"Doppler Shift" reflected from fixed objects?

It RF modulates using SSB, then de-modulates using a carrier offset by 5Hz.

This detunes the harmonics somewhat, it's not a pure pitch-shift.

Summing signals is not amplitude modulation. The only available nonlinear element might be the listener's ears.

Right. Superimposing delayed echoes creates no new spectral lines. Summing might sometimes look like AM, but it's not.

If the observer moves around, there will be Doppler effects.

You can't get a frequency change from passive refelctors. But it could be that the congregation changes the intensity the reflection at particular frequencies or that it produces fill sound that changes the perceived sound.

No need for RF even. Mix AF with the 3-5Hz and select upper or lower sideband (as in quadrature image reject mixer).

Modern sound systems with feedback reduction seem not to use frequency shifting but instead very sharp notch filtering done digitally.

piglet

It's quite impossible for there to be a Doppler shift in such a situation. If there were, it would mean that there was an ever increasing number of cycles of the emitted sound that had yet to arrive back after being reflected.

Sylvia.

The apparent drift in pitch was very small, so we aren't looking for a mechanism that could generate frequencies that were greatly different from the original tone.

There is another factor which hasn't been considered: the modulation envelope of the sound. These effects were only noticed after the chior had sung the last note., so they occurred on the falling edge of an amplitude modulation envelope.

Suppose a short burst of constant frequency was combined with a time-shifted version of itself; the constant amplitude section in the middle would be a sine wave with its phase half way between the two components. At the end, when one component was falling in amplitude before the other, the resultant wave would shift towards the phase of the larger amplitude waveform; this would result in a phase shift and effectively generate a new frequency that was only slightly different from the original. This is consistent with the theory of sidebands generated by amplitude modulating a sine wave.

The effect could not occur continuously, but it would show up during changes in amplitude. In this acoustic case, the delay was many wavelengths and the decay in amplitude could have been a second or two, during which time the effect was noticed.

Imagine a stack of vertical sheets of reflecting material at a distance. The observer emits a sound and it is reflected off the first sheet. That sheet is removed in a fraction of a second to reveal the second sheet which is a bit further away. The second sheet is immediately removed to reveal the third sheet - and so on. If it was done rapildy enough with many thin sheets, would that not have the same effect as a single moving sheet?

Alternatively, imagine a single reflective sheet moved by a linear stepper motor, so that most of the time it was stationary - that should still produce Doppler shift (albeit with a few other artefacts).

If these two examples are sufficient to demonstrate that actual movement is not the key factor, it is possible that a non-moving arrangement might generate the effect of a Doppler shift in certain circumstances.

In the case I described, the reflection off the first pillar would be dying away when the reflection from the second pillar arrived, so the effect of a virtual moving reflector might be generated by the mixing of two slowly-decaying signals of different amplitude and phase.

-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply)

This is a transient phenomonon at the end of a note. The effect of multiple reflections would be to blend many time-displaced copies of the original, so there would be a lot more cycles coming back (most of them being repetitions of previous cycles). It may not be Doppler shift as originally defined, but it might produce the same effect from a slightly different mechanism.

No. the reflected waves all have the same frequency. These would get added up. This means you add many sine waves with the same frequency and different phase. The sum of sines with the same frequency is always a sine with exactly this frequency. But what you get with this arrangement is that the reflected waves interfere and you get suppression of certain frequencies and amplification of others. The arrangement of pillars creates some sort of metamaterial, acting as a filter.

The note sung is not pure, there'll be some vibrato (FM) and tremolo (AM), so the note has a frequency content either side of its fundamental pitch. The reflected note has been HF cut to some extent so sounds a bit flat. With a congregation, there's less reflected sound.

I just made that up, but is sounds(!) plausible.

Cheers

I found this interesting:

If we're talking electrical engineering, signals-and-systems, there are no new frequencies created by fixed reflectors. But if the issue is psychoacoustics, people might hear frequency shifts when presented with a lot of echoes, especially when the source is complex, like a chior. Echoes can't create new spectral lines, but they can change the shape of a complex spectrum, which people will perceive as an overall pitch change.

Since I'm tone deaf, and don't like music, that's about as much as I can pontificate.

I disagree with the term 'always'.

Suppose a number of sinewave sources were available, each one slightly phase-retarded from the previous one. Fade in Source 1, then fade in Source 2 until it reaches the same amplitude; the phase of the composite wave will be half way between the two. Next fade out Source 1 and the phase of the composite wave will be the phase of Source 2; it will have retarded a little.

Now do the same from Source 2 to Source 3 and so-on until the phase has completed 360 degrees and you are back to Source 1. In the time it has taken you to do all that, the waveform will have lost a complete cycle, even though all the sources were at a fixed frequency. If you could cycle rapidly enough, you would produce a lower frequency composite wave than the frequency of the sources.

The effect occurs without a congregation, but disappears when a congregation is present.

Exactly, the effect disappears because there's much less reflected sound with a congregation and probably the decay time is shorter.

No frequencies are altered, but the relative amplitudes of the many frequencies involved would be, and pitch is a perceptual thing, not quite the same as frequency.

Cheers

If the reflectors are fixed at different positions in space the combined reflections would give one fixed-frequency and fixed-phase resultant under steady-state conditions. If the initial wave varied in amplitude, the amplitude variations would return from different reflectors at different times, so the effect of combining them would be phase (and hence frequency) variations in the resultant sound..

On May 1, 2018, Chris Jones wrote (in article ):

This is exactly how a phased-array radar works, and the idea did come from Christiaan Huygens? wave theory of light.

And a ?chirped grating? can turn a click into a chirp, and vice versa. No new frequencies are generated, but it sounds like there are.

Joe Gwinn

No. An invariant linear transfer function, even with delays, can't create frequencies that weren't in the original source.

A complex time-variant waveform can sound like the overall pitch changes with time, and echoes can change that perception.