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Tuesday, 16 February 2016

Sound Design: How to Make a Shepard's Tone

If you are not familiar with it, a Shepard's tone is an audio illusion whereby a sound seems to continuously rise (or fall) in pitch. This is a really cool trick that you can use to make pads or riser sounds, and it's often used in films to create a sense of tension.

Basically, this is the audio equivalent of a barber's pole. So what are the mechanics behind it and how do we make one?

Step 1: Create four sine waves that are an octave apart from each other.

Start with an A3 note, as this equals 440Hz and is nice and easy to calculate from a mathematical point of view. Next create a copy of this that is one octave higher (A4) and two copies that are one and two octaves below respectively (A2 and A1). You should have four evenly spaced sine waves at the following fequencies: 110Hz, 220Hz, 440Hz and 880Hz.

Step 2: Automate the pitch of each sine wave so that it's going up one octave.

So for a rising sensation we need to increase the pitch of each sine wave. You might see where this is going, we want the tone at 110Hz to rise to 220Hz, then the tone at 220Hz to rise to 440Hz and so on. Below you can see an instance of Operator with the transpose control automated up as per these instructions. You could use an envelope to do this, however you will get an audible resetting of the pitch at the end of the envelope, which will make it sound un-natural in comparison to what we would like.

Step 3: Add volume fades to create a continous loop.

Now the finishing touch, automate the volume of the highest tone (the one that begins on 880Hz and ascends to 1760Hz) so that it's gradually fading out. Rinse and repeat with the lowest tone, except this time automate the volume so that it's fading in rather than out. 

Now you should have a constinously rising loop, to which you may want to add reverb or other effects and processing. See the clip below for an example. Of course, this is just the basics of how this effect works, experimenting with other sounds (not just sine waves) can lead to some really interesting results. Personally I find that this effect seems to work really well with string sounds. 


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