Muffling sound
Have you had a busy day? Have you done lots of work? Perhaps you had calculations to do in maths or a piece of text to read for English with questions. Maybe you researched something for a history project or tried a scientific investigation. When you are performing work like this you need to be able to concentrate. If you can't, you may make mistakes and have to redo the work. It has been found that if people have to work in noisy places, they cannot develop their full power of concentration and often make mistakes. It is important, then, that where people work they can have quietness to help them concentrate.

When people are resting they need quietness then too. If people cannot get the rest they need they become easily upset or quickly lose their temper. If people have to live and work in very noisy surroundings, the loudness of the sounds can do permanent damage to hearing. Quietness then is also needed for good health.

Let us have a look at the things around us that cause loud sounds and what is done to protect us from them. Some of the loudest sounds are made by the jet engines of aircraft. The roar comes from the burning gases in the engines and little can be done to reduce the noise, because huge amounts of burning gas are needed to lift the aeroplane into the air. Sound waves lose their power as they move away from their source, so an ideal place for an airport would be on land far from towns where few people would be disturbed by the noise of the aircraft. However, this would increase the length of passenger journeys so airports are built much nearer to where people live. One way to reduce the noise level and help the people who live close by is not to allow aeroplanes to take off or land in the evenings. This gives everyone near the airport the chance of a peaceful night's sleep.

One of the noisiest machines that ever enters your neighbourhood is the road drill. It is brought in to dig up the tarmac so that new pipes and cables can be laid beneath the road. The road drill uses compressed air to raise and lower a tough metal chisel very rapidly. As the drill cuts its way into the road it makes an almost painful noise. If you look at the person operating the drill you will see they are wearing ear protectors. These prevent the full power in the sound waves from reaching the ears and damaging them.

When people discover that someone is using a road drill in their street, they close the windows. If the window is open the sound waves from the road drill can travel right into the home. When the window is shut, some of the sound is reflected away but some still passes through the glass so the road drill can still be heard. If the windows are double-glazed the road drill will sound quieter. This is because of the way the double-glazed window is made. It is formed from two sheets of glass. They do not press up close to each other but have a gap between them. When the sheets are fastened together some of the air is removed from the gap. A place where some or all of the air has been removed is called a vacuum. A sound can pass through air because the air particles swing, press on each other and carry the sound wave along. In a vacuum there are fewer air particles so they cannot carry sound waves as efficiently as in air. As a consequence less sound passes through a double-glazed window than a single-glazed window.

After the road drill, probably the noisiest machine in the neighbourhood is the car engine. In a car engine petrol vapour is exploded with air. This makes the gas mixture push down on pistons in the engine and they in turn give power to the wheels to make the car move. The waste gases pass out of the engine along a pipe called the exhaust pipe. A large vibration is created in the gases when they explode and this travels down the exhaust pipe as sound waves. At the end of the exhaust pipe is a metal box called a silencer. Inside the silencer the power of the sound wave is reduced. There are several ways of reducing sound waves in a silencer. In many of them there are materials which absorb the energy in the sound waves and so make the car much quieter. If there is a hole in the silencer it will make a loud roaring sound because the energy of the sound waves is not being absorbed.

Although you may not realise it, you are surrounded by materials which absorb sound. These materials have soft, rough surfaces with air spaces in them. When sound waves strike them the materials absorb some of their power so making your surroundings quieter. What are these materials? They are the carpets, curtains, cushions and chair coverings in the room. You even carry sound absorbing materials with you - they are your clothes.

How do people at airports cope with the noise?
Those people who have to go out onto the runways to guide in the aeroplanes, or transport luggage and supplies to the aeroplanes wear ear protectors.

How do ear protectors work?
Ear protectors are a pair of plastic cups, which fit over the ear. They are lined with a soft material, which presses against the skin around the ear when the protectors are in place. The energy in sound waves is absorbed by the materials so that only quieter sounds reach the ears.

Can people wear earplugs to reduce the level of sound reaching their ears?
Yes, in some factories the workers wear earplugs instead of ear protectors. In one kind of plastic earplug the plug is shaped to fit across the hole in the ear and prevent most of the sounds entering. Another kind of earplug is made from plastic foam. The holes in the foam trap the sound waves and as they bounce around they are absorbed by the plastic material. You should not wear earplugs unless you are advised to do so by a responsible adult.

Do soft clothes and soft furnishings really make a difference to the sounds in a room?
Yes, they do. Go into a kitchen or a bathroom where there are few soft surfaces to absorb sound and make a noise by either shouting or banging a metal object. Now make the sound again in a living room or bedroom where there are plenty of soft, sound-absorbing surfaces and notice the difference. When you are in a shopping mall you may be aware of the sounds reflected from the hard surfaces of the floor, walls and ceiling. If you then walk into a clothes shop you will sense that it is much quieter due to the surfaces of the large number of clothes absorbing the sound.

Is it possible to make a room very quiet by using sound absorbing materials?
Yes, it is. These rooms are called anechoic chambers. They have walls and the ceiling covered in wedges of soft material. Any sound that is produced strikes a wedge where some of it is absorbed and some of it is reflected. The reflected sound does not travel very far before it strikes another wedge where more sound is absorbed and reflected. In a short time all the energy in the sound wave is absorbed. An anechoic chamber is used to test loudspeakers. In the test, only the sounds produced by the loudspeaker need to be measured. Reflected sounds would affect the results so they are removed by the wedges of soft material.
If you watch a pop star in a recording studio you may notice that the walls seem to have stripes. They are in fact covered with a soft material and the stripes are really grooves in the material. The walls are covered in this material to absorb sound and prevent any small echoes. If the echoes were recorded they would spoil the sound of the singer's voice.