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# Sound

## 05 Reflection of Sound

#### SONIC BOOM

Sonic boom is an explosive noise caused by the shock waves from an aircraft (or any other object) which is travelling faster than the speed of sound.

#### WHICH IS THE FASTEST? SOUND OR LIGHT

• The speed of light is greater than that of speed of sound.
• The speed of sound in air is about 344 m/s whereas the speed of light in air is 3 x 108 m/s.

#### IN RAINY SEASON, WHY DOES LIGHTNING ALWAYS COME BEFORE THUNDER?

It is due to the very high speed of light that we see the flash of lightning first and it is due to comparatively low speed of sound that the thunder is heard a little later.

#### REFLECTION OF SOUND

Reflection of the sound is the bouncing back of sound when it strikes a hard surface.
If we take two tubes and position them against a wall, as shown in the figure below. Upon placing a speaker or any other source of sound near one end of the tube, we receive the sound at the end of the other tube. This activity proves that the surface of wall reflects the sound wave.

#### LAWS OF REFLECTION OF SOUND

The laws of reflection of light are obeyed during the reflection of sound.

• The incident sound wave, the reflected sound wave, and the normal at the point of incidence, all lie in the same plane.
• The angle of reflection of sound is always equal to the angle of incidence of sound.

#### APPLICATION OF REFLECTION OF SOUND

The reflection of sound is utilised in the working of devices such as:

• Megaphone
• Bulb horn
• Stethoscope, and
• Soundboard.

#### MEGAPHONE AND BULB HORN

• A megaphone is a large, cone-shaped (or funnel-shaped) device.
• It is used for amplifying and directing the voice of a person who speaks into it.
• When a person speaks into the narrow end of the megaphone tube, the sound waves produced by his voice are prevented from spreading by successive reflections from the wider end of the megaphone tub. Due to this the sound of the voice of the person can be heard over a longer distance.

#### STETHOSCOPE

Stethoscope is a medical instrument used by the doctors for listening to the sounds produced within the human body, mainly in the heart and the lungs.

#### SOUNDBOARD

• The soundboard is a concave board (curved board) such that the sound source is at the focus.
• It is placed behind the speaker in large halls or auditoriums so that his speech can be heard easily even by the persons sitting at a considerable distance.
• Sound waves are made to reflect uniformly throughout the hall or an auditorium thus enhancing their quality.

#### The human ear

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