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Is pitch variation in reverb an illusion?

Started by David Pinnegar, October 15, 2010, 04:07:22 AM

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David Pinnegar

Hi!

The other evening I was listening to a disc of the organ at St Maximin. Philippe Bardon always says how special the acoustic is there, and says that reverberation is ruined by a cruxiform format to a building and or a dome.

At the end of a large chord, it's apparent that one hears the wall of sound hitting one end of the building and then the other, bouncing back between the ends, but when I heard it the other evening it almost sounded as though the pitch changed minutely as the sound passed in each direction.

http://www.organmatters.co.uk/reverb.wma

Is that perception an illusion or a reality?

Best wishes

David P

revtonynewnham

Hi

Interesting.  the pitch change is noticeable.  I wonder if there's an air current in the building causing some sort of doppler-type effect - or maybe the sound at the microphones is modified by something moving close by?  Or perhaps it's a phase anomaly caused by multiple reverb path lengths?

Is it audible listening to the organ live?

Every Blessing

Tony

David Pinnegar

#2
Quote from: revtonynewnham on October 15, 2010, 02:53:03 PMInteresting.  the pitch change is noticeable.  I wonder if there's an air current in the building causing some sort of doppler-type effect  . . .
Is it audible listening to the organ live?

Dear Tony

:-) In the old days we would have thought it was just tape wow . . .

I think it probably is audible live and I'm thinking that it's on account of the shockwave of sound actually travelling cleanly from one end of the building to the other and back, so passing in different directions past one's ears or the microphone. This is an organ on a west wall, with a straight bounce from end to end. Were the organ to be in the middle somewhere, the sound-front would go in both directions at once and one would hear the two concurrent waves in both directions simulataneously and merged.

Perhaps this is why the St Maximin acoustic is so special?

Paul - have you any ideas on this?

Best wishes

David P

NeilCraig

Hello all,

This is not an illusion, nor is it a recording artefact.  In the summer I visited Tewkesbury Abbey and, both sitting 1/3 of the way down the north side of the nave and also listening to the voluntary from the crossing, in front of the Milton Organ case, there was a pronounced flattening of pitch during the decay.

At Tewkesbury I could not hear the sound bouncing off the west wall and coming back, so I don't think it's due to that; in fact at York Minster, where I've both sung and played many times, the organ, particularly the 32' Sackbut, is very audible as it rebounds from the back of the nave, but there is no frequency droop.

I've noted this effect in numerous acoustics and it seems to me that it is most pronounced when the width and height of the nave are similar.  York is wide but nowhere near as wide as the 92' of the vault.  Tewkesbury, being Romanesque, is very squat.  My local church, although not very reverberant, also exhibits this phenomenon and is roughly square in cross-section of the nave.

Warm Regards
Neil

pwhodges

In the sample clip, there is no change of pitch - I've just been analysing the spectrum.

I have a nagging recollection that there might be a real effect that can cause pitch change in reverberation (which I believe I've heard, too), but I can't at present find any reference to it.  I'll keep looking...

Paul

David Pinnegar

Dear Paul

Thanks - most intriguing - I wonder how many of us think they can hear a pitch shift - as I certainly thought I did first time I heard it and it's been good to find that I'm not unique in this. Has anyone any explanation for what is going on?

Best wishes

David P

David Pinnegar

#6
Hi!

Following Paul's test I have processed the reverb tail to remove the diminution:
http://www.organmatters.co.uk/unreverb.wma

It's very strange as clearly there was no pitch change in reality but when I first heard it on the recording the perception of a pitch change seemed striking.

Best wishes

David P


Postscript - here's the spectral analysis:

I wonder if the phenonomen happens on account of progressive loss of high frequencies?

The first 0.3 seconds should be discounted in terms of analysis as level processing starts after around then. It looks as though there are spikes 0.1 seconds apart which suggests a distance of around 100ft. It looks as though, starting with the peak at around 1 second, there is a negative phase peak at 1.8, a positive phase peak at around 2.6 and a negative peak at around 3.4. Is the length of St Maximin basilica around 800feet?

pwhodges

#7
I've looked around a bit, and found some references to changes in pitch perception with volume.  (Some references also seem to be tying the effect to changing volume, but I've not read the primary documents yet.)  Most of the references are to the work of a researcher called Thomas Rossing.  Just to be clear, there is no physical effect - this is a perceptual illusion.

In summary, a tone with exponentially increasing volume will be heard as rising if it is above 2kHz, and falling if it is below this, the amount of rise or fall depending on the distance of the frequency from the central one (sometimes called "Stevens's Rule").

These tests have been done with pure tones; in the case of complex, musical, sounds, the effect is less clear.  It appears that a low note with lots of harmonics will rise, because the bulk of the energy is above 2kHz, for instance.  Parkin (in "Pitch Change during Reverberant Decay", J Sound Vib. 32, 530 [1974]) noted the sharpening effect after the release of a final organ chord in a reverberant building, which is where this thread started.  I also recall a discussion (which I can't find) of choirs singing flatter in some buildings, presumably because of the reverberation in those buildings having a spectrum that causes a perception of flattening.

You can easily hear the change in pitch with volume by striking a tuning fork hard and holding it very close to your ear.

Paul

revtonynewnham

Hi

Now you mention it, I've heard of this effect.  I guess it is behind the observed phenomenon.

Every Blessing

Tony

David Pinnegar

Hi!

It's very interesting that our perception of sound which in this case is an illusion actually resembles The Compton Effect in respect of electrons and photons in quantum physics. http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/comptonscattering.htm Perhaps our _perceptions_ of phenonomae might give us an insight as to what really does happen in the invisible world . . .

Best wishes

David P