�鶹������ҳ���

• A sound wave is a vibration that travels through a solid, liquid or gas such as the air or water.

• A loud sound has a large amplitude, a high pitched sound has a high frequency.

• Musicians and scientists record and analyse sounds using wave traces.

When something shakes, scientists call it a vibration. All sounds are made by something that is shaking or vibrating.

Sometimes you can see the source of the sound vibrating, like a guitar string, drum or a loudspeaker, but other times it’s notso obvious, like the floor vibrating when you drop something on to it.

Watch this video to learn about what happens to sound waves when you record a song.

You don’t hear a sound as soon as it is made. It takes time for it to reach you.

Click on the slide show below to see how a sound travels through the air to reach your ear.

When there is a sound wave, the air particles don’t travel directly from the object making the sound to your ear. Sound waves are vibrations being passed on between particles. In the example above, the vibrating rubber band pushes on the air particles next to it.

The air particles start vibrating and push on the air particles next to them, so the vibrations arepassed on. Look at one particle (for example, the dark blue one), although the particle moves back and forth, it doesn't actually go anywhere, it just passes the movement on.

The particle moves one way and then moves back in the opposite direction, so ends up back where it started.The particles vibrate in the same direction as the wave travels. Sound is an example of a longitudinal wave.

If you pluck a stretched rubber band hard, it makes a loud sound. If you pluck it gently, it makes a quiet sound.

The loudness of a sound depends on how big the vibration of the air is.

• If air particles move back and forth a lot we hear a loud sound.
• If the source of a sound makes the particles vibrate more gently, we hear a quieter sound.

The more stretched a rubber band is, the quicker it vibrates and the higher the pitch of the sound.

The pitch of a sound depends on how quickly the air vibrates.

• When air particles move back and forth quickly we hear a high pitch sound.
• When the air vibrates less quickly we hear a low pitch sound.

The number of vibrations each second is called the frequency and is measured in a unit called hertz (Hz). The lowest pitch sound that most humans can hear is 20 Hz. The highest pitch sound the average human can hear is 20,000 Hz. The pitch you can hear varies with your age.

To record or analyse a sound, scientists and musicians use a microphone to turn the sound into an electrical signal. The electrical signal can then be displayed on a device called an oscilloscope and it produces a graph called a wave trace.

Wave traces appear on an oscilloscope graph as a transverse wave, but it is important to remember that because they are a sound, they are actually a longitudinal wave.

Wave traces are graphs that show how big the vibration of the air particles is on the vertical axis against the time on the horizontal axis.

A loud sound makes the air move back and forth a lot and so the peaks on the graph are big. A quiet sound makes the air vibrate back and forth less, so the peaks are smaller.

A high pitch sound has a high frequency and makes the air vibrate quickly , so the wave peaks are closer together.

A low pitch sound makes the air particles vibrate more slowly and so the peaks are further apart.

On a wave trace:

• If there is no sound, the wave trace is a flat line.
• How high the peak of the graph is compared to the flat line is called the amplitude.
• The higher the pitch the more peaks there are (the frequency of waves).