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So, I assume everyone here probably is aware of the "shepard tone" audio illusion, which creates the impression of an ever-decreasing or ever-increasing pitch. What I wonder is, if you had a series of tones of equal pitch, that are located differently with regards to the left and right audio channel, and slow move in one direction, with new ones fading in at one extreme and old ones fading out at the other, would that create the illusion of a tone that is forever moving in one direction?

Or are our brains much better at discerning the absolute orientation of a sound than they are of discerning absolute pitch, which would make this not work?

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I'm going to hang my hat on a nope answer to this, for the following simple reasons.

Our ears are particularly bad at localizing continuous tones, but rather good at localising transients. This is because of the fact that the ear/brain complex localises sound primarily based on time-of-arrival data that is derived by the brain based on the data provided to the brain by the ears.

With continuous tones there is nothing to distinguish one part of the signal from another and therefore there really isn't any useful time-of-arrival data that can be derived.

With transients however, it is much easier for the brain to determine time-of-arrival differences, consequently we are much more able to localise these particular types of sounds.

A "shepard" tone is a signal based on continuous tones and would therefore fall into the former category of sounds that are very difficult to localise.

Nice idea though!

  • Hey Mark! I agree the tones can sometimes be difficult to localize, especially if the tones are rising or falling very fast. But either in mono or stereo, panned left or right, the pitch illusion is still apparent. – Joel Pinteric Apr 24 at 6:33
  • Yes I agree, the pitch illusion is always apparent, but I don't believe that's what the OP was after. He's after a constantly changing localisation effect along with the pitch effects, which I am fairly sure isn't possible. There's only so far you can go in one direction before it turns into a circle! – Mark Apr 24 at 6:52
  • Well, that seems a lot like it could be achieved as surround sound, but probably not the best way to approach it. If it is about spatialisation then just use a surround panner or binaural processor. The Shepard-flanger effect could be a good effect on a continuous sound though, it gives the illusion of continuous movement rather than a loud tonal sound. – Joel Pinteric Apr 24 at 7:04
  • I'm going to go find that one and check it out. Until now, was unaware of it. Thx mate. – Mark Apr 24 at 10:29
  • That's just a name I gave to the Endless Series effect - the Shepard tone set to flanger FX Mode. olilarkin.co.uk – Joel Pinteric Apr 25 at 11:32
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Yes it is possible, and it is common for shepard tones to be generated in a stereo space.

There are plug-ins today like Oli Larkin's Endless Series that allow custom generation of these tones, even as stereo effects onto audio input, as a shepard-flanger or shepard-phasers.

You'll find many parameters that affect the illusion, such as centre frequency, modulation speed, bell curve, number of harmonics etc. It appears that the stereo effects are similar to phase rotation or a delay. It's probably best to experiment with what works best, as some settings sound better than others.

The main reason why it is possible under many different settings, is because our brains are constantly grouping the rising (or falling) frequency overtones together as though they are related to one sound, this appears to be independant of the stereo space. My hypothesis is that this is due to our ears identifying the harmonics as related, usually by their octaves, but there are more relationships to apply.

With the aforementioned plug-in, you can actually modulate the pitch-rise format to scales/steps, and even randomly modulate the pitch rise/falls with success, so our brains can easily be fooled.

With this Binaural Shepard Tone Generator, each additional Shepard tone can be mixed in separately.

  • hey mate - I don't think this was what the OP was asking - he's asking more about creating a tone that simulates constantly changing localisation in the stereo soundstage. Certainly possible to create 'stereo' tones, but not possible to localise them continuously in one direction. – Mark Apr 24 at 6:50

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