Bangs and booms

The idea

Rapid heating makes air or gases expand very rapidly, creating a sudden density disturbance that travels outward and is heard as a loud sound pulse; an aircraft flying faster than sound produces a sonic boom.

A bang with no vibrating string

A firecracker bursts with a loud bang. No string or drum is vibrating. Such sounds are made in a slightly different way.

What do you think produces the loud sound?

• Air or gases heated rapidly and expanding very quickly
• A tight string inside it vibrating
• A drum skin inside it vibrating

Two ways to make sound

Drag each sound to the way it is made.

A sudden push in the air

When air or gases are heated rapidly, they expand very rapidly in a short time. This makes a sudden disturbance in the air density that travels outward. When it reaches our ears, we hear a loud sound pulse. A supersonic aircraft flying faster than sound makes a similar but more complex disturbance.

When air or gases are heated rapidly, they expand very rapidly. This makes a sudden disturbance in the density that travels outward and is heard as a loud sound pulse. An aircraft flying faster than sound (a supersonic aircraft) makes a loud sound called a sonic boom.

Notes

Rapid heating makes air or gases expand very rapidly, a sudden density disturbance heard as a loud sound pulse. An aircraft faster than sound makes a sonic boom.

Check yourself

What produces the sudden loud sound of a firecracker or a clap of thunder?

What travels outward from the place where the air expands very rapidly?

When is a sonic boom produced?

Which of these makes sound by rapid expansion of heated gases rather than by a vibrating object?

• A string vibrating inside. These sounds are not made by a vibrating string. They are made by air or gases expanding very rapidly when heated.
• The slow cooling of the air. The sound comes from rapid heating, which makes air or gases expand very rapidly.
• Rapid expansion of air or gases that are heated rapidly — correct. Yes!
• A sudden disturbance in the density of the air — correct. Yes!
• A stream of hot air particles that reaches your ear. What travels outward is a disturbance in the air density. The particles do not travel with it.
• A steady hum of constant density. The disturbance is sudden, and we hear it as a loud sound pulse.
• When an aircraft flies slower than sound. A sonic boom is produced by an aircraft flying faster than the speed of sound.
• When an aircraft flies at a speed higher than the speed of sound — correct. Yes!
• When a string is plucked very hard. A sonic boom is made by a supersonic aircraft, not by a plucked string.
• A plucked string. A plucked string makes sound because it vibrates.
• A struck tuning fork. A tuning fork makes sound because its prongs vibrate.
• A firecracker exploding — correct. Yes!