Note on Reflection and Refraction of Sound

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When sound travels from one medium to another medium, it strikes the surface of the medium and bounces or return back in some other direction, this phenomenon of returning back of sound is called the reflection of sound.

Echo and reverberation are the products of reflection of sound.

Echo:

The repetition of sound, which is reflected from walls of large room or distant surface is called echo.

Conditions required for echo:

  • The distance between the source of sound and reflecting body should be more than 17m
  • The reflector should be hard and of large area.

Reverberation:

If the distance between the source of sound and reflecting body is less than 17m, original sound and reflected sound mix and the sound is prolonged. This process is known as reverberation. It is heard in an empty room or newly built building.

Noise and Music

A sound which produces an unpleasant and jarring effect on the ear is called noise. The sound which produces a pleasing sensation on the ear is called musical sound. Musical sound is characterized by a regular, continuous vibration while a noise does not have any of these characteristics.

Differences between Musical sound and Noise

Noise Musical sound
It produces an unpleasant effect on the ears. It produces a pleasant effect on the ears.
It has abrupt, harsh and discontinuous sound. It has regularity and rhythm.
Changes in amplitude may be sudden. Changes in amplitude are not sudden.

Sound pollution

Noise pollution or noise disturbance is the disturbing or excessive noise that may harm the activity or balance of human or animal life.

Effects of sound pollution
  • It increases blood pressure and sugar level in blood.
  • It may make people deaf and can cause ear problems.
  • Communication based on sound is disturbed in noisy places.
  • It makes people tired, nausea etc.
Remedy of Sound Pollution
  • Cinema halls and industries should be established far from residential areas.
  • Use of private vehicles should be reduced and public transport should be used.
  • Afforestation should be done in living and working areas.
  • Sound absorbing materials such as glass, wool, carpet, etc. should be used.
  • Unnecessary use of horn of vehicles should be avoided and silencer should be used in vehicles.

Refraction of sound

Refraction is the process of bending of waves when they pass from one medium to another where their speed is changed or different. Refraction of sound is not important as the refraction of light but the refraction of sound is an interesting phenomenon in sound. As we know that sound in heard clearer at night than in day- time. This is due to the refraction of the sound wave. During the day- time, the temperature of the air near the land is more than the temperature of the air above it. As we go up, the layer of air behave as a denser medium and hence the ray of sound diverges upwards from the source and as a result less sound is heard by the listener. This phenomenon is just opposite during the night. The layer of air near the land or ground behave as a denser medium and the upper layer as a rarer medium. Here, the ray diverging upwards from the source is refracted away from the normal and undergoes total internal reflection and travels downwards. As a result, sound is heard more distinct and clear by the listener.

  • The repetition of sound, which is reflected from walls of large room or distant surface is called echo.
  • If the distance between the source of sound and reflecting body is less than 17m, original sound and reflected sound mix and the sound is prolonged. This process is known as reverberation.
  • A sound which produces an unpleasant and jarring effect on the ear is called noise.
  • The sound which produces a pleasing sensation on the ear is called musical sound.
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Very Short Questions

The repetition of sound produced due to the reflection from walls of a large room, hill, mountain, etc. is called an echo.

The required conditions for echo are as follows:

  1. The distance between the sources and sound and reflecting surface should be more than 17m.
  2. The loudness of the sound should be sufficient enough so that it can be heard after reflection.

When the reflected sound mix with the original sound and the sound is prolonged, then this phenomenon is called reverberation.

The conditions for reverberation are as follows:

  1. The distance between the sources and sound and reflecting surface should be less than 17m.
  2. There should be number of reflecting surfaces close to one another.

Echo Reverberation
The reflected sound is repeated. The reflected sound is prolonged.
The distance between the sources and sound and reflecting surface should be more than 17m. The distance between the sources and sound and reflecting surface should be less than 17m.

In big auditorium or cinema hall, the reverberation is easily occurred and it creates the sound confusion. To avoid this, walls of auditorium and cinema hall are covered with sound absorbing materials.

For echo, the distance between the sources of sound and reflecting surface should be more than 17m. Hence, it is not heard in a small room.

In a furnished room, the furniture and other materials absorb sound a lot and in a newly constructed room, almost sound is reflected and reverberation takes place. So, reverberation is experienced more in newly constructed room than in a furnished room.

The total internal reflection of sound occurs at night but it is impossible at day time. So, sound is heard more distinct at night than at day time.

Musical sound Noise
1. It produces a pleasing effect on our ears. 1. It produces an unpleasing effect in our ears.
2. It has regular and rhythmic sound. 2. It has harsh, irregular and discontinuous sound.

The sound which is not pleasant to our ears is called noise. The pollution caused by noise is called sound pollution. The barking of dogs, sound from vehicles, etc. are the examples of sound pollution.

We can reduce it by following ways:

  1. Controlling the production of unnecessary sound from the sources
  2. Use of sound absorbing materials
  3. Using noisy sources away from residential area

Any four effects of sound pollution are as follows:

  1. It may make people deaf and can cause ear problems.
  2. It makes difficult to talk.
  3. It makes people tired and distressed.
  4. It makes people irritated and short tempered.

Solution:
Here,
The time to strike the bottom of sea and return back is considerably equal.
∴ Time = 3/2 = 1.5 s
Velocity of sound in water (v) = 1500m/s
Depth of sea (d) = ?
We have,
d = v x t
= 1500 x 1.5
= 2250 m
Hence, the depth of the sea is 2250m.

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  • Why do we hear a sound when we strike a bell with a hammer?

    Due to vibration in air molecules.
    Due to vibration in the bell.
    Due to vibration in the hammer.
    Due to vibration in our ear drum.
  • The difference between transverse wave and longitudinal wave is

    Difference in the direction of vibration of particles.
    Difference in intensity of vibration.
    Difference in energy required to propagate wave.
    Difference in the energy they transmit.
  • Which is not source of sound?

    Explosion in Space
    Vocal cord
    Bell
    A Speaker
  • What is name of especial microphones that pick up sounds in water?

    Waterphones
    Aquaphones
    Hydrophones
    Liquidphones
  • Speed of sound is greatest in

    Water
    Curd
    Wool
    Steel
  • When people shout at the top of their lungs which type of sound is produced?

    Acoustic sound.
    Ultra sound.
    Infra sound.
    High pitch sound.
  • Intensity of sound wave is define as ______.

    Rate at which sound waves carry energy away from a source through a unit area at right angle to the direction of travel of wave is called intensity of sound wave.


    Rate at which sound waves carry energy away from a source through a unit area at parallel angle to the direction of travel of wave is called intensity of sound wave.


    Rate at which sound waves carry particles towards a source through a unit area at right angle to the direction of travel of wave is called intensity of sound wave.


    Rate at which sound waves carry frequency away from a source through a unit area at right angle to the direction of travel of wave is called intensity of sound wave.


  • The factor to form echo or reverberation is dependent on

    Intensity between sound and listener.
    Amplitude between sound and listener..
    Pitch between sound and listener.
    Distance between sound and listener.
  • For persistence hearing what condition must be followed?

    Sound must have carry energy at rate of 10-12 watt/m2
    Sound must be repeated more than 100 times per sec.
    Sound must be repeated more than 10 times per sec.
    Sound must have frequency of 1KHz.
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