Yamaha QL5

How Sound Works – Frequencies, Compression & Rarefaction

Every good Live Audio Engineer needs to know a bit of the science behind what all their fancy equipment is actually doing. 

The first thing we need to understand is frequency. Understanding frequency means that you will be able to equalise, or EQ a mix, and identify and reduce any unwanted sounds from your mix.

So let’s start at the beginning.

The primary school physics beginning…

So what is sound?

If you’ve had the privilege of completing primary school already, you probably have some fuzzy recollection of a physics class with a teacher trying to explain something to better your mind and teach you the facts of life. Though all you really wanted to learn was how to turn yourself invisible so you could crawl out the window and get on with everything else you had planned for the afternoon, which didn’t include listening to his fuzzy voice.

Forget that, it obviously didn’t help you anyway.

I’m going to use some technical terms here, then I’ll explain what they mean:

“compression” and “rarefaction”

These two terms relate to the physical change that happens to the air molecules when the object next to them moves, pushing them together.

Think about what happens to rain in a crazy gusty storm – as the rain falls the wind pushes it together, creating dense sheets of raindrops as it falls. 

sheet of rain

Like this, your vocal cords push the molecules of the air together, creating compression, and the space now left with not many molecules between is rarefaction.

Now this one’s a little hard to get your head around: A waveform is a just a chart that demonstrates the amount of compression or rarefaction of the air molecules. So in the below diagram, the line below the midline is showing how much the air is rarefying, while the line above the centre line is demonstrating how much the air is compressing. As the line reaches the top of the grid, it is at its maximum point of compression and the molecules are as close together as they can be.

Now the term wavelength, describes the physical distance between two corresponding points of the same compression or rarefaction. This is called one cycle. It is measured in meters for low frequencies and in centimeters for high frequencies.

This distance defines the frequency of a sound. If the cycles are very long, this will be a low frequency, like the rumble of bass or a truck driving past. If the cycles are very short, this will be a high frequency like the whistle of a kettle. We measure the frequency of sound as cycles per second. 1 cycle per second is 1 Hertz (hz) and 1000 cycles per second is 1000 Hertz or 1 Kilohertz (kHz). 1 Kilohertz has 1000 hertz in it, just like a kilometer has 1000 meters in it.

The human ear can hear a range of sounds from 20Hz to 20kHz, but this decreases as our hearing wears out with age and abuse. 

Now, take a big breath, that’s all you’re going to need to know for the moment to understand frequencies. 

Go have a walk around and listen to the sounds around you and try to take a guess what frequency range they’re in. Remember that sounds are usually made up of lots of frequencies in a particular range.

Or better still, pull out an EQ, put some music through it and increase the volume on particular frequencies to hear what they sound like!

Next, we’ll learn about volume or Amplitude.

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