What makes a sound?

The idea

Sound is produced by vibrations, the periodic to and fro motion (oscillations) of an object such as a string, membrane or air column; the object that produces sound is its source.

A hum that stops

Meena stretches a rubber band between her fingers and plucks it. It hums. When she touches the band with a finger, the hum stops.

Why do you think the hum stopped?

• The band stopped moving to and fro
• The band ran out of sound stored inside it
• The band became too hot to make sound

Vibrate or keep still

Choose a source. Then switch its vibration on and off.

To and fro, again and again

In each case something moves to and fro again and again. This is a vibration. A string, a membrane (a stretched sheet like a drum skin) and the air inside a pipe can all vibrate. The object that makes the sound is its source.

Sound is produced by vibrations, the periodic to and fro motion (oscillations) of an object such as a string, a membrane or an air column. The object that produces sound is called its source.

Notes

Sound is produced by vibrations, the to and fro motion of a source such as a string, membrane or air column.

Check yourself

A rubber band is stretched and vibrating, and it makes a sound. Its vibration stops. What happens to the sound?

When you blow through a bansuri (flute), what vibrates to produce the sound?

What is the object that produces a sound called?

Which of these is a way of making a vibration?

• The sound goes on, because the band is still stretched. As long as the band is stretched and vibrating, sound is produced. Once the vibration stops, so does the sound.
• The sound stops too — correct. Yes!
• The sound gets louder. When the vibration stops, the sound stops. It does not get louder.
• The player's lips, not the instrument. In a bansuri the sound is produced by the vibration of the air inside the hollow pipe.
• The flat stand the bansuri rests on. The sound of a bansuri comes from the vibrating air column inside the pipe, not from a stand.
• The air inside the hollow pipe — correct. Yes!
• The source of the sound — correct. Yes!
• The medium of the sound. A medium is the material through which sound travels. The object that produces sound is its source.
• An oscillation. An oscillation is the to and fro motion itself, not the object that produces the sound.
• Keeping a stretched string still. A string that is not moving is not vibrating, so it does not produce sound.
• Pulling a stretched string and letting go — correct. Yes!
• Leaving a metal object untouched on a table. A metal object makes a sound when it is struck, because striking makes it vibrate. Left untouched, it is not vibrating.