Precisely - that is the actual situation. The choir is producing a change in amplitude and the long delays of the different air paths cause the progressive ramping down of the reflections.
Whether this is enough to account for the observed drop in pitch is another matter, but phase change from a linear system is not a physical impossibility.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
I have a concept of a 'virtual reflector' in a position intermediate between two real physical reflectors. Under steady-state conditions, the resultant sound would be a combination, in both amplitude and phase, of the two reflections; it would be indistinguishable from the sound coming from a single reflector at an intermediate position. Now suppose the amplitude of one reflection is decreased very slightly: the virtual reflector will appear to move away from that reflector and towards the other one by a small distance.
Apply this to a whole series of reflectors and a sound of smoothly decreasing amplitude which effectively fades-out one reflection after another. Although the physical reflectors are discrete items, the virtual reflector will move smoothly between them as the sound levels vary; the resultant sound will change gradually in exactly the same way as if it had come from a real reflector that was moving away.
The combination effect works in stereo as the sound moves across the sound stage; I am imagining the same effect moving towards and away from the observer.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
She is a clarinettist and recorder player, so we might try that (with the permission of the abbey authorities) if we visit Tewkesbury again.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
On May 2, 2018, Adrian Tuddenham wrote (in article):
This is called a serrodyne effect in radar. Google on serrodyne to be flooded with papers. It was first used with TWT (Travelling Wave Tube) RF amplifiers, but any linearly varying phase shift (or time delay) will do.
Joe Gwinn
[...]
Thanks for the information, it does seem as though the serrodyne effect could have a lot in common with the effect I am describing. Your comment about Google is unfortunately true: a search produces a flood of papers describing devices using the effect, but no explanation or definition of what it actually is. :-(
I am beginning to come round to preferring the idea of Doppler shift from a virtual reflector as the most likely explanation. The above explanation is too dependent on exact wavelengths and phases.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
Would this be a virtual Doppler shift with a virtually moving reflector?
The serrodyne effect is real, but sound travels at a constant velocity, certainly at the pressure levels involved here.
We really ought to get back to the real world.
-- Mike Perkins Video Solutions Ltd www.videosolutions.ltd.uk
It seems a lot more likely that the effect is perceptual, caused by the more rapid dissipation of the higher harmonics. Even when the fundamental does not change frequency, the change in harmonic mix changes the perceived pitch.
The other factor is that most mechanical resonators have harmonics that aren't quite harmonic; they're sharp. Pianos are stretch-tuned to counter the effect of this inharmonicity. The high keys can be as much as two full semitones sharper than a pure multiplication of the equivalent low keys, otherwise there is an audible clash with the harmonics of those low keys.
As the (sharp) high harmonics fade, the (flat) fundamental remains, and the perceived effect is that the whole note falls flat.
Until you produce measured spectral analysis showing a pitch shift in individual spectral lines, I'm going with this explanation.
Clifford Heath.
No.
When singers who aren't used to them record in headphones, they sing flat. For ... reasons[1], the sound you hear in your head is sharp relative to what it really is. What (s)he is desribing is the actual sound.
[1] Little's Law reasons? I don't remember. Bone is denser than air, yadda yadda...Human hearing is mostly made up by various cortices in the brain.
it can't be doppler because nothing's moving.
-- Les Cargill
It also works with sine waves -- the perception of tone itself slows down at high frequencies, so that, say, 10kHz doesn't sound an octave above 5kHz, it just sounds higher, shriller, sharper.
It's also harder to pick out harmonic relationships, because two tones need to be very well matched, and pure, to percieve a beat.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: https://www.seventransistorlabs.com/
On May 2, 2018, Adrian Tuddenham wrote (in article):
Serrodyne is a simple effect. Consider a sinewave generator which has a linearly varying phase k*t in addition to the omega*t term:
Sin[ omega*t + k*t ] -> Sin[ (omega+k)*t ]
The instantaneous frequency is k+omega, and the constant k may be positive or negative. The difference is that (unlike for omega) one cannot do this for forever, so the phase term k*t is periodically reset, so the added phase forms a sawtooth waveform. To be more precise, that term is actually k(t-t0), where t0 is periodically reset, at the start of each serrodyne ramp. The frequency is omega + k say 90% of the time, and the other 10% of the time is ignored because the serrodyne ramp is retracing, soon to start over.
.It isn?t really a doppler shift, because nothing is actually moving. It?s a click turning into a chirp, as with the reflection off the angled corrugated wall. The row of columns is a perfectly good angled corrugated wall.
The only difference between a click, a chirp, and white noise is the phases of the various sinewave terms. In a click, all the phase are zero at exactly one place. In a chirp, the phase varies linearly with frequency. In white noise, the phases are random.
Joe Gwinn
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