# Important Questions for Class 9 Chapter 12 - Sound

Important questions based on NCERT syllabus for Chapter 12 - Sound:

Question-1: A person shouts near a wall and heard the echo after 10 sec. What is the distance of the wall from the person? (take speed of sound=346 m/s)

Solution: Speed of sound, v=346m/s
T=10sec
Distance travelled by the sound=v × t = 346 ×10 = 3460m
In 10s sound has to travel from person and wall and back. Hence, sound travels twice the distance
Distance between person and wall=3460/2=1730m.

Question-2: How are compressions and rarefactions produced in air near a source of sound?

Solution: Sound is a mechanical wave and needs a medium to travel. The vibrations created travel parallel to the direction of propagation of sound and thus create regions of high and low pressures.

When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression (C). This compression starts to move away from the vibrating object.

When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R).

Question-3: Explain the uses of the reflection of sound.

Solution: 1. Megaphones or loudspeakers, horns, musical instruments such as trumpets and shehanais, are all designed to send sound in a particular direction without spreading it in all directions.

1. Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs. In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflections of sound.

2. Generally the ceilings of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the hall.