L2 - Sound Waves

objectives

When a loudspeaker cone vibrates, it moves forwards and backwards very fast. This squashes and stretches the air in front.
As a result, a series of compressions (’squashes’) and rarefactions ('stretches') travel out through the air. These are sound waves. When they reach your ears, they make your ear-drums vibrate and you hear a sound.

Sound waves are caused by vibrations Any vibrating object can be a source of sound waves. As well as loudspeaker cones, examples include vibrating guitar strings, the vibrating air inside a trumpet, and the vibrating prongs of a tuning fork.
Also, when hard objects (such as cymbals and steel drums) are struck, they vibrate and produce sound waves.
Sound waves are longitudinal waves The air oscillates backwards and forwards as the compressions and rarefactions pass through it. When a compression passes, the air pressure rises. When a rarefaction passes, the pressure falls. The distance from one compression to the next is the wavelength.
Sound waves need a material to travel through This material is called a medium. Without it, there is nothing to pass on any oscillations. Sound cannot travel through a vacuum (completely empty space).
Sound waves can travel through solids, liquids, and gases Most sound waves reaching your ear have travelled through air. But you can also hear when swimming underwater, and walls, windows, doors, and ceilings can all transmit (pass on) sound.
Sound waves can be reflected and refracted
Sounds waves can be diffracted You can hear someone through an open window even if you cannot see them. That is because sound waves are diffracted by everyday objects: they spread through gaps or bend round obstacles of similar size to their wavelength (typically from a few centimetres to a few' metres).

Sound cannot travel through a vacuum. When the air is removed from this jar, the bell goes quiet, even though the hammer is still striking the metal. (The rubber bands reduce the sound transmitted by the connecting wires.)

Sound waves can be displayed graphically using a microphone and an oscilloscope as on the right. When sound waves enter the microphone, they make a crystal or a metal plate inside it vibrate.
The vibrations are changed into electrical oscillations, and the oscilloscope uses these to make a spot oscillate up and down on the screen. It moves the spot steadily sideways at the same time, producing a wave shape called a waveform.
The waveform is really a graph showing how the air pressure at the microphone varies with time. It is not a picture of the sound waves themselves: sound waves are not transverse (up-and-down).

Hard surfaces reflect sounds and can cause echoes. In large rooms and halls, the soft materials in curtains, carpets, and padded furniture help reduce the problem by absorbing the energy in sound waves.

Looking like giant mushrooms, these acoustic diffusers hang from the ceiling of the Albert Hall in London. Made of fibreglass, their job is to scatter reflected sounds so that echoes don't spoil the music being performed below.

The bricks, wood, and steel used in buildings are all good transmitters of sound waves. To stop unwanted sounds getting in or passing from one room to the next, panels packed with foam or fibrewool can be used to cut down sound transmission.

If you live near an airport, double (or even triple) glazed windows are essential in situations like this. Glass is a good transmitter of sound waves, but glass sheets with an air layer sandwiched between let much less sound through.

6.03---Sound-Waves.docx

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