# Physics

### Chapter : 5. Sound

#### Practical Applications of Multiple of Reflection of Sound

Stethoscope :Stethoscope is a medical diagnostic instrument based on multiple reflection of sound waves. This is used by doctors for listening to the sounds produced inside the body, particularly in the heart or lungs.

Megaphone : Megaphone is a horn-shaped tube. Megaphones are used for addressing a small group of people.
Speaking tube is a hollow tube– one end is the speaker's end, whereas the other one is the listener's end.
In these devices, the sound waves suffer reflection repeatedly and the energy of the waves remains confined to the tube. The sound waves are prevented from spreading out.

It is for this very reason that loudspeakers also have horn-shaped opening.
b>Ear Trumpet (or Hearing Aid) : Ear trumpet or hearing aid is used by the persons who are hard of hearing. The sound waves received by the wide end of the trumpet are reflected into a much narrower area, leading it to the ear. This increases amplitude of the vibrating air inside the ear and helps in improving hearing.
Sound Boards and Curved Ceiling and Walls in Large Halls : The arched ceiling and walls of large halls or auditorium often reflect the sound waves. These reflected sound waves interfere with the words of the speaker. This problem is solved by hanging curtains, putting up screens or by using sound boards. A sound board is often a concave rigid surface. The speaker is located at the focus of the sound board placed behind the speaker. The sound board the reflected sound waves parallel. This enable the sound to reach large distances.

The sound board prevents the spreading out of the sound waves in different directions.
Example : 10
A girl hears the echo of his own voice from a distance hill after 3 second. The speed of sound in air is 340 m/s. What is the distance of the hill from the girl ?
Solution. Let d be the distance of the hill from the girl. Total distance travelled by the sound in going and coming back = 2d
Now,

d = 510 m