Multiple Small Subs - Geddes Approach

[gedlee]

Originally posted by john k...


Earl,

What I was saying about MP having noting to do with decay was in regards to a system undergoing forced oscillation at some frequency and then when the forcing function is removed it will decay with oscillation at its natural frequency if the system is sufficiently underdamped. That characteristic is independent of whether the system is MP or not. That's all.

John

Seemed like you were saying more than that at the time.

xpert:

Your arguument is not concrete enough for me to discuss. I have made my argument by simply pointing out the discussion in Morse. You seem unwilling to go there to understand the points made there so I don't see any point in continuing. If you read Morse, and can find an error, or a point of disgreement, then I would be happy to discuss it with you. But I don't think that it is my job to explain it to you when it is readily available (and something that anyone who wants to argue acoustics should have read already - Morse is a (the) fundamental text on the subject).

[xpert]

Originally posted by gedlee
xpert:
Your arguument is not concrete enough for me to discuss. I have made my argument by simply pointing out the discussion in Morse. [/B]

Sorry Earl,

I won't discuss a clear misunderstanding of Yours without Your collaboration. You've got Morse wrong all to clear. You don't differ to much from audiophiles in using matrial wording. But You avoid to answer to concrete queries on what Your talking is all about. Thus Your thinking remains arcaine. Wellcome to the club.

I will cite myself, and - give up.

s far as I have understood Morse from Your very (brief) citation, he does NOT support any of Your statements. First of all, what is decay within a cavity described by eigenmodes (may be) and some sort of damping on the reflective surfaces? Do You try to derive Your claims from an understanding of that situation? What is different compared to, example given a one dimensional oscillator mass/compliance type, electric circuit bild up with reactive impedances or much simpler an organ pipe? The concept of eigenmodes encourages to look at the whole systems as a sum of parts that superimpose linearly. Do You try to tell us, that the eigenmode concept is not valid here? To only flutter with some papers/books is not very kind. Do You think john_k does understand Your objections? If not why not try to explain the basic concept behind Your doubt.

[JPV]

Originally posted by gedlee


I'll stand by my statement and claim that it is supported by Morse. If you need more information to understand it then LOOK IT UP.

Is it not correct to say that the output of even a simple electrical lumped constants system with resonnance at 51hz will drift from 49 to 51 hz if the 49 hz input will be switched off. The switching off ( multiplication by a step function) is broad band and will contain enough energy at 51 hz to see a 51 hz decaying transient if the resonnance is sharp enough. This drift is not specific to acoustics but is pehaps more important in a system with distributed constants and propagation.


JPV

[gedlee]

Originally posted by JPV


Is it not correct to say that the output of even a simple electrical lumped constants system with resonnance at 51hz will drift from 49 to 51 hz if the 49 hz input will be switched off. The switching off ( multiplication by a step function) is broad band and will contain enough energy at 51 hz to see a 51 hz decaying transient if the resonnance is sharp enough. This drift is not specific to acoustics but is pehaps more important in a system with distributed constants and propagation.


JPV

I thought about this exact example myself and I will say that I believe that you are correct. However I'm not a EE so thats not my expertise. On the other hand, the acoustics I know well and I am 100% confident in my position and won't be goaded into an argument that makes no sense. It seems likely to me that ALL systems would have to decay at their resonant frequencies when the excitation is turned off. The excitation simply become the inital conditions for an unforced problem (i.e. a homogeneous Diff Eq.) All homogeneous Dif EQ solutions decay at the natural frequencies since there is no driven frequency.

[xpert]

Originally posted by JPV


Is it not correct to say that the output of even a simple electrical lumped constants system with resonnance at 51hz will drift from 49 to 51 hz if the 49 hz input will be switched off. The switching off ( multiplication by a step function) is broad band and will contain enough energy at 51 hz to see a 51 hz decaying transient if the resonnance is sharp enough. This drift is not specific to acoustics but is pehaps more important in a system with distributed constants and propagation.


JPV

Hi,

It might have been due to my bad English that Mr. Earl Geddes didn't get my objections right. He did get Yours wrong too.

The clue here is, he forgot about the Fourier integral. That not only the "switch" is a broadband signal but also the decay itself has to be described as broadband. Depending on the Q, not only on the switch.

Geddes: "All homogeneous Dif EQ solutions decay at the natural frequencies since there is no driven frequency."

If it does decay, losses are part of the DifEq. Then the solution is a Fourier integral instead of a spot frequency. That easy.

Replace "at" by "around", eliminate the swift idea, replace it by convolution of broadband signals with a transfer function.

For me I found out, that I don't know how to solve the wave equation with lossy walls if the systems is more than 0-dimensional. The losses have to be taken into consideration right from the start because a possible steady states is an equilibrium between losses and source. Losses and source seperated. Simulation is easy, but a handy formula? Any input?

Thank You

[Rudolf]

Originally posted by xpert

Geddes: "All homogeneous Dif EQ solutions decay at the natural frequencies since there is no driven frequency."

If it does decay, losses are part of the DifEq. Then the solution is a Fourier integral instead of a spot frequency. That easy.

Replace "at" by "around", eliminate the swift idea, replace it by convolution of broadband signals with a transfer function.

Xpert,
I have to admit that I don´t fully understand the mathematics behind this decay process, but I even less understand the problem you have with Earl Geddes argument.

If I remember correctly this whole discussion started with some CSD which showed ringing to change from one frequency to another within a short time segment. It goes without saying that this change cannot take place abruptly in the moment, when the excitation has been switched off, but has to take some time. Laws of physics would not allow otherwise. In the light of this your demand to <Replace "at" by "around"> would be purely cosmetic. Everybody following this discussion from the cited CSD on will have been aware of this transitional situation.

If this is not what you are talking about, please just forget about my objection.

Rudolf

[john k...]

I think you are all arguing over semantics and not physics.

[BHTX]

Originally posted by john k...
I think you are all arguing over semantics and not physics.

Exactly. I can't believe I wasted this much time trying to figure out what the hell was going on.

[xpert]

Originally posted by Rudolf


I have to admit that I don´t fully understand the mathematics behind this decay process ... discussion started with some CSD which showed ringing to change from one frequency to another within a short time segment ... Laws of physics would not allow otherwise ...

Sorry to say, that all isn't about cosmetic or semantics. It is about some basics that should be understood in the first place. A thoroughly used wording should reflect that comprehensive understanding.

That said very homogenious DifEq has in case of losses - that is here so by definition - a solution that isn't a single frequency but a spectrum. Given a starting condition that is not null the spectrum will be constant over time, no shifts, period.

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

[xpert]

Originally posted by BHTX


Exactly. I can't believe I wasted this much time trying to figure out what the hell was going on.

Hi,

The Geddes approach for smooth low bass response is worth it. And he asked for this discussion that You couldn't figure out:

Originally posted by gedlee

Well you are both wrong. Morse is one of the formost Physicists of the 20th century and doesn't get things like this wrong. Read it in his book - Morse, P.M., "Vibration and Sound" - last page of the book pg. 429 Do a "further analysis" if you dare! If you have the capability to do that! Show its wrong or recind your comments!

As I posted before, no offense. But I couldn't rescind. For me the outcome isn't to bad:

To really calculate all that decaying modes within a room is a challange. An algorithm to bettern the acoustic situation by trial is worth a thought. To validate Geddes' algorithm it should be tested in daily life. As far as I saw here it performed very well if not perfectly, not at least budget wise.

Why talk about "phase" issues at all?! From psycho acoustics it can be shown that stuff as such is by far irrelevant here. But IF one talks about it, one should avoid sloppy wording.

so long