What is Sound?


Sound is the name for pressure waves that travel through air or water (or indeed any gas, liquid, solid or plasma). These waves aren't like the up and down waves you see on the surface of the sea: instead they travel along by pushing molecules to and fro, making them bump into the next ones in line and causing a chain reaction such that each bumped molecule bumps the next one beyond it, as you can see in the illustration above. Alternatively, if a the wave starts by molecules being pulled in the opposite direction, they leave gaps which other molecules try to fill, so you get a pulling wave rather than a pushing one, as you can see with every second wave. Look carefully at what's happening: sometimes the molecules all move to the right, while other times they all move to the left, but the actual waves move from left to right across the screen every time. Watch them for a while until you understand this.
It isn't actually as simple as the illustration shows though. You need to imagine gas molecules in the air and the ways that they actually collide with each other. Air molecules rarely hit each other straight on, so they can bounce off each other at all sorts of angles: there is actually just a general movement of molecules in one direction or the other, and this makes the pressure of the air go up or down for a moment as the wave passes through. If it's a pushing wave, then more molecules are forced into the same space. The higher pressure then pushes some of those molecules onwards again, so after the wave has passed through the air, all the molecules will have been shifted a short distance away from the object that made the sound. If on the other hand it's a pulling wave, then molecules rush into the lower pressure area as the wave arrives, leaving another space behind them as they do so, and more molecules then rush into that space in turn, so once the wave has passed through, all the molecules have moved a short distance towards the object that made the sound. Sounds are normally made by things vibrating back and forth, so in reality most sounds involve a whole series of push and pull waves which make the molecules of air shift backwards and forwards lots of times, and they tend to end up more or less where they started.
It is important for you to realise that the air molecules do not spring back to where they started: they will only move back again if a wave of reduced pressure (a pull wave) follows a wave of high pressure (a push wave), and that depends entirely on the object which made the sound moving back to where it started first. This means that if the object making the sound moves backwards and forwards very quickly, as happens with things that make high-pitched sounds, then the molecules of air will also keep changing their direction of travel very quickly to match, but if the object moves backwards and forwards very slowly, as happens with things that make low-pitched sounds, then the molecules of air will change their direction of travel equally slowly. If two objects vibrate at the same speed, but one of them does so much more energetically than the other, then the two sounds will both be made at the same pitch, but the more energetic one will create waves of much higher and lower pressures than the other object: the sound it makes will be much louder as a result, so the volume of a sound depends on size of the pressure differences generated. A sound wave spreads out as it travels along, so it weakens with distance and the pressure differences get smaller and smaller until the sound is eventually too faint to be heard any more: the same energy is still spreading out through the air, but there's less of it at any particular point, so it moves each molecule less and less energetically the further you go from the source of the sound, thus making the sound quieter and quieter.
Atoms and molecules are always moving about in all directions and knocking against the atoms around them, so your eardrum is being hit by air molecules all the time from both the outside and the inside. So long as the air pressure is the same on both sides, the eardrum won't move and you will hear no sound, but if the pressure goes up or down on the outside, the eardrum will be pushed inwards or outwards, and that movement is what makes you hear a sound. The air on the inside of the eardrum is normally kept at the same pressure as the average pressure outside, and it is given a chance to readjust to the average outside pressure whenever you swallow. Try swallowing now and you may feel (and hear) this happening. There are tubes connecting these inner chambers of air behind the eardrums to the outside air by way of your mouth, but they are normally kept closed: if they were open all the time, pressure waves of sound would be able to reach both sides of the eardrums and that would stop them moving about so well, reducing your ability to hear. If an explosion goes off, it may burst your eardrums, but if you know that an explosion is about to happen, you may be able to reduce the damage by keeping your mouth open: this helps the pressure wave to force its way through into the inner chambers of air behind the eardrums, and it's just possible that it may get there just in time to stop them bursting.
Sounds are made in a number of different ways, and some of them are too low or high for us to be able to hear. Elephants make low noises (called infra-sound) which we can't hear, but if you stand near an elephant when it's making an infra-sonic noise you can actually feel the vibration going through you. Bats go to the opposite extreme by making sounds that are too high for us to hear (called ultrasound), although children can usually hear their lower sounds, and I could still just hear bats when I was thirty. Ultrasound has its pressure waves spaced too close together for our eardrums to keep up with the pulses of atoms knocking against them, so they don't vibrate with the sounds and we hear nothing (although children's eardrums are smaller and more flexible so they can vibrate fast enough to hear the low end of ultrasound which adults can't detect). These inaudible sounds, plus the audible ones that we make when we speak, shout, sing or whisper, are made by such means as pushing air through tight gaps to make it vibrate quickly in a co-ordinated way, but noises can also be made by knocking things together so that the air molecules are jolted into moving fast in the same direction, or slapping things together so that the air is accelerated to high speed as it is pushed aside. Thunder is made when lightning heats up air, and this very hot air is made to expand so fast by the heat energy that it produces violent pressure waves of sound.
When sound waves enter your ear, they first make your eardrum move about (it's very fragile, so never stick anything deep into your ear), but then this vibration is passed on by way of some small bones (including a "hammer" and an "anvil") to the "cochlea" (pronounced "cock-lay-ah"), a liquid-filled chamber where little hairs stick out into the liquid. When the liquid is moved by a vibration, the hairs are moved with it, and they can detect extremely small movements of the water. The hairs turn the vibrations into signals that can be sent on to the brain. If you spend a lot of time listening to loud music, these sensitive hairs become damaged and they will eventually stop working. Many of the friends I had at school are now unable to hear noises which I wouldn't even describe as quiet, but they simply aren't aware that they aren't hearing them: they are already well on the way to going deaf. If you can't hear someone talking to you when you're listening to music, it's too loud: turn it down until you can make out what they're saying, and then tell them to be quiet so that you can hear your music.
Sound travels through air at about seven hundred miles an hour (or 1100 kilometres an hour), but in water it can travel four times faster because the molecules are all packed close enough together to be touching each other all the time, so it is possible for the waves to be passed on without the molecules spending any time zig-zagging about. The quality of sound is also maintained better in a liquid, and it loses a lot less energy as it goes along because it's more difficult for any parts of the wave to get ahead of or to fall behind the rest of the wave in the way that happens when sound waves travel through air. The result of this is that a whale can sing at one side of an ocean and still be heard by a whale thousands of miles away at the other side. Next time you go swimming, send a friend to the far end of the pool with a waterproof watch and get him to set off the alarm while holding it underwater: you'll have no difficulty hearing it if you get your head underwater (though you do need to get the water right into your ears). The sound of the beeps will fill the entire pool, so make sure your friend knows to shut it up after a few seconds so as not to annoy people. Arrange a system of waving before you start so that you know when to stick your head under: you don't want to have to do it too many times, and you'll need to do it at least twice because your friend will also want to have a turn at listening from the other end when you set the alarm off.
Sound can travel through some solid substances even faster than it does through water: four times faster through steel than water. It is said that you can tell if a train is coming by putting your ear to one of the metal tracks of a railway line: you will hear it through the track long before it can be heard through the air. I've never tried doing this, and I'm not that keen to try it: one person who did give it a go was killed by an electric current travelling along the track, even though it wasn't an electric railway line. Don't trust these things and stay well clear of them: you can never tell what some idiot might have done to the track out of sight round the next corner, and be aware that high-voltage electricity can jump, so it can kill you even if you don't touch the metal. If you want to try a safe way to hearing sound travelling through something solid, just put your ear to a table and get someone to tap the far end: you're much less likely to get injured or killed doing that.

Try to remember the following points:-


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