Production Technique #5

Shepard Tone

Bradley Ellis
Jul 28, 2017 · 4 min read

Intro

Whilst brainstorming for this blog post, I thought I’d research and discuss the numerous ways white-noise is used to create risers in dance music. I’d watched a series of great videos like the one below and was keen to explore.

However, whilst watching a video on a personal hero Hans Zimmer, I was inspired to do something entirely different.

In the video below, Vox explores how Hans used a clever auditory illusion to create tension in his newest soundtrack for the film, Dunkirk.

The Shepard tone

According to the Journal of the Acoustical Society of America:

A Shepard tone is a sound consisting of a superposition of sine waves separated by octaves. When played with the bass pitch of the tone moving upward or downward, it is referred to as the Shepard scale. This creates the auditory illusion of a tone that continually ascends or descends in pitch, yet which ultimately seems to get no higher or lower.

Here is a video of the Shepard scale in action:

The application for this in music is endless, whether it be to create tension in a war thriller like Dunkirk, or merely to invoke suspense in a dance track.

Implementation

For help in creating this effect, I’ve once again turned to the tutelage of Tom Cosm. He has a 2-part video series explaining how to achieve it in Ableton.

I have veered slightly from the example of Tom Cosm though. Instead of using pure sine waves, I have used the Grand Piano pack, native to Ableton. I wanted to re-create the effect in the vein of Hans Zimmer, so I kept the sound classic.

Here is the sound I created:

Breakdown

I begun by creating a 4-bar loop, consisting of pairs of 1/4 triplet notes.

Beginning at A, the sequence of notes ascends until it completes 1 octave.

Next, I duplicated this track five times, and adjusted the octave of each of those new tracks so that none were the same.

Eventually, I had a tracks beginning through the range A0 — A5.

At this point the illusion is almost complete. While there are overlapping frequencies, the volumes of each octave remain the same and so a clear start and end point is still obvious.

The final piece of the puzzle then is to create the crossover. To achieve this, I added volume automation on the outermost tracks (A0 & A5 respectively).

Why It Works

We can think of each track both as an octave but also as a range of frequencies. Our first track A0 represents the frequency range 60–120 Hz; A1 represents the range 120–220 Hz and so on until A5, which represents 1760–3520 Hz.

As the notes all ascend identically through the octaves, there exists overlap at all times between 120–1760 Hz. The only range where there isn’t overlap is in our outermost tracks.

That is why A0 ramps up in volume. Initially you cannot hear the tones of A0 but by the end of 4 bars, A0 has reached 120 Hz; simultaneously, A1 snaps back from 220 Hz to 120 Hz, before it begins its ascension once more to 220 Hz.

This crossover explains why there appears to be a constant ascension from 120 Hz through to 220 Hz; the ascension as it were was started by A0, and was then passed on to A1.

If you follow this logic all the way up through the octaves, we arrive at A5 where the volume ramps down, such that by the time it snaps back from 3520 Hz to 1760 Hz, the snap-back is inaudible; what is audible instead is the ascension continuing from A4 below, which before snapping back to 880 Hz itself, has climbed to 1760 Hz. Thus it also ‘passes on’ an ascension, this time at 1760 Hz to A5.

Summary

The logic involved in all this, is that because we can always hear at least 2 tones rising in pitch at the same time (in my case it’s at least 5 tones) our brains are tricked into perceiving a constant, ascending tone. Once those tones are looped together, the result is a piano scale which sounds as if it is continuing into infinity.

One other point worth mentioning, is that I found lowering the tempo helps sell the illusion. I recorded my example at 70 bpm and it was far more convincing there than at say, 120 bpm.


Bradley Ellis

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Audio Engineer || Music: https://goo.gl/9r4Ajm