Pitch of Sounds

Sounds can be high like a whistle or low like a big drum. We call this the pitch of a sound. Pitch tells us how high or low a sound is.

Every sound starts with something vibrating. Vibrating means shaking back and forth very fast. When a guitar string is plucked, it shakes. When you tap a drum, the drum skin shakes. These shakes travel through the air to your ears.

The pitch depends on how fast the shaking happens. Fast shaking makes a high pitch. Slow shaking makes a low pitch. Scientists call the speed of shaking the frequency. A high frequency means many shakes each second, and that sounds high. A low frequency means fewer shakes each second, and that sounds low.

Short things, thin things, and tight things shake faster, so they make higher sounds. A short rubber band plucked tight makes a higher note than a long, loose one. That is why a tiny bird chirps high and a big dog barks low.

Have you ever wondered why a flute and a tuba sound so different? The secret is the air inside them.

When you blow into an instrument, the air inside starts to vibrate. The air bounces back and forth in a special pattern called a standing wave. The size of the instrument decides how the air can vibrate, and that decides the pitch you hear.

A short tube of air vibrates faster, so it makes a higher pitch. A long tube of air vibrates slower, so it makes a lower pitch. That is why a little flute plays high notes and a giant tuba plays low, rumbly notes.

You can hear this pattern with everyday things too. Blow across the top of a bottle. Then add some water and blow again. The water makes the air space shorter, so the pitch goes higher. Musicians use this same trick all the time. A trombone player slides the tube longer and shorter to change the pitch. Try it with bottles at home and see if you can play a tune.

You know the sound a siren makes. It rises up and falls down, over and over. That sound is made by a simple machine, and it shows exactly how pitch works.

The heart of a siren is a spinning wheel with holes in it. The wheel sits inside a cover that also has holes. Air gets pushed through the holes. When a hole in the wheel lines up with a hole in the cover, a puff of air shoots through. When the holes do not line up, the air stops.

Each puff of air is a tiny vibration. When the wheel spins slowly, only a few puffs come out each second, so you hear a low pitch. When the wheel spins faster, many more puffs come out each second, and the pitch climbs higher. Faster vibration, higher pitch. Slower vibration, lower pitch.

The siren just lets you hear the change happen right in front of you.