Multiple Small Subs - Geddes Approach
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Pan
diyAudio Member

Join Date: Sep 2002
Quote:
 Originally posted by gedlee [B]I still only see the single mode/node discussion as being misleading. Even at a node a source will excite other nearby modes since all modes are excited by all frequencies to some extent.
A 20Hz sine will not excite a 20k standing wave, and a 20k sine will not excite a 20Hz standing wave.

Will 20.1Hz see reinforcement form a 20Hz standing wave? Yes! Will a sub placed in the node of a 20Hz standing wave excite the second order resonance/standing wave at 40Hz when the signal to the sub is shifted from 20Hz to 40Hz..? Of course!

I don't see what this has to do with the things being discussed.

Quote:
 Then there is the direct sound, but careful study will show that this direct sound is in fact the contribution of all the modes (Welti got this wrong in his paper).
The sound that reaches a specific point in a room is the direct sound +/- the reflections and room resonances. Nothing new there.

Quote:
 This goes back to something that I said a long time ago that nobody accepted. Free space has to be thought of as a continuum where the modal density goes to infinity - not zero.
Could the reason be that what you write makes no sense at all?

I think it would be correct to say that high modal density means "tightly packed resonances in the form of standing waves". Standing waves occur between two or more surfaces that trap/reflect the propagating soundwave. Wihout opposing surfaces (or rather no surfaces at all) there will be no such resonances.

Quote:
 Hence, even outdoors in free space the direct field is carried by the modes (which are now infinite in density) just as it is in a small room. But in a small room the modes get sparse and hence the ability for them to carry the energy goes down.

Can you tell us what it is that makes you think this is the case?

Quote:
 The sound from a LF source does not - let me repeat - does not travel in all directions away from the source.
Yes it does IF the radiating source is small compared to the wavelength. If you can show the opposite I'll be glad to see you at the Nobel dinner next year. ;-)

Quote:
 (There is what is called an evanescent wave sent out, but this disapates in time and space exponentialy so it is a very small factor.
And this is something only you are aware of?

Quote:
 The sound wave can travel only in a discrete number of directions defined by the modes that it excites.
Nope! It travels in all direction but the sum of the pressure in a specific point at a given time depends on the reflections and standing waves within the room.

Quote:
 This means that the energy emitted by this source in a real room is not the same as the energy emmitted by this source in free space. This can be seen in the radiation impedance for a small room which is NOT the same as that for a source in free space.
Yes it is , more or less. An electrodynamic driver sub is controlled by the compliance and the mass of the driver and the box it is mounted in. Acoustic impedance has little effect.

Quote:
 People want to think that you take a free space source and bring it into a room and that it emites sound the same way, but that this gets amplified by the modes at certain frequencies. This is not correct. The presence of the room changes everything and not until the source sees a high model density does it begin to behave as it does in free space.
Again this makes no sense at all. Since you claim things not known to the rest of the world and not backed by known physics maybe it would be in order to back your claims up by a rational explanation, a model or some measurements??

/Peter

 13th December 2008, 10:40 PM #202 doobius   diyAudio Member   Join Date: Aug 2008
FrankWW
diyAudio Member

Join Date: Jul 2004
Location: n/a
Earl,

Quote:
 Free space has to be thought of as a continuum where the modal density goes to infinity - not zero. Hence, even outdoors in free space the direct field is carried by the modes (which are now infinite in density) just as it is in a small room. But in a small room the modes get sparse and hence the ability for them to carry the energy goes down.
In other words, a room, whatever size, is a modal filter?

Thus, a very large room filters fewer modes than a very small small room?

Quote:
 This means that the energy emitted by this source in a real room is not the same as the energy emmitted by this source in free space. This can be seen in the radiation impedance for a small room which is NOT the same as that for a source in free space.
Quote:
 The presence of the room changes everything and not until the source sees a high model density does it begin to behave as it does in free space.
That is, the source will start to see an impedance similar to what it sees in free space?

 13th December 2008, 11:10 PM #204 HiFiNutNut   diyAudio Member   Join Date: May 2004 Location: Sydney Good to read all the debates / discussions. With all this enormous amount of information I still feel like I am unable start doing it. You don't need to convert me now as I am already converted. I have opted to using Geddes multiple sub approach. I am just right now designing my own PCB on veroboards after having just completed the sub woofer EQ, allpass filter (worked out 6-160dgree phase shift by turning a 100k trimpot) on LTSpice. What XO and slope? This is the question asked many times and nobody has given an answer, or the answer would be like "they are selected based on the result of measurements". Well it is true, but ideally some samples can be given to serve as a guide. I am considering these factors: 1. A typical room has the model region 35Hz-200Hz. 2. The subs should cover as much the model region as possible. 3. We can localize sound source starting from 150Hz. 4. Gentle slope (like LR2, BW2) may be better than steep slope (like LR4) because the model region has a transitional period and we want to maintain a smooth frequency response. If we can localize sound source starting from 150Hz, that means the subs should be cut off at 150Hz. And we know, unless the SPL is at least 30dB down, it would have an influence on the main speakers. Suppose we have a 2nd order LP at 50Hz, at 100Hz it is -12dB, at 200Hz it is -24dB, at 400Hz it is -36dB. This may hide the sub and make it not localizable. But it would not have sufficient output from 100Hz to 200Hz to work with the room modes. Suppose we have a 2nd order LP at 100Hz, at 200Hz it is -12dB, at 400Hz it is -24dB. The sub would probably have the right amount of output to smooth the model region but obviously, it is very localizable. This points me to BW3 and LR4. However, since Dr. Geddes recommends using commercial subs, and most of the commercial subs support only 2nd order LP, I can't see how the job can be done. Regards, Bill
gedlee
diyAudio Member

Join Date: Dec 2004
Location: Novi, Michigan
Quote:
 Originally posted by markus76 Looks like you had access to the ominous journal describing practical methods to achieve cold fusion and world peace.

No - I did a PhD thesis on the topic and learned a couple of things along the way.
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gedlee
diyAudio Member

Join Date: Dec 2004
Location: Novi, Michigan
Quote:
 Originally posted by FrankWW Earl, In other words, a room, whatever size, is a modal filter? Thus, a very large room filters fewer modes than a very small small room? That is, the source will start to see an impedance similar to what it sees in free space?

Correct as the room gets larger and larger its modal density increases until eventually it becomes free space. I'm not sure that I would use the term "filter" - I actually thought of using it before, but then I declined as its not quite the same thing.

You are precisely correct about the source impedance.
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Earl Geddes Gedlee Website

gedlee
diyAudio Member

Join Date: Dec 2004
Location: Novi, Michigan
Quote:
 Originally posted by HiFiNutNut What XO and slope? This is the question asked many times and nobody has given an answer, or the answer would be like "they are selected based on the result of measurements". Well it is true, but ideally some samples can be given to serve as a guide. This points me to BW3 and LR4. However, since Dr. Geddes recommends using commercial subs, and most of the commercial subs support only 2nd order LP, I can't see how the job can be done. Regards, Bill

The Xo location can only be determined in-situ, but the slope can be decided. I would use 2 nd order even if I were designing the system myself. I have not found localization issues in any bandpass sub usage, but then these, acoustically, end up being 4th order LP. But that probably does not happen arround the cutoff since it would be unusual that the elctrical filter and the acoustic one were at the same frequencies. Thats why, to me, 4th order is too sharp if it designed such that all four poles are nearly the same frequency. Maybe thats why I like bandpass and 2nd order - eventual sharp cutoff, but more gradual at the cutoff.
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Earl Geddes Gedlee Website

 14th December 2008, 04:33 AM #208 HiFiNutNut   diyAudio Member   Join Date: May 2004 Location: Sydney Dr. Geddes, I use sealed boxes and I guess I could shift the poles of the 4 order LP and make a BW2 simulating the rolloff of the bandpass box and another one standard BW2, which is the 2nd order XO you referred to. So what is the usual bandpass rolloff frequency of your subs? I guess that the precision of rolloff does not matter because it is virtually impossible to match the SPL and phase of the drivers with the numerous room modes. If random placement of subs works it simply suggests that none of the subs need to conform to any particular response and phase curves, but the random effects would make the final response flat. I guess that a first 2nd order BW2 LP may be placed around 60-80Hz and a second 2nd order LR2 at around 175Hz, and that would probably work. If the XO points needs to be adjusted then it would be the first one. What do you think? Regards, Bill
gedlee
diyAudio Member

Join Date: Dec 2004
Location: Novi, Michigan
Quote:
 Originally posted by HiFiNutNut Dr. Geddes, I guess that the precision of rolloff does not matter because it is virtually impossible to match the SPL and phase of the drivers with the numerous room modes. If random placement of subs works it simply suggests that none of the subs need to conform to any particular response and phase curves, but the random effects would make the final response flat. What do you think? Regards, Bill

I think that this is well said.

I wish that others were as perceptive and had this level of understanding. Its hard for "deterministic" people to understand the world of "random". It took the world nearly 50 years to accept Quantum Mechanics. Many still don't accept it. Einstein never did.
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 14th December 2008, 07:33 AM #210 soongsc   diyAudio Member     Join Date: Mar 2005 Location: Taiwan I had a very interesting experience. A parade when by while I was by the side of the road. lots of drums of various sizes. The sound was not particularly loud to me, but there was one type of drum that made my chest vibrate along with it. It was not the drums with the lowest frequency causing this. I have never noticed this kind of experience in a room even when I played back different individual frequencies. __________________ Hear the real thing!

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