Wave motion & sound - 04

Reflection of sound: 

The bouncing back of sound when it strikes a hard surface is called the reflection of sound. 

Echo:

repeatation of sound caused by the reflection of sound waves is called echo 

Minimum Distance of hear an echo: 

The human ear can hear two sounds seperately only if they reach the ear after an interval of 1/10 th of a second.

Calculation of minimum Distance of hear an echo: 

We know that: 

Speed of sound in air = 340 m/s

Time taken = 1/10 s. ( persistence of hearing)

Distance travelled= ?

1) The distance travelled by sound from a source of sound to the sound reflecting surface , and  coming back to this source of sound should be 34 meters.

2) the distance from source of sound to reflecting surface = 34/2 = 17

3) We can conclude minimum distance from sound reflecting surface to hear echo is 17 meters

Relation between speed of sound , time of hearing  echo from reflecting body:

 d = distance between sourse of sound and reflecting body 

S = source of sound

O = observer

t = time 

distance travelled by sound = 2d

time taken = 1 sec

Reverberation:

The repeated multiple reflection of sound in any big enclosed space is called reverberation.

(Or)

The persistence of sound in a big hall due to repeated reflection from the walls ceiling and floor of the hall is called reverberation.

To reduce the reverberation, the walls nd ceiling of halls are covered with sound absorbing materials 

Ex: rough plaster, fiber board , loose woolen or cotton cloth 

Applications of echo:

SONAR (Sonar Navigation and Ranging)

1) The reflection of sound can be used for measuring the depth of sea.

2) SONAR is a device which is used to find the depth of sea or locate the under water objects like enemy's  submarine, shoals of fish. 

Musical sound:

1) musical sound is produced by periodic vibrations

2) it has regular wave forms

3) it produces  a pleasant effect on the ear.

4) musical sound is produced by musical instruments Like sitar, violin and drum

Noise: 

1) Noise produced by non periodic vibrations.

2) it has irregular wave forms

3) it produces unpleasant effect on years

4) Noise is produced by machines in factory, moving train by the traffic at a bussy crossing roads and horns by trucks

Difference between musical sound & noise:

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 Q.NO .(1) 

A boy stands  80 m. in front of a High wall and then blows a whistle. Then the time interval when he hears an echo is ( speed of sound = 340 m/s)

 Q.NO.(2)

A man stands at a distance of 255 m. in front of cliff. He fires a gun and heard an echo after 1.5 sec. What will be the velocity of sound 

 Q.NO.(3)

A boy stands at 166 m. in front of building and then He blows a whistle. It's interval when he hears an echo. ( Take speed of sound = 332m/s)

 Q.NO.(4)

A boy stands at 85 m. in front of high wall and then He blows a whistle. Then the time of   interval when he hears an echo is ______×1/10 sec ( speed of sound 340m/s)

 Q.NO.(5)

A boy stands at 66.5 m. in front of building and then blows a whistle. It's interval when he hears an echo. (  speed of sound = 332m/s)

Q.NO.(6)

An echo returned in 3 sec. Then the distance of the reflecting surface from the source is ( speed of sound = 342m/s)