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Class nine science Sound Extra Questions-answers

Chapter 12: Sound Extra Questions-answers

Speed of Sound in Different Media: The flash of lightning due to collision of clouds is seen much before the thunder, although both occur simultaneously. This happens because the velocity of light is greater than the velocity of sound. The speed of sound depends on the properties of the medium through which it travels. The medium can vary in (I) elasticity (II) density (III) pressure and (IV) temperature. The speed of sound decreases as it moves from solid to a gaseous state. But in any medium, the speed of sound increases with increase in temperature.

Reflection of Sound: The bouncing back of sound when it strikes a hard surface is known as a reflection of sound.

Laws of reflection of Sound :

(i) The incident sound wave, the reflected sound wave and normal at the point of incidence, all lie in the same place.

(ii) The angle of incidence of sound is equal to the angle of reflection of Sound.

ECHO

An echo is the phenomenon of repetition of the sound of a source by reflection from an obstacle. The sensation of sound persists in our brain for about 0.1s. To hear a distinct echo, the time interval between the original sound and the reflected one must be at least 0.1s. The speed of sound in air is 344m/s. The distance travelled by the sound is

0.1s = Speed × time

= 344 × 0.1 = 34.4m. So, the echo will be heard if the minimum distance between the source of the sound and the obstacle = 34.4/2

m = 17.2 m.

To hear an echo, our distance from the reflecting surface should be 17.2m. This distance will change with the change in temperature.

REVERBERATION

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

To reduce reverberation, the roof and the walls of the auditorium are generally covered with sound-absorbent materials like compressed fibreboard, rough plaster or draperies. The seat materials are also selected on the basis of their sound-absorbing properties.

Uses of Multiple Reflection of Sound: Some Simple devices based on multiple reflections of sound are Megaphone, Stethoscope, Bulb horn, soundboards etc.

Megaphones: Megaphone is based on multiple reflections of sound. Megaphone is a horn-shaped tube designed to send amplified sound in a particular direction. Megaphone is used to address a small gathering.

The narrow end of the megaphone is used for speaking and the wide end delivers the amplified sound to the audience.

Stethoscope: It is a medical diagnostic instrument based on multiple reflections of sound waves. This is used by doctors for listening to the sounds produced inside the body, particularly in the heart or lungs. The sound of heartbeats (or lungs) reaches the doctor’s ears by the multiple reflections of sound waves through the stethoscope tube.

Bulb Horn: A bulb horn is a sound instrument which is used by drivers of trucks, autos, buses, etc for signalling. When the rubber bulb is pressed, the air is forced out through a slit. This produces some sound.

When this sound passes through the metallic tube, it suffers multiple reflections. The amplified sound then comes out from the mouth of the horn.

Soundboard: It is a concave board (curved board) placed behind the stage in big halls so that sound after reflecting from the soundboard, spreads evenly across the width of the hall. The speaker is made to sit/stand at the focal point of the soundboard. When the sound waves

(from the speaker) fall on the concave soundboard, they get reflected as a parallel beam of sound towards the audience.

This prevents the spreading of the sound waves in various directions. As a result, the sound gets distributed evenly in the hall even at the far – off the end of the hall.

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