Psychoacoustic illusions

[taudo]

This rumination is presented for your consideration - just for fun!

Sometimes I think we enthusiasts get carried away with our technical/engineering approach to the listening experience (that would certainly be me!) and forget that what we are really trying to do is create a realistic and pleasing _ acoustic illusion_ and not necessarily create the perfect system that is "phase and frequency flat from DC to ultraviolet".

To be sure, the human acoustic system usually responds favorably to the engineering approach. But do we overlook something by being so attached to it?

I am reminded of something I saw in grad school many years ago - and of which I may get certain specifics wrong. However, I'm certain of the basic point illustrated.

It was, I believe, Edmund Land (of Land Camera fame) who in the 50's or 60's demonstrated that our "conventional" understanding of color vision had to be wrong. He did this by making B&W transparencies through various monochromatic filters, then projecting them through a different monochromatic filter. Thus, the only information projected onto the screen was 1) brightness (via B&W density) and 2) a single monochromatic color. Given this, our conventional theory of color vision (at least when I was in grad school in 1978 or so) predicted that subjects would see only something like a sepia monochrome image (or red, or whatever monochromatic filter was chosen for projection)

What Land did was to show that superimposing one such B&W projection on top of another taken with a different filter caused the viewer to see full color although there was only one color actually present in the projection.

In short, Land was able to use a non-theoretical approach to produce an amazing visual illusion of color. I've seen it, and while it isn't highly saturated, it IS real color where none should exist.

Now, I haven't revisited this subject in 25 years or so - perhaps the Land Color Illusion has been explained by now. The point is, it was _very real_ despite being completely at odds with then current theory.

The Land Color Illusion is not the only such example in human sensation & perception - there are many such. Maybe we should be looking for them.

I got to thinking about this when I came across the February 2003 post on this board by "blackdog" who claimed to have witnessed a very realistic psychoacoustic illusion implemented on Altec equipment (How Far Can Altecs Go?) Wouldn't you love to hear that system and decide for yourself? And if it was the great illusionist that blackdog said it was - wouldn't you just love to pack that room with test equipment and try to find out _why_ it works so well?

I sure would!

taudo

[Fitts]

Oh dear, should we be talking about the reality (or lack) of what we hear or are we supposed to be dealing with the logistics of trying accurately and faithfully replicate sound patterns with speakers and cabinets and crossovers? (or can we do both)

Here's my shot at it:

Our ears don't have flat response, we put tone controls on just about every with a volume control so why do we try so hard to make sound systems that have perfectly flat response characteristics? (I never run with the tone controls flat, do you?)

Why do cars go down the road sounding like they have giant flat tires, thump, thump, thump...? (probably one of those acoustic illusions)

Isn't everything just one big acoustic illusion anyway? (I think, therefore I am)

'Cause its fun, right ! What else you gonna do if you don't tweak you system? Where else can you have this much fun for the price of an occasional donation to Todd? (yes, I know I'm over due)

Would I like to hear a psyco acoustic illusion? I probably have before like when I cross wired my home made active crossover and didn't realize what I had done. (heck it only took me a whole week to figure out why the sound was dancing all over the place) Or maybe it is that darn ringing I hear day and night. (the Doc calls it tinnitus)

Could I tell you how to make a psyco acoustic phenomenon ? No way, I haven't a clue. Is it done with a Bose radio?

Does anyone else know? ...

Thanks for listening (well, actually reading)

[GM]

>Our ears don't have flat response,

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True, but we have the world's most sophisticated on-board processor to EQ it. ;(

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>we put tone controls on just about every with a volume control so why do we try so hard to make sound systems that have perfectly flat response characteristics? (I never run with the tone controls flat, do you?)

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Because we want a flat in-room response with a minimum of correction, with none in our critical hearing BW (~250-5kHz).

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>Why do cars go down the road sounding like they have giant flat tires, thump, thump, thump...? (probably one of those acoustic illusions)

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Because the sub is exciting the car's resonant frequency.

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>Isn't everything just one big acoustic illusion anyway? (I think, therefore I am)

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Yep.

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>Could I tell you how to make a psyco acoustic phenomenon ? No way, I haven't a clue. Is it done with a Bose radio?

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Haven't auditioned one, but the man's well aware of how to 'enhance' stereo.

GM

[thinking]

>Could I tell you how to make a psyco acoustic phenomenon ? No way, I haven't a clue. Is it done with a Bose radio?


OK, I'm going to weigh in over my head now, but I was under the impression that stereo, and every derivative thereafter was a psyco acoustic phenomenon. There are no stereo violins, clarinets or cymbals, but with the correct micing, recording and playback, it creates the illusion that each piece is being played from it's proper location on the stage, which "appears in our mind" inbetween the speaker components. As further evidence of this, there are some home stereo units around that had a mono/stereo button, (although I've never been sure why they included it, it's either recorded in stereo or it's not) just press that button to mono in the middle of a well recorded stereo session, and see what it does to your psyco acoustic illusion.

[thevott]

The mono (or monaural) button is primarily present to clean up noisy FM radio stations which, the best of my understanding, rely on a comparatively noisy AM subcarrier to insert the stereo 'differential' signal---correct me if I'm wrong on the latter.

[taudo]

"thinking" is, of course, exactly correct when he says;

>I was under the impression that stereo, and every derivative thereafter was a psyco acoustic phenomenon. There are no stereo violins, clarinets or cymbals, but with the correct micing, recording and playback, it creates the illusion that each piece is being played from it's proper location on the stage, which "appears in our mind" inbetween the speaker components.

Our well known and appreciated "stereo imaging" is exactly the kind of very useful illusion I was pondering.

In the early days of stereo, "flat" or "mechanistic" thnking lead to the common practice of placing two mics at the front of the stage, and playback was supposed to place two speakers at the same positions. The common sense notion was that this would provide correct spatial imaging. Of course, we all know that common practice today bears no resemblance to that old method.

Film movies use a rapid succession of still images to give the illusion of motion. TV uses raster scanning. FM multiplex broadcasting gives the illusion of true stereo. Surround Sound processors give the illusion of larger halls. All these things are quite useful.

Might we be overlooking some other ways of enhancing our listening experience?

Just some enjoyable musing while trying to figure out how to rebuild my old BR enclosures!

taudo

[dennis gilliam]

FM stereo, or at least as it stands today, is transmitted in "matrix" format.

From 50 Hz to 15kHz in the carrier, you have L+R. Everything common to both channels, and is compatible with a straight mono FM receiver.

At 19 kHz, is a 8 to 10% reference signal, mostly a square wave, to give your radio's decoder something to synchronize to. It also triggers that little red light usually labled stereo.

From 23 to 53 kHz is the portion of the FM spectrum carrying the difference, or Left minus Right signal. Everything that is NOT common to both channels, in other words.

This is done to make it easier to have your receiver keep playing through times when signal degredations occur. Algebraically, it makes good sense to transmit it this way, as you can reconstitute it back:

(L+R) + (L-R)=2L
(L+R) - (L-R)=2R

Your decoder chops this primitive digital (BUT VERY ROBUST) stream up at a clock rate of 19,000 hZ to give you discrete left & right.

This leaves nothing to be desired in terms of separation, better than some 40 dB, but the brick wall filters @15 kHz have FM sounding a bit flat compared to CD (20kHz top end) audio.

I hope this sheds a little light on FM...

DG

[GM]

Yep, in my mind I was referring to the brain enhanced 'stereo' that we perceive due to us having two offset ears (we hear in mono below ~800Hz depending on head size), but being tired at the time it appears I didn't make it clear in print. Indeed, all mono recordings I've listened to had enough ambiance to give a bit of stereo 'space', some to the point of sounding more 'real' than their later stereo counterparts. B***, as well has some others have come up with all manner of acoustic/electronic designs to 'trick' us and I'm confident that there's others that haven't been invented yet.

I figure it's just a matter of time before digital becomes so sophisticated/powerful that sonic holograms are the norm. At one time I had a Carver C-9 Sonic Hologram Generator and it gave me the hots for a really transparent one that works on all recordings.

GM

[Steve Schell]

Taudo, you have awakened in me an old memory.

When I was about 10 years old I would often stay awake most of the night in the summertime watching old movies on television. One night, quite late, a commercial for the soft drink Squirt came on. I didn't pay much attention until the announcer said something like "Squirt is such a remarkable soft drink that you may actually be seeing color on your black and white television right now." I did! Some sort of swirling pattern revealed red, yellow and green fringing as I recall.

When I told family and friends about this later, they said that it was impossible, that I had dreamed it. After awhile, they almost had me believing them, until I saw the same commercial once more, again late at night, a couple of years later, so help me.

[ckdf]

I once had an interesting article about that. It stated that the color information that the receptors in your eyes detect is sent to the brain as a pulse encoded signal. By appropriately switching a white light source on and off you can generate a similar pattern and fool your brain to "see" color where there is none. The article contained a drawing of a white disc that had some black segments of circles of a certain pattern on it. You could cut out that disc with some scissors, put it on e.g. on a fan to quickly rotate it and when doing that with the right speed you could see rings of different colors.