Multiple Small Subs - Geddes Approach

[Pan]

Originally posted by jason_watkins
A sort of mushy thump. Not sure of your point.

A a short transient signal that will excite resonances.

In responce to Janneman:

The transient is a high-freq component and will not go through the sub anyway. The transient will go to the high freq driver. The transient will not excite the room modes.

/Peter

[jason_watkins]

I tire of semantic debates. Transients have bandwidth, you can't make blanket statements without referring more specifically to what you're talking about. A gunshot will clearly have little to no content in the subwoofer region.

Transient response is a property of a system, one that modes in the subwoofer frequencies have minimal impact on.

[Pan]

As for EQ'ing room response. Yes it works to some degree since the EQ will be a inverse function more or less of the room resonance which will correct both time and frequency. No compressor is needed for this.

Problem with EQ is that you can not get the nulls up by applying gain. The amplifiers will be severly taxed and loudspeaker distortion will be excessive. And also you will get horrible peaks at other locations than the one you are correcting for.

The only way to avoid nulls is the use of multiple sources or absorbtion in the room. This will reduce/eliminate the build up of the resonances in the first place.

Practicall experience has shown that a combination of multiple sources, absorbers* and some EQ gives excellent results.. or so I read in a old suspect paper in some foreign language that mysteriously went up in smoke jsut as I had finsihed reading it.. :-)

*Absorber can be tube traps, helmholtz traps, panel/membrane absorbers or of course suitable design of the elements that makes up the listening room itself.

Someone did critique my approach since "four subs can't deal with all modes" or something like that, well we don't necessarily need "perfect" results, just good enough to get the systems performance up to some level. Some modes can be avoided by adjusting teh listening positions for example and again, the last bit may be EQ'd.


It's also worth to mention that the peaks (as always) is much more annoying than nulls. So by only using an EQ with a standard set up, taking down the extreme peaks will give much improved results.


/Peter

[Pan]

Originally posted by jason_watkins
I tire of semantic debates. Transients have bandwidth, you can't make blanket statements without referring more specifically to what you're talking about. A gunshot will clearly have little to no content in the subwoofer region.

Transient response is a property of a system, one that modes in the subwoofer frequencies have minimal impact on.

Well that's exactly my point in my response to Janneman! ;-)

If not transients is applicable to woofers one might 1) wonder why the whole business is talking about transient response of various LF alignments and 2) where we draw the limit for transient vs. non-transient.

Shoud we say 179.2Hz? ;-)


/Peter, not very found of semantics either!

[gedlee]

Originally posted by Pan


If some people could drop the prestige, stop taking it personal and accept facts when it is presented and realise they were wrong I think the climate would be better on this forum. What I see is several members of the forum dropping out of the discussion (or changing subject) as soon as someone points out errors in their reasoning. Better would be to continue the discussion and sort out the misunderstanding for the benefit of all readers.

Peter

Peter

IMO you are a major source of the problem here. As I have pointed out before, I will not respond to your posts because of your attitude. You could start by not tying to push the problem onto other people and learn to show some respect. Of all the people who post here it is you that I find to be the most inaccurate and misleading and yet you speak with such authority. I have trouble resolving this in my mind. Either I misunderstand the situation or you do. I'll leave that for you and the other readers to sort out.

As to the use of multiple subs, Youngho and a few others have it exactly right

I think that Geddes' approach is the most practical and cost-effective method for achieving smooth bass (yes, better than Welti's, yes, cheaper than BassQ) and it's difficult if not impossible to argue with the results that have been presented with the current approach. It would be cool to incorporate some brute-force method a la SFM for calculating the optimal phase and volume settings for each subwoofer in the future, but this is DIYaudio, right?

There is not much that I can add here except that if you don't agree with the above statement then you misunderstand something.

[markus76]

youngho, I guess you misunderstood my comments.

Originally posted by youngho
"How should multiple subs decrease the decay time?" (frequency and time domains of bass are related because of minimum phase phenomena-like behavior of bass, page 241, 243, 244)

Originally posted by youngho "You can't and you don't want to avoid the excitation. The decay time itself stays the same" (mode cancellation and node placement can address this minimum phase phenomena, 244)

I should have written "rate" instead of "time". If you switch of the excitation signal all modes will still be there. They will decay at the same rate independend of the excitation signal's level.

Originally posted by youngho "[multiple subs] don't change a room's modal field" (they can through mode cancellation, page 224, 225, 229)

The modal field is an attribute of a room's physical properties. You can couple energy to modes in different ways so the frequency response becomes smooth but you can't get rid of modes (without absorption).

Originally posted by youngho "In a real room there will be no such thing as a not excited mode" (there is when a subwoofer is placed in the node of a mode, as with midwall or quarter-wall placement, or when two subwoofers are placed in such a way to provide mode cancellation, page 221, 224, 225).

I'm always looking at ALL low frequency modes up to let's say 100 Hz. If you "cancel" one mode you will excite another one.
Talking of "cancellation" and Toole, take a look at the picture on page 221. When you place a sub at a null, that particular mode is "cancelled". But that means that we will hear nothing at all. I don't think that this is what we want. What we want is to smooth the whole low frequency band for a given listening position or area.

Best, Markus

P.S. Still searching for the journal describing practical methods to achieve cold fusion and world peace.

P.P.S. I have the feeling that some people do not know that modes can be found in the whole audible frequency band and not only at low frequencies. In fact there are thousands of modes above the Schroeder frequency.

The number of modes in an idealized room measuring 4m x 5.2m x 2.4m:

[Pan]

Originally posted by markus76

I should have written "rate" instead of "time". If you switch of the excitation signal all modes will still be there. They will decay at the same rate independend of the excitation signal's level.

Markus, if you drive woofers positioned for maxium cancellation of a standing wave between two surfaces the mode will NOT be there, it is NOT excited. :-)

The modal field is an attribute of a room's physical properties. You can couple energy to modes in different ways so the frequency response becomes smooth but you can't get rid of modes (without absorption).

Again that is not correct. When you use multiple suibs you can view the room as being divided into several new small rooms.. sort of. Makes sense?

You can not get smooth frequency response wihtout improvement in the time domain. Time domain and frequency domain are the same thing looking from two different angles when we deal with minimum phase devices.

I'm always looking at ALL low frequency modes up to let's say 100 Hz. If you "cancel" one mode you will excite another one.

Nope! With a corner placement of a single sub you excite all axial modes, by moving the sub to a null you side step the excitation of that particular mode but do not excite a new mode that was not there before. Using multiple subs is taking this further.

Talking of "cancellation" and Toole, take a look at the picture on page 221. When you place a sub at a null, that particular mode is "cancelled". But that means that we will hear nothing at all. I don't think that this is what we want. What we want is to smooth the whole low frequency band for a given listening position or area.

And that was the "essential" part I mentioned earlier. That's not the way it works. How do you think we can hear deep bass in headphones? In a car? Outdoors?

Placing a sub in a null does not mean there will be no output, it only means we will free ourselfs from that particular mode.

P.S. Still searching for the journal describing practical methods to achieve cold fusion and world peace.

I burned it, would cause to much of a buzz and confusion!

P.P.S. I have the feeling that some people do not know that modes can be found in the whole audible frequency band and not only at low frequencies. In fact there are thousands of modes above the Schroeder frequency.

Not only in the audible band but as far up as you can count. The thing is that what we are most sensitive about is modal density. When there's a wide gap between the modes the resonances stand out much more than if they are tightly packed. That's the reason the first axial modes is the most important ones to fight.


/Peter

[youngho]

Originally posted by markus76
youngho, I guess you misunderstood my comments.
I should have written "rate" instead of "time". If you switch of the excitation signal all modes will still be there. They will decay at the same rate independend of the excitation signal's level.

Yes, rate and time are different. The rate of decay for a bass mode is essentially the same as for other frequencies in the bass region, so the way it was written was confusing.

The modal field is an attribute of a room's physical properties. You can couple energy to modes in different ways so the frequency response becomes smooth but you can't get rid of modes (without absorption).

I guess I interpreted "modal field", which was otherwise undefined at the time, as the complex frequency-dependent pressure distribution field determined by the room dimensions and shape and stimulated by a sound source. Yes, you're right. I was taking the Zen position: if a mode is not stimulated or if it is cancelled, does it exist for the listener?

I'm always looking at ALL low frequency modes up to let's say 100 Hz. If you "cancel" one mode you will excite another one.

I think what you're trying to write is that if you cancel some modes without addressing others, the remaining modes become more obvious and problematic?

Talking of "cancellation" and Toole, take a look at the picture on page 221. When you place a sub at a null, that particular mode is "cancelled". But that means that we will hear nothing at all. I don't think that this is what we want. What we want is to smooth the whole low frequency band for a given listening position or area.

That is incorrect. If you place a subwoofer at the node for a mode and have it play the frequency for that mode, you will hear that frequency, but the mode will not be activated. The relative level will be low because it's not reinforced by standing waves, but you will still hear it. But I agree, one would like "to smooth the whole low frequency bandwidth for a given listening position or area as much as possible."

P.S. Still searching for the journal describing practical methods to achieve cold fusion and world peace.

I'm glad the joke was appreciated.

P.P.S. I have the feeling that some people do not know that modes can be found in the whole audible frequency band and not only at low frequencies. In fact there are thousands of modes above the Schroeder frequency.

I'm sorry if I have been focussed on modes in the bass region. The thread title is about multiple small subs, so I was only thinking about modes that the subs would be energizing. Unfortunately, users may perceive many problems with bass reproduction due to modes above 80 Hz and below the transition frequency/region causing issues with harmonics and timbre. However, this isn't a problem with the setup of subwoofer(s) itself.

Anyway, I've learned a few things reading this thread and participating a little. Thanks to all, no semantics for me, happy listening, and have a great weekend!

[gedlee]

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. 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). 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. 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.

The sound from a LF source does not - let me repeat - does not travel in all directions away from the source. (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. The sound wave can travel only in a discrete number of directions defined by the modes that it excites. 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.

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.

[markus76]

Looks like you had access to the ominous journal describing practical methods to achieve cold fusion and world peace.