.

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Ultrasonic sounds are those sounds that have frequencies more than 20,000 Hz.
Infrasonic sounds are those sounds that have frequencies less than 20 Hz.

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The following are the two uses of ultrasonic sound:
(a) They are used in industries to detect the cracks in metals.
(b) They are also used to measure the thickness of a material.

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Shrillness is the effect of sound on the ear. The difference in various musical notes is recognisable due to the difference in their pitches or shrillness.

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Two characteristics of sound are as follows:
(a) Pitch, which depends upon the frequency of vibration.
(b) Loudness, which depends upon the amplitude of vibration.

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Galton's whistle is an instrument which generates ultrasonic vibrations. It is used to train dogs.

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Air from the lungs makes the vocal cords vibrate.

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 Pitch Loudness 1. Pitch determines the shrillness of a sound. 1. Loudness of sound determines the degree of sensation of a sound. 2. It depends on the frequency of the vibrating body. 2. It depends on the amplitude of vibration.

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Frequency: Frequency of a vibrating body may be defined as the number of vibration made per second.
Time period: Time period of a vibrating body may be defined as the time taken by the body to complete one vibration.
Amplitude: Amplitude is the maximum distance travelled by a vibrating body from its mean position to either side of the extreme positions.

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Sound pollution is the disturbance produced in the environment by the excess of noise.

Two reasons of sound pollution are:

1. Industries: They use many heavy machines which are a major source of noise pollution.
2. Vehicles: In cities, large numbers of vehicles run at the same time, creating a lot of noises through horns and engines.

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The following are three ill effects created by noise pollution:

1. Chronic exposure to noise pollution can result in loss of hearing.
2. Noise pollution can cause anger, tension and may also affect the sleep pattern of a person.
3. Noise pollution reduces concentration and results in loss of work efficiency.

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(a) To prove that sound can move in water, take a bucket full of water and a squeaking toy in a polythene bag. Now, hold the toy inside the water and squeeze it. When we hold our ear against the side of the bucket, we can hear the squeak. This proves that sound can travel through liquid.

(b) To prove that sound can move in solid, take a wooden stick and hold one end of it near your ear. Now, ask any one to gently knock at the other end of the stick. We will see that we can hear the sound of knocking clearly. This proves that sound can travels through solid.

(c) To prove that sound cannot travel through vacuum, suspend an electric bell in a bell jar and connect it to a battery. As the bell rings, we can hear its sound. Now pump out the air from the jar through an outlet in the jar using a vacuum pump. On ringing the bell, we cannot hear anything once the air has been pumped out. This shows that sound cannot travel through vacuum.

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To measure the speed of sound in air, we will need two wooden blocks, a measuring tape, a scale and a friend to help with the experiment.
Find a large open area and choose two spots on the opposite sides of the area. Give the wooden blocks to your friend. Make yourself stand at one spot and your friend at the other. Measure the distance between the spots using the measuring tape. Signal your friend to hit the blocks against each other. Start your stopwatch as you see the blocks hit each other. Note the reading on the stopwatch the moment you hear the sound from the blocks.

We can calculate the speed using the formula speed=$\frac{\mathrm{distance}}{\mathrm{time}}$.

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Sound pollution is the disturbance produced in the environment by the excess of noise.

Two reasons of sound pollution are as follows:
1. Industries: They use many heavy machines which is a major source of noise pollution.
2. Vehicles: In cities, large number of vehicles run at the same time creating a lot of noises through engines and horns.

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 Infrasonic sounds Ultrasonic sounds 1. Infrasonic sounds are the sounds that have frequencies less than 20 Hz. 1. Ultrasonic sounds are the sounds that have frequencies more than 20,000 Hz. 2. Whales and snakes uses infrasonic sound to communicate with each other. 2. Bats produces ultrasonic to communicate with each other. 3. They are utilised for monitoring earthquakes, charting rock and petroleum formations below the earth. 3. They are utilised for detecting cracks in metals, to find the depth of the sea, etc.