xpert said:
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.
gedlee said:
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/publications/IOARP21.pdf
Thank You
xpert said:
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.
gedlee said:
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/publications/aes116th_2.pdf
gedlee said:
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
JPV said:
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.
gedlee said:
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
john k... said: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
john k... said:
Hi JPV,
Actually I wasn't referring to your posts. They were just what I had considered posting. A light of sanity. I was just amused that this comes down to agreeing with what I posted 5 days ago on page 42.
That's the problem with Inet discussions. The feed back loop has too long a delay. Sitting around a table and this would have ended before it began.
JPV said:
Yes, of course there is a shift, its just that this is usually not seperated out as a factor in acoustics. Its just part of what we call "resonance", which are "eigenvalues" and always contain the damping.
john k... said:Five days of "quién es más macho".
We all know how smart you are John and how you know all of this, but this kind of comment does nothing to add clarity to the discussion. A gentleman got confused and wouldn't let go. You added to the confusion with your comments, but you did nothing to help clear it up.
john k... said:Five days of "quién es más macho".
john k... said:
Hi all,
Mr. Geddes still won't admit his mistake (wording?) regarding the gradual shift whilst decay. There is no gradual shift. He asked for a discussion that he got. It has been shown from different perspectives that he was (is) fairly wrong.
Now courtesy of JPV we have found a new"shift". It should be emphazised that this new shift has no connection to the "shift" Mr. Geddes claimed for decaying room resonances.
I would expect appologies for in some way insultant suggestions to read the books, including the gedlee litrature. But I'm afraid it's a hopeless attempt. Style ain't a must in science, but honor is.
Thank You
JPV said:
Oh most deffinately. If you read my work you will find that I believe that LF damping is the single most important thing that you can do to a room to improve its sound. Multiple subs help, but nothing truely makes the sound field smooth like damping. Its just very difficult to get LF damping, especially without incurring too much HF damping. It has to be done within the structure itself. No add-on can really do this.
gedlee said:
There are several types of solutions that works very well.
- Helmholtzabsorbers
- Quarter wave absorbers
- Resistive/reactive devices like Tubetraps
- Membrane/panelabsorbers
I have used the helmholtz and membrane type and IME the membrane type is more effective in relation to its size.
I have used eight pcs. of the RPG corner traps for many years but even two of them makes a clear difference in quality if placed in the right position. The front is 60x60cm so they are not very big.
So an add on can and will do it and do it very effective indeed.
Now if you got the means to build these function into the structures of the room that's of course nice but many can not do that and then the various kinds of bass absorbers is a very good way towards high performance audio for those who rent their appartment or setting up a temporary project studio for example.
/Peter
JPV,
I would reference you to the works of both Geddes and Toole regarding damping and the proper way to do it. Resistive membranes which are the most common on the market attenuate the upper registers much better than the low ones. An ideal situation, like Dr. Geddes mentioned, is in structural pieces which will allow your walls to flex. If this is done you keep a live room while damping the low end. This is not what most people try to do in their rooms these days but all the evidence supports having a live room which the common absorbing materials kills.
I would reference you to the works of both Geddes and Toole regarding damping and the proper way to do it. Resistive membranes which are the most common on the market attenuate the upper registers much better than the low ones. An ideal situation, like Dr. Geddes mentioned, is in structural pieces which will allow your walls to flex. If this is done you keep a live room while damping the low end. This is not what most people try to do in their rooms these days but all the evidence supports having a live room which the common absorbing materials kills.
gedlee said:
http://www.rpginc.com/products/modexplate/
Based on http://www.ibp.fraunhofer.de/akustik/ra/vpr/index_e.html
Best, Markus