A clap or snap approximates an impulse, and an impulse includes multiple, theoretically all, frequencies. But the corrigated wall creates no new frequencies.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
In steady-state conditions, the reflections will still be the same. If you change the amplitude and/or phase of the source, it is not more in steady state.
You can create a frequency change with two sources at the same frequency but phase-shifted (ideally by 90 degrees) and change the amplitudes of the sources with another pair of signals on a different frequency and phase-shifted by 90 degrees. This creates a complex multiplication of the two frequencies, creating a sum or difference frequency.
-- -TV
The acoustic conditions in that abbey are ideal for creating multiple overlapping phase-shifted echoes of the original sound. At the end of a note, the conditions are no longer steady state, so this then fulfils all the conditions of your explanation.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
Under steady-state conditions this is true, but when the amplitude varies there will be sidebands created. The resultant is a combination of single identical frequencies of different phases which change individually in amplitude and cause the phase of the resultant to shift. The regular spacing away from the choir of the reflecting surfaces (pillars) gives a smooth transition of phase from one reflection to the next as the decreasing amplitude of the original sound reaches each reflector in turn.
I would be very surprised if this was just a psycho-acoustic phenomenon; my friend has an exceptionally good ear for real pitch and has identified other effects in the past which have turned out to have a solid factual basis.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
Given a signal source, passing it through a nonvariant linear transfer function will not cause frequency shifts or create new spectral lines.
But ears and brains are nonlinear.
"A good ear for real pitch" is fundamentally psycho-acoustic. He is not a spectrum analyzer.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
so check it with a spectrum analyzer. there are spectrum analyzer apps that can run on your phone m
An impulse reflected from a grating will generate a cyclic wave. If the source of the impulse is close to the grating such that the angle varies with distance, you'll get a chirp.
-- Grizzly H.
No.
As long as the source and listener are steady and the source frequency and amplitude stay put, there are no new frequencies, regardless how many echoes you combine. It will still stay as a linear operation, and the result is a single frequency at the original frequency, but its amplitude and phase will be different from the original.
-- -TV
This is not a nonvariant system, the choir was modulating the amplitude before the signal was launched into the reflecting area. My friend said she only noticed the effect on the reverberation tails, not during the steady sounding of the note.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
The critical points are:
The amplitude was changing and the effect was only audible on the reverberation tails when the conditions were not steady-state.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
I said that the transfer function was invarient, not the signal source. The linear transfer function could be the time-invariant acoustic properties of a non-moving building.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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The linear transfer function of the building in isolation may have been invariant but the building was only part of the system. The effect occurred when the choir stopped singing a note, so they were part of the transfer mechanism too. Their amplitude modulation seems to have caused the transfer function of the building to exhibit properties that would not have shown up on a purely steady-state signal.
Transfer functions that include time delays can give very complex effects on transients, especially when the delays are equivalent to many wavelengths of the signal.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
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Try drawing the waveform that would be produced by such a stepwise moving r eflector and I think you will see it is *nothing* like a signal frequency s hifted by a moving reflector. It will be of a single frequency, but phase shifted by varying amounts in a stepwise fashion. If the steps are reduced in time duration as well as distance, then, yes, they will approach the co ntinuously moving object. Your point that the object is not moving is not particularly useful because the reflector *is* moving, but in a stepwise fa shion. The reflected waveform only becomes similar to that reflected from a continuously moving reflector when the motion approaches that of a contin uously moving reflector.
Regardless, none of this is relevant to the case at hand of multiple statio nary reflectors.
Rick C.
This is a passive, linear system. Using terms like amplitude modulation, without some non-linearity, isn't something we are going to see.
If you can show how some form of linearity can be established then that would be most helpful.
I have already remarked how missing frequency components, through selective filtering, can cause the perception of a change in pitch. That mechanism doesn't require any non-linearity.
A very good example is the Shepard tone. No amplitude modulation, no sidebands, just filtering.
-- Mike Perkins Video Solutions Ltd www.videosolutions.ltd.uk
Agreed
I didn't think that was the case put forward by the OP.
-- Mike Perkins Video Solutions Ltd www.videosolutions.ltd.uk
^^^^ Correction non-linearity
-- Mike Perkins Video Solutions Ltd www.videosolutions.ltd.uk
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And what is your point? You can use a series of infinitely short pulses of some sizes, each one with a frequency content of an impulse (infinitely br oad) and it could approximate any signal you wish. As in your example, the se components are not a fixed frequency. By ramping them up and down you a re altering their frequency content.
Rick C.
I vaguely remember reading about non-harmonic distortion in strings (musical instruments). IIRC the gist of it was that a note close to the harmonic of another string would start that string vibrating and this would be measured as non harmonic distortion.
IFF there was something in the area that could vibrate at close to the frequency of the singer this *might* be audible. Adding a large amount of meat into the space dampens the acoustics and might be responsible for the inability to hear the sound when the room was occupied.
Just a thought though - I have never seen it in practice.
-- Using Opera's mail client: http://www.opera.com/mail/
In message , Adrian Tuddenham writes
Higher frequencies are attenuated more with distance, so the ensemble average pitch of a reflected complex sound will sound lower. As an aside, that's why bagpipes sound better from a distance.
It may also be that the higher frequencies are not reflected as well, giving the same effect.
Ask your friend to whistle a single short tone and see if the reflection shifts . Be aware that some sects consider whistling in a church a sin, so she might be safer to borrow a recorder and play a short sharp note.
Brian
-- Brian Howie
The resultant sound was a mixture of several different reflections, each with a different (relatively long) time delay. During steady state, the resultant would have the same frequency as the source and a phase which can be taken as 'reference'.
At the end of the sound, as the first reflection begins to die down, the second reflection will take precedence and the phase will shift slightly towards that of the second reflection, which is likely to be different from the first. As the second reflection dies, the phase will shift to that of the third reflection and so-on. Thus a perfectly linear system with delays is capable of producing phase shift during a transient (the end of a sound) and if these phase shifts are all in the same direction they will correspond to a frequency change.
The building does not disobey anything we may have been told about a linear system because the 'rules' we were taught only apply to a steady state. The non-linearity is coming from the choir when they stop singing; that is a form of amplitude modulation, a non-linear process.
-- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
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