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.
The repetition of sound, which is reflected from walls of large room or distant surface is called echo.
Conditions required for 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. It is heard in an empty room or newly built building.
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
|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.|
Noise pollution or noise disturbance is the disturbing or excessive noise that may harm the activity or balance of human or animal life.
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 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.|
'Echo is not heard in a small room'. Why?
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.
|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.|
A ship receives a clear echo after 3 second when it transmits sound. Calculate the depth of the sea if the velocity of sound in water is 1500m/s.
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) = ?
d = v x t
= 1500 x 1.5
= 2250 m
Hence, the depth of the sea is 2250m.
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