The ‘Shepard Tone’ named after psychologist and cognitive scientist Roger Shepard, is an auditory illusion involving a sound that appears to ascend (or descend), in pitch without actually leaving its preset range. Also an artist, here are some of Roger Shepard’s more famous (and disturbing), sketches of visual illusions.
The shepard tone has been used extensively in movies, music and games to create a sense of building tension or foreboding. In movies, it has been used several times by composer Christopher Nolan when working with Hans Zimmer. They employed the technique in ‘Dunkirk’, The ‘Prestige’ and Nolan also used the phenomenon as the sound of Batman’s motorbike in ‘Dark Night’. It has featured in several adventure games including the score for the endless staircase in Super Mario 64 and in music a version on this theme was first utilised by J.S.Bach in his ‘Endlessly Rising Canon‘ 400 years ago! Modern examples are Pink Floyd’s ‘Echoes‘, Franz Ferdinands ‘Always Ascending‘ (great example), and is used extensively in modern dance tracks to build tension in the lead up to the ‘drop‘. Have a quick watch of the video below which explains how Hans Zimmer and Christoper Nolan used the Shepard Tone to create the dark mood in Dunkirk. This clip also explains some of the basic theory behind this ‘sonic barbers pole’ illusion!
On our recent Sound Replacement Project, Dr Duck mentioned that he thought a shepard tone would work well to build the tension leading up to the amp explosion as Marty McFly is gaining up the equipment. We already had some ascending amp hum that Shay and I had created (and placed in the clip), which could then be blended with this shepard tone to accentuate the rising tension in the clip. Dr Duck had always wanted to make one (I had not even heard the term before), given his love of Trance music so we set about researching how it was done.
The theory sounded simple enough (this was a misjudgement), in that all that appeared to be involved was to have three chromatic scales of the same key play simultaneously one octave in pitch apart. The middle scale is to have a constant amplitude. The lower one begins louder and decreases in amplitude as it moves up the scale. The higher scale begins softer crescendo’s as it travels. The amplitude is an integral part of the illusion as science and tech writer for the Daily Mail Ellie Zolfagharifard explains: “Changes in volume cause the listener to focus on certain notes while ignoring the others. The illusion works because each tone seems to sound lower than the preceding. The listener judges subtle changes in tone by comparing it to the preceding note, not to tones from twenty or thirty seconds ago” (Daily Mail, 2014). Simple? Um…not so much!
As Dr Duck is most comfortable in Logic (although proficient in all DAWS!), we opened up a session and prepared our scales using a sample of the guitar hum to match the pitch using Midi. Following the theory perfectly, we found to our dismay that whilst there was indeed the basis of the auditory illusion present, the general ascension was not fluid (you could still hear individual notes), and when the sequence restarted after one revolution the transition was quite jarring (particularly on the lower octave scale). Back to the research we went. What we didn’t realise at the time was that whilst we had the theory for the shepard tone right, what we were actually seeking (and the version of the tone used in movies and music), was the Shepard–Risset glissando, a version of the original shepard tone created by electronic composer Jean-Claude Risset (1938-2016). This version is more fluid and the individual notes are inaudible, so the whole experience is more like a pitch bend than a note by note progression. Please listen to a great example below:
After hitting a few YouTube clips that just reconfirmed what we had already achieved, we found one that explained something new! We moved to Pro Tools, replicating the basic theory we had employed in Logic with several notable additions to our technique:
- The new session was matched to the frame rate used in our Back to the Future clip (24/sec).
- We used a longer sample of our guitar hum (6.5sec) and duplicated this sample 3 times.
- Beginning the sequence one semitone lower to the original starting point of the scale and ending it one semitone higher hence making the looping transition markedly smoother and more fluid.
The end result mixed in below the amp hum, can be heard below from the 40sec point:
Our journey had been arduous, but exciting as we took each small step toward our goal. It was really quite a satisfying achievement. The result is a tone that is not perfect, but certainly served our purpose. I say this because I can still hear the point at which the sequence transitions, particularly as the lowest scale loops and begins its journey but after listening to and analysing so many examples of this illusion during this process, I can now hear this same transition point in all of them! The shepard tone however is certainly a remarkable phenomenon and as it is usually used to compliment other sounds in a movie or song (placed to the rear of the mix), is a technique worth understanding and knowing how to create as the overall compliment to other elements in building tension is definitely evident.
Feature image courtesy of https://youtu.be/S_v63x56uRI Retrieved on 28/11/17
Roger Shepard sketches courtesy of http://im-possible.info/english/art/classic/shepard.html# Retrieved 29/11/17
Ellie Zolfagharifard quote courtesy of http://www.dailymail.co.uk/sciencetech/article-2547978/Hearing-believing-These-incredible-audio-illusions-make-head-spin.html Retrieved 30/11/17