The Colours of Noise - Grey

It’s a common misconception that audio engineers only deal in sound: there’s an entire rainbow of noise colours and textures out there, each with their own unique properties which engineers use to identify and manipulate sound.

Thinking of noise in terms of colour gives us greater control over the sounds we want to attenuate or amplify, which is why the noise colour spectrum is a valuable tool for a range of engineering tasks.

Think about the difference between music and daily noises like the kettle boiling, stomping boots, the whirr of an air-conditioning unit or a car engine. Music is attractive to our ears because the frequencies are spaced at intervals the ear can easily understand, creating a harmonic structure that sounds pleasing. But the noises we hear around us every day are made up of chaotic waveforms, a random distribution of frequency and amplitude. This makes them hard to analyse.

At HVAC Attenuation we design silencers for heating, cooling and ventilation products, so our engineers need to be able to do this work of analysis in order to identify exactly how to silence the annoying hum, buzz or whine of certain machines.

We’ve heard about white, pink, blue and brown noise. But what about grey?

Winter seems an appropriate time to think about the colour grey, which conjures images of bad weather, rain and dreary overcast days. It also calls to mind certain metals and durable substances like aluminium and steel.

Grey is associated with textures and ideas that have a feeling of density or opacity, which might be why we use it to describe a texturally flattened sound. Grey noise is a mix of frequencies at equal loudness, which creates a homogenous soundscape. Fittingly, it sounds a lot like an intense downpour.

A working definition of grey noise is a random output of noise mapped out over a loudness curve, giving the listener the impression that it’s equally loud at all frequencies. If you listen to a grey noise sample you might think it sounds like a wall or shower of noise, and that’s because we’re hearing this uniform loudness. Believe it or not, many grey noise samples are actually recorded from natural sources which create this loud, opaque, wall-like quality - the “grey noise” application on Amazon’s Alexa devices, for example, is taken from a recording of a Swedish waterfall.

Grey noise is commonly used for hearing tests, and provides relief in the treatment of tinnitus. It is said to help some people focus, sleep and meditate. Next time it’s pouring rain outside, think about it: you’re listening to grey noise.