The Nature of Sound: by Ahmed Al-Qarawi [AQ Series: Episode 1]
Published on February 12th, 2012 | by Nariné (Kuwait Music)
We all know the experience of sound, and we all learned in school that it is a vibration of air molecules that stimulates our eardrums. People who work with sound every day tend not to think about the science of sound and take it for granted. But unless you have assimilated a good understanding of the nature of the medium in which you work, how are you ever going to make it really work for you?
Sound starts with a vibrating source, commonly the vocal cords, musical instruments and loudspeaker diaphragms, as far as we are concerned.
Let us think of a loudspeaker diaphragm. It vibrates forwards and backwards and pushes against air molecules. On a forward push, it squeezes air molecules together causing a ‘compression’, or region of high pressure. On pulling back it separates air molecules causing a ‘rarefaction’, or region of low pressure. The compressions and rarefactions travel away from the diaphragm in the form of a wave motion.
Wave motions are all around us, from the water waves we see in the sea (best viewed from a ship – the breaking effect near the shore disguises their true nature), to all forms of electromagnetic radiation such as x-rays, light, microwaves and radio waves.
The child’s toy commonly known as the ‘slinky spring’ can display a wave very much like a sound wave. The slinky is a spring – the metal versions work best – of around 15 cm in diameter and perhaps 4 m long when lightly stretched. If two people pull it out and one gives a sharp forward and backward impulse, the compression produced will travel to the end of the spring and – if the other person holds his or her end firmly – reflect back.
This demonstrates a longitudinal wave where the direction of wave motion is parallel to the direction of the motion of the actual material (we can call the motion of the material the ‘particle motion’). A sound wave is a longitudinal wave.
Contrast this with a water wave where the wave moves parallel to the surface of the sea, but water molecules move up and down. This is a transverse wave. Electromagnetic waves are transverse waves too.
One feature that the water wave demonstrates perfectly is that if you look out from the side of a ship at a piece of flotsam riding the wave, the wave appears to travel from place to place, carrying energy as it does so, but the flotsam simply bobs up and down. Other than wind or tide acting directly on the flotsam itself, it will bob up and down all day without going anywhere.
This is true of sound too. A sound wave leaves a loudspeaker cabinet, but this doesn’t mean that air travels away from the cabinet. The air molecules simply vibrate forwards and backwards, never going anywhere. (When air molecules travel from one place to another that is called, in purely technical terms, a wind!).
If this were not so then either a vacuum would develop inside or around the cabinet and there would be a danger of asphyxiation. Obviously this doesn’t happen. Oddly enough, if you put your hand in front of a bass loudspeaker you will feel a breeze, if not a full-on wind, on your hand. This is an illusion since you feel the air molecules when they press on your hand, but not when they pull back.
In a transverse wave, such as a water wave, the direction of particle motion is at right angles (‘perpendicular’) to the direction of wave motion. In a longitudinal wave, such as sound, the direction of particle motion is parallel to the direction of wave motion.
Although the longitudinal wave in the slinky spring is similar to a sound wave, it doesn’t quite tell the whole story. The slinky wave is confined within the spring whereas a sound wave spreads out readily. It is possible to think of each air molecule (actually oxygen, nitrogen and an increasing amount of carbon dioxide) that vibrates under the influence of a sound wave as a sound source in its own right.
Molecules are of course very small, and it is a feature of small sound sources – or point sources – that they emit sound equally in all directions, or ‘omnidirectionally’. So where light travels over great distances in straight lines, sound has a tendency to follow a straight-line path, but readily spreads out from that path in an ever-widening arc, particularly at low frequencies.
[Regarding point sources - it is also worth considering the example of a small loudspeaker emitting a low frequency tone. If the speaker is small in comparison with the wavelength being emitted, then it will have the characteristics of a point source and will obey the inverse square law - sound pressure halves for every doubling of distance from the source.]
Attention: This article is copyrighted to Ahmed Al-Qarawi, please collect his approval before referring to it.
About Ahmed Al-Qarawi
I’m Music Producer / composer & Owner of sound vision Production company in Kuwait for over 15 years now, I have been scoring music for Serials /Commercials / short film and Musical Albums for over 10 years.I would be delighted to discuss any future opportunities.