Which sound is called noise?

Rattling trucks, roaring jets, squeaking chalk, and creaking doors don’t make music. They make sounds that disturb us - sounds we don’t want to hear. These unwanted sounds are called noise.

Noise is hard to stop. Like other sounds, noise travels through air and through solid things - even through walls.

But some materials actually soak up noise. They absorb sound waves and keep them from travelling. Inside a building, rugs and curtains soak up sound. The soft threads and tiny air spaces in the material help trap the vibrations. Special ceiling tiles can trap sound vibrations, too. The tiles are full of tiny holes, like a sponge. When sound waves strike the tiles, they bounce around inside the holes until they get weaker and die away.

People who work with aeroplanes and other heavy machines wear special helmets and earmuffs to cover their ears while they are working. The sound-absorbing material shuts out most of the noise that could bother them or even hurt their ears.

There is a special branch of science that deals with the way sound affects people. This science is known as acoustics. Acoustics helps people design theatres so that music sounds good. It also helps them work out how to control harmful noise. Scientists even use acoustics to study how we make and understand sounds.

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How can we measure the intensity of sound?

Sounds can be high or low. But they can also be loud or soft. What makes a sound loud or soft?

Sounds are the vibrations, or back-and-forth movements, made by moving objects. Strong vibrations make strong sounds. Vibrations are strong when an object moves a lot.

You can make a sound by stretching a thick rubber band and then plucking it with your fingers. If you pluck the rubber band hard, the back-and-forth movements are bigger. Bigger movements make a louder sound. If you pluck the rubber band lightly, the back-and-forth movements are smaller. So the sound is softer.

Scientists measure the strength of a sound in units called decibels. A sound of zero decibels is the weakest sound a person with normal hearing can hear. A sound of 140 decibels hurts your ears.

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What are reflected sounds?

What happens when you shout a big “hello” near a mountain, or between tall buildings, or in a large, empty hall? Well, you may hear an echo, another “hello” just like the one you shouted.

Sound bounces off hard, smooth things the way a ball bounces off a wall. The echo of your “hello” is reflected sound - sound that bounces back to you.

Why don’t you always hear echoes? It depends on how far the sound goes before it bounces. In a small room, the sound you make travels only a short distance before it bounces. It comes back so fast that it seems like part of what you are saying.

But in a very large room, the sound travels a while before it bounces back. By the time the sound comes back, you have finished speaking. So you hear the sound a second time.

You can hear the sound you make again, and again, and again! For example, if you shout between two tall buildings, the sound bounces back and forth between the walls. When that happens, the sound is reflected back to you from more than one spot. You hear “hello . . . hello . . . hello . . .” from each reflected sound. As the reflections get weaker and weaker, the sound dies out and the echoes finally stop.

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Why does there a change in the whistle of train when it rushes by?

Have you ever noticed a change in the “whistle” a train makes as it rushes by? If you listen to a passing train, you’ll notice that the sound gets higher and then lower as the train goes past you.

Actually, the whistle makes the same sound all the time. The sound seems to change because the train is passing you.

The sound spreads out in all directions from the train. But because the train is moving, each sound wave starts a little ahead of the place where the last one started. This makes the sound waves ahead of the train bunch up so that more of the waves reach your ear every second. And the more waves that reach your ear in a second, the higher the sound.

But behind the train, the waves are spread apart. As the train speeds away, fewer waves reach your ear each second - so the sound gets lower.

Most things never catch up with the sound waves they make. But some jet planes do. Supersonic planes can fly faster than sound travels. When they fly this fast, they slam into the waves of air they have made. This creates a tremendous air wave called a shock wave. The shock wave spreads out behind the plane in a funnel shape. Travelling at the speed of sound, the shock wave crashes into the air and the ground. This makes a huge exploding noise called a sonic boom.

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Why are the sounds different?

Zzzeee goes the tiny mosquito as it zips past your ear. VROOOM growls a big tractor as it rumbles past you. The sound the mosquito makes is much higher than the sound of the tractor. When something vibrates, sound travels outwards from it in waves. Each vibration - each complete back-and-forth motion - makes a single sound wave. The faster something vibrates, the more sound waves it makes and the higher the pitch.

A mosquito makes high-pitched sounds because its wings vibrate very fast - about a thousand times a second!

A tractor makes low-pitched sounds because its heavy metal parts vibrate slowly. The slow vibrations make only a few sound waves every second - the low rumble that you hear.

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