Sound needs something to travel through

The idea

Sound propagates through a medium, which may be a solid, a liquid or a gas, and cannot propagate in vacuum, a space with no matter.

A bell in a jar

An electric bell rings inside a glass bell jar. A vacuum pump slowly sucks the air out of the jar. You can see the bell ringing the whole time.

What do you think you will hear as the air is pumped out?

• The sound becomes fainter and fainter
• The sound stays as loud as before
• The sound becomes louder

Pump out the air

Use the slider to take the air out of the jar. Then let the air back in.

No medium, no sound

The bell kept ringing, yet the sound faded as the air went. The material that carries sound is called a medium. A space with no matter is a vacuum. Sound cannot travel through a vacuum.

Sound propagates through a medium, which may be a solid, a liquid or a gas. It cannot propagate in vacuum, a space with no matter.

Notes

Sound needs a medium (a solid, liquid or gas). It cannot propagate in vacuum, a space with no matter.

Check yourself

A bell rings in a jar and the air is slowly pumped out. What do you hear?

Why can astronauts on a spacewalk not directly hear each other speak?

What is a space with no matter called?

Spoons tapped together under water can be heard. What does this tell you?

• The sound gets louder. Removing the air takes away the medium, so the sound gets fainter.
• The sound gets fainter — correct. Yes!
• The sound does not change. As the air is sucked out, the sound becomes fainter, and in a near vacuum almost no sound can be heard.
• Sound cannot travel through gases. Sound can travel through gases such as air. In outer space there is almost no matter to carry it.
• Their voices are too soft in space. The reason is that sound cannot propagate in the near vacuum of outer space. It is not about how loud they speak.
• Outer space is a near vacuum, so sound cannot propagate — correct. Yes!
• A vacuum — correct. Yes!
• A medium. A medium is the material through which sound propagates. A space with no matter is a vacuum.
• A source. A source is the object that produces sound. A space with no matter is a vacuum.
• Sound cannot travel through a liquid. The sound reached you after travelling through water and air, so it does travel through a liquid.
• Sound can travel through a liquid — correct. Yes!
• Sound travels through a liquid but not through a gas. The sound also travelled through air to reach you, so it travels through a gas as well.