(a) The propagation of sound does not occur through a vacuum.

(b) Noise pollution is a social issue.

(c) The sound which is disagreeable to the ears is called noise.

(d) Noise has adverse effects on our health/mind/ears.

If the silencer of a motorcycle is broken then it should be repaired immediately to reduce noise pollution. Driving such a motor cycle should be avoided till the silencer is repaired or replaced.

• If a factory in the surroundings is producing continuous loud noise, we should inform about the loud noise to the authorities of the factories.
• If there is any problem with machinery in the factories, it should be repaired by the owners of the factory.
• If they are not paying any heed, then we can lodge a complaint against the factory to a local government body citing the troubles faced by the residents around and the danger it is causing to the environment.

The to and fro motion of a body about its mean position is known as vibration. Vibrating objects have a particular speed.

Sound propagates with a maximum speed through the solid medium.

• We can hear the sound of a train which is far off by placing our ear to the railway track.
• If we knock at the door, the person inside the room can hear it clearly.
• Stethoscope is another example of sound travelling through solids. Doctors use stethoscopes to listen to our heartbeat.

The excess of unwanted sound, continuous unpleasant sound in the environment is a Noise pollution. It can adversely affect human and other living beings health. 

• Use of loudspeakers as well as horns of buses and trucks should be minimized.

• The entertainment of our homes should not be trouble for others. Therefore, the volume of TV or radio should be kept at minimum. It should be restricted only to those who are watching or listening to the programmes.
• Engines of unmaintained vehicles produce lot of unwanted and harsh sound.

• Therefore, regular maintenance of automobiles should be done so that noise produced by them can be kept under check.

• Factories, airports, railway stations and bus stands, etc. can create lot of noise. Therefore, they should not be located in the residential areas. But they should be away from localities where people stay.