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Old 4th May 2009, 02:47 PM   #1101
gedlee is offline gedlee  United States
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Location: Novi, Michigan
Quote:
Originally posted by xpert

Alas, nowbody will gain an insight into signal theory by cutting MDF boards. It isn't meant to be harsh, by the way. I really would appreciate someone competent in acoustics solve the 1-dim wave equation with a source somewhere between lossy boundaries. Morse? Any handy formula?

cheers
Insults are unlikely to lead to the answers that you desire. The problem that you speak of is trivial and solved in many texts - including my own. That you don't know this is not very supportive of your claims to superior knowledge.

I would like to ask that we get off of this discussion of mathematical details to the main topic, since none of this is constructive.
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Old 4th May 2009, 03:04 PM   #1102
xpert is offline xpert  Afghanistan
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Quote:
Originally posted by gedlee


Insults are unlikely to lead to the answers that you desire.
The problem that you speak of is trivial and solved in many texts - including my own. That you don't know this is not very supportive of your claims to superior knowledge.

I would like to ask that we get off of this discussion of mathematical details to the main topic, since none of this is constructive.
Mr. Gedlee,

Won't You please admit an error regarding frequency shift with decay ? Is that the insult You mention? You Yourself asked for a discussion as cited above.

I never ever claimed "superior knowledge". It's a trival rethoric figure with wich You try to insult me by irony.

Thank You.


In the end the topic is done, isn't it? On phase distortion I suggest the following - free - reading:

http://www.genelec-ht.com/documents/...ns/IOARP21.pdf

Thank You
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Old 4th May 2009, 03:42 PM   #1103
gedlee is offline gedlee  United States
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Quote:
Originally posted by xpert

Won't You please admit an error regarding frequency shift with decay ? Is that the insult You mention? You Yourself asked for a discussion as cited above.
I was refering to "Alas, nowbody will gain an insight into signal theory by cutting MDF boards." which seemed directed at me since this discussion is between us and no one else.

I do not admit to any errors. I thought that I made that clear. There IS a frequency shift in a decay, thats clear. What you seem to be saying (and its never been clear to me) is that I am in error because the decay is not at a "pure tone" but occurs over a spectrum. That is correct, (everything is a spectrum!) but does not change the fact that the pure tone of excitation disappears and what appears in its place is a "spectrum" which has peaks at the resonance frequencies the widths and heights of which depend on the damping and the relationship between these frequencies and the driven frequency. This is what I said and this is what happens.
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Old 4th May 2009, 03:57 PM   #1104
xpert is offline xpert  Afghanistan
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Quote:
Originally posted by gedlee

I do not admit to any errors. I thought that I made that clear. There IS a frequency shift in a decay, thats clear. What you seem to be saying (and its never been clear to me) is that I am in error because the decay is not at a "pure tone" but occurs over a spectrum. That is correct, (everything is a spectrum!) but does not change the fact that the pure tone of excitation disappears and what appears in its place is a "spectrum" which has peaks at the resonance frequencies the widths and heights of which depend on the damping and the relationship between these frequencies and the driven frequency. This is what I said and this is what happens.
Earl,

I gave it up earlier. If You don't see that very mistake yet I'm not Jesus to make You see by wonder. You ever gave additional input, e/g the homogenious DifEq. You DO know that the non trivial solution is a spectrum, and that this spectrum is temporally constant irregardless of its starting state, isn't it? No shift! Otherwise it was non linear ... I never tried to insult You. I simply do not take it personally.

To end it up, I really appreciate Your algorithm on optimizing in room bass response. An algorithm is defined as a correct methodology that end with a propper result. Why should I long for more? The yield is mostly equivalent to optimum, so what? But as You knock on peoples heads quite often when they babble audiophile BS, I felt free to gave in the first time a very friendly hint. Regarding some misleading wording. Don't take it to serious, ain't worth it.

so long

a more recent publication on the related side topic

http://www.genelec-ht.com/documents/...aes116th_2.pdf
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Old 4th May 2009, 07:35 PM   #1105
gedlee is offline gedlee  United States
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I don't think that the Genelec papers tell us anything that is surprising or new, their results are to be expected from simple psychoacoustics, but its good to see people doing real work in the area.
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Old 4th May 2009, 08:46 PM   #1106
JPV is offline JPV  Belgium
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Quote:
Originally posted by gedlee


I was refering to "Alas, nowbody will gain an insight into signal theory by cutting MDF boards." which seemed directed at me since this discussion is between us and no one else.

I do not admit to any errors. I thought that I made that clear. There IS a frequency shift in a decay, thats clear. What you seem to be saying (and its never been clear to me) is that I am in error because the decay is not at a "pure tone" but occurs over a spectrum. That is correct, (everything is a spectrum!) but does not change the fact that the pure tone of excitation disappears and what appears in its place is a "spectrum" which has peaks at the resonance frequencies the widths and heights of which depend on the damping and the relationship between these frequencies and the driven frequency. This is what I said and this is what happens.

Let's consider a second order system underdamped ( typical of acoustical systems)

When the force function is switched off, what remains is of course the transient. In this case, it is easy to calculate analytically and any text book will show that this transient is a exponentially decaying sinusoid depending only on the system and the initial conditions and not on the force function except for the initial amplitude which depends on the value of the force function when it is switched off.

This decaying sinusoid is not a pure tone from a spectrum point of view of course but the decaying sinusoid is at a fixed frequency not related to the force function frequency.

This fixed frequency in not the natural frequency of the underdamped system but is the natural frequency times a coefficient depending on the damping ratio z of the system.
This coefficient is sqrt ( 1-z²). Therefore if the system has a very low damping ( z<<1), this frequency is very close to the natural one. If the system is close to critically damped ( z=1), then the decaying sinusoid will be at very low frequency with respect to the natural frequency of the system.


JPV
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Old 5th May 2009, 12:16 AM   #1107
gedlee is offline gedlee  United States
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Quote:
Originally posted by JPV

This decaying sinusoid is not a pure tone from a spectrum point of view of course but the decaying sinusoid is at a fixed frequency not related to the force function frequency.

This fixed frequency in not the natural frequency of the underdamped system but is the natural frequency times a coefficient depending on the damping ratio z of the system.
This coefficient is sqrt ( 1-z²). Therefore if the system has a very low damping ( z<<1), this frequency is very close to the natural one. If the system is close to critically damped ( z=1), then the decaying sinusoid will be at very low frequency with respect to the natural frequency of the system.


JPV

I agree completely but left off the part about the shift in natural frequency because of damping (this is usually a small effect for acoustic room modes, not so for mechanical systems). The point is that the decay is not at the excitation frequency but at the "resonances" which in room acoustics are the damped modes. In acoustics the difference in natural frequency and the damped frequency are never considered because the modal frequency needs to include the boundary conditions which will always include the damping since all damping in room acoustics occurs at the boundary. Short of this subtle difference I agree completely.
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Old 5th May 2009, 07:07 AM   #1108
JPV is offline JPV  Belgium
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Quote:
Originally posted by gedlee



I agree completely but left off the part about the shift in natural frequency because of damping (this is usually a small effect for acoustic room modes, not so for mechanical systems). The point is that the decay is not at the excitation frequency but at the "resonances" which in room acoustics are the damped modes. In acoustics the difference in natural frequency and the damped frequency are never considered because the modal frequency needs to include the boundary conditions which will always include the damping since all damping in room acoustics occurs at the boundary. Short of this subtle difference I agree completely.
I understand. This is the difference between lumped parameters with differential equation and only influence of initial conditions and distributed one with partial differential equation that must include boundary conditions.

But at low frequencies in small rooms, the acoustical system is more like a lumped parameter one and there, don't you think that this damped resonance frequency effect is existing?


JPV
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Old 5th May 2009, 10:31 AM   #1109
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Five days of "quién es más macho".
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John k.... Music and Design NaO Dipole Loudspeakers.
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Old 5th May 2009, 11:11 AM   #1110
JPV is offline JPV  Belgium
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Quote:
Originally posted by john k...
Five days of "quién es más macho".
I don't want to show beeing macho. I am only an amateur in this interesting field.
But I asked this question because at low frequencies in small rooms it is said that there is nearly no damping. Therefore introduction of a little bit of damping could ( if what I thought is right about a more mechanical system at low frequencies) decrease the decay but also decrease the damping frequency of the transient and shift it to less audible. This must have been experimented I suppose.
It is a reason more to try to damp the listening room at low frequencies. Because this is also the living room, the question is if any small trial there will be worthwhile.

This is in fact my real question.

JPV
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