What is Sound?

This post will cover the very basic rules of sound. One of the most important things to realise is that if you do not know the basic definition of sound and how it works, you’re career as a sound engineer will be very limited! So without further ado, What exactly is sound?

A sound is generated by vibration. Any moving object can cause sound to be created, and this sound is transferred by the vibration of air particles around a given object. Think of it as the ripple effect you get if you drop a pebble into some water. The same thing happens with the air around the source of a sound. The image below should help to visualise this.

So these ripples, are actually more commonly referred to as sound waves. To understand how sound waves are plotted on a graph we must first look at how the air particles are affected by the source of the sound. If you look at the image above, you can see the ripples clearly, and you can see the spaces in-between the ripples. If we think back to the water example, the ripples actually contain more water than the spaces in-between them, creating the visual affect you see above. The same is true of the air particles affected by the source of a sound, except of course there is no visual effect.

It is at this point important to note that sound waves, and ripples in water are technically different. Ripples within water are known as transverse waves, where as sound waves are actually longitudinal waves. The difference being that in a transverse wave (water) the particle displacement is perpendicular to the direction of wave propagation, whereas in a longitudinal wave the particle displacement is parallel to the direction of wave propagation.

So, air particles will bunch together at the height of the wave, and move further apart between the peaks of the wave creating alternating high and low pressure. This is known as compression (bunching together to create high pressure) and rarefaction (moving apart to create low pressure). This is the fundamental reason that we are able to hear sound.

The diagram above shows the compressed (or condensed) air as the darker, more dense specs that correspond with the peaks of the sound wave. The rarefaction can be seen as the more sparse lighter specs corresponding with the troughs of the sound wave. These specs represent the number of air particles, but it is important to note that it is NOT the air particles that are moving, it is the disturbance. The individual air particles are simply oscillating back and forth from their original position (known as their equilibrium).

So, these waves of alternating high and low pressure are what travel through the air, at a speed of ~340 meters per second, towards your ear. We will talk about exactly how these sounds are captured in more depth in a later post, both by your ear and by a microphone. But for the sake of completeness, your ear has a drum with a very fragile membrane stretched across which moves in and out according to the alternating air pressure. Your brain then receives this signal as an audible sound. Again, this is an incredibly simplistic explanation of a very intricate and complex process, so more information on this will be coming soon.

I think this post sums up the very basics of what sound is, so I shall leave it there. You must remember that sound is a very complex thing so I will try to cover things one small step at a time. Next post I will go into more detail about sound waves, and the various properties and elements that eventually translate into a pitch that you can hear.

For now, thank you very much for reading. If you see any mistakes or have any feedback, I cannot encourage you enough to let me know in the comments below or to my twitter account @sammio2

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iOS Developer & blogger

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