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Physics for Kids

Basics of Sound


How does sound move or propagate?

The vibration is started by some mechanical movement, such as someone plucking a guitar string or knocking on a door. This causes a vibration on the molecules next to the mechanical event (i.e. where your hand hit the door when knocking). When these molecules vibrate, they in turn cause the molecules around them to vibrate. The vibration will spread from molecule to molecule causing the sound to travel.

Sound must travel through matter because it needs the vibration of molecules to propagate. Because outer space is a vacuum with no matter, it's very quiet. The matter that transports the sound is called the medium.

Speed of Sound

The speed of sound is how fast the wave or vibrations pass through the medium or matter. The type of matter has a large impact on the speed at which the sound will travel. For example, sound travels faster in water than air. Sound travels even faster in steel.

In dry air, sound travels at 343 meters per second (768 mph). At this rate sound will travel one mile in around five seconds. Sound travels 4 times faster in water (1,482 meters per second) and around 13 times faster through steel (4,512 meters per second).

What is the Sound Barrier?

When airplanes go faster than the speed of sound (also called Mach 1), it's called breaking the sound barrier. Most airplanes don't go this fast, but some fighter jets do. When they pass through the speed of sound, the airplane sheds water drops that have condensed on the plane creating a cool looking white halo (see the picture above).

When planes break the sound barrier they also create something called a sonic boom. This is a loud noise like an explosion that is generated from a number of sound waves that are forced together as the plane is now traveling faster than sound.


The volume of sound is the measure of loudness. To quantify volume we use decibels. The more decibels, the louder the sound is. A soft sound, like a whisper will measure around 15-20 decibels. A loud sound like a jet engine is more like 150 decibels. The threshold of pain occurs at around 130 decibels.

Loud sound can actually damage your ears and cause loss of hearing. Even sounds as loud as 85 decibels can ruin your ears if you listen to them over a long period of time. For this reason, it's a good idea to not listen to loud music or have your headphones turned up too loud.

For more on the Science of Sound: Sound 102

Sound Experiments
Sound Pitch - Learn how frequency effects sound and pitch.
Sound Waves - See how sound waves propagate.
Sound Vibrations- Learn about sound by making a kazoo.

Waves and Sound
Intro to Waves
Properties of Waves
Wave Behavior
Basics of Sound
Pitch and Acoustics
The Sound Wave
How Musical Notes Work
The Ear and Hearing
Glossary of Wave Terms
Light and Optics
Intro to Light
Light Spectrum
Light as a Wave
Electromagnetic Waves
The Eye and Seeing

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