Multiple Small Subs - Geddes Approach

I certainly am not saying that mode cancellation is necessarily the goal, that it is the necessarily most effective way of dealing with a room's problems, or that it will necessarily yield in the best statistical bass response. I don't know that anyone said so.

I was responding to Markus's comments. "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), "[multiple subs] don't change a room's modal field" (they can through mode cancellation, page 224, 225, 229), "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), "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). Markus asked for data, and all of these points were addressed in Toole's book, along with data from real rooms for each point. Lastly, Markus said, "it has nearly no practical relevance in a real room," but these measurements were made in real rooms, two of which were Toole's own home listening rooms Again, Welti included data from an investigation in a real room in his Harman white paper.

Markus is right, it really isn't a discussion because it's already been addressed in Toole's book.

Have a nice weekend, all!
 
soongsc said:
Let's take the cello for example: Most of the time change in direction of the bow creates a small transient that allows you to pinpoint where the cello seems to be; however, if the note is low that excites room modes, then you will not get a feeling that the cello is at that original location. Whereas, if you listen to cello played in a hall, you will almost hear the note coming from the cello where it is located.

Cello is a terrible example. It has little output is the true bass region. Markus is right, the examples you're choosing have transients with fundamentals and harmonics outside of the bass frequencies. Choose another, please.
 
youngho said:


Cello is a terrible example. It has little output is the true bass region. Markus is right, the examples you're choosing have transients with fundamentals and harmonics outside of the bass frequencies. Choose another, please.
rooms modes start as high as 300Hz. I'm talking about the effects of exciting room modes. the effect is the same regardless what instrument you talk about. It's fine if you prefer a different kind of sound.
 
Then, yes, you're right, subwoofers will not address the rooms modes outside of their reproduced frequency region, nor will they reproduce transients at 300 Hz, at least not if they're set up properly. I don't know why you consider this to be a profound point, but I'm sure everyone will concede it to you. If you have been listening to "subwoofers" playing up to 300 Hz, I'm not surprised that you have experienced "focus shifts" and "one instrument floating around." Was this by any chance a Bose "subwoofer"?
 
Cello is a terrible example. It has little output is the true bass region. Markus is right, the examples you're choosing have transients with fundamentals and harmonics outside of the bass frequencies. Choose another, please.

That was my point in choosing it. It's lowest note is 65hz but with all the harmonics and transients you can be sure the sound of a cello will get every driver in the system moving. I don't understand what soongsc is trying to say, but it seems to be some variation on "room modes = bad" or "subs = bad because they excite room modes". A real cello played in your living room will also excite them of course. Unless you're content to listen to a speaker with no output below 200hz room modes are going to be excited. It's not caused by subs or something to be avoided, it's just a fact of listening to music (live or recorded) in small rooms.
 
gedlee said:
Not a very pleasing discussion.

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.


I will say that I do not consider "mode cancellation" to be the goal, nor do I consider this approach to be effective.

Well it's exactly what's going with your sub set up and it is very effective as can easily be shown with measurements.

For every mode that is cancelled another is acentuated - i.e. doubled.

No, typically you push the problems up above where the subs is active. Also the lowest axial modes is typically those who stand out the most and are most annoying.

This does not lead to a smooth overall response.

Yes it does as can be shown (and has been shown)

Talking about cancelling "axial" modes in only one direction also seems like a dead end to me as there are lots of other types of modes and there are three directions. What about all those modes? Quite simply, the problem is too complex and multi-dimensional to treat in anything but a statistical manner.

Nope, no-one suggest dealing with axial modes in only one direction. Ideally you try to adress all axial modes in the sub range in several dimensions and the more subs you have the more you can cancell and smooth out room response by doing this. The oblique and tangential modes are typically much lower in level than axial modes and those are also taken down in level by multiple sources at different locations for low bass output.


Viewed statistically cancelling a mode is not going to yield the best statistics. You have to look at the total picture of all the modes and all the sources combined into a single system. The discussion gets lost when you try and narrow it down to the point of talking about a single mode in a single direction.

No-one is focusing on only one axial mode, that would be a mistake since there are three (or more) dimensions in a room and several room resonances that stand out.


/Peter
 
poptart said:


That was my point in choosing it. It's lowest note is 65hz but with all the harmonics and transients you can be sure the sound of a cello will get every driver in the system moving. I don't understand what soongsc is trying to say, but it seems to be some variation on "room modes = bad" or "subs = bad because they excite room modes". A real cello played in your living room will also excite them of course. Unless you're content to listen to a speaker with no output below 200hz room modes are going to be excited. It's not caused by subs or something to be avoided, it's just a fact of listening to music (live or recorded) in small rooms.

If you want a clear view into the recording (HiFi) you must do something about the standing waves or the result will be mediocre at best. So yes, to some degree it should be avoided to excite room modes as it imparts a fingerprint on the recorded material as it is reproduced. To much and strong influences of the listening room on the recording will mask the early reflections and ambience of the recorded material.

Of course it's another story if you want to playback dry material and want to place the orchestra/ensemble/instruments in your listening room. I mostly listen to recordings that have spacial information though and I want to hear into the recording as a virtual space that opens up behind and slightly around the loudspeakers.

Strong room colorations impart a sense of sameness to the recordings, a kind of rectifications one could say, and that takes away much of the magic of sterephony IMO.


/Peter
 
Re: mode cancellation

With real rooms and pseudorandom placement ala SFM/Geddes mode cancellation, if any, is not simply on/off, but some modes may be tamed only a little while others almost completely. Add to this 'filling in' of some nulls and we get a smooth response if the setup is just right. Am I right?
 
Dipole cat in pigeons?

Wrt to even in room response, how would either of the multiple box subs options compare with a pair of dipoles of equal bass depth?

(ie simplifying the issue by putting aside the need for more Vd and EQ - but only allowing a pair of dipoles)?

Cheers
 
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Pan said:
A transient will excite resonances and a transient is not necessarily a HF signal but any signal of relatively short duration as compared to/in contrast to a stationary signal.


/Peter


A transient/short duration signal is by definition an hf phenomenon and, as explained by Mr Fourier, will be routed by the xover to the mid/hf driver. If you for instance Spice a xover and feed it with a drum kick type signal, the lf component appears at the lf output of the xover while the transient or attack component appears at the mid/hf output. So it is difficult to see how messing around with the subs would change localisation etc which is depending on the transient part.

Jan Didden
 
While I don't agree that the definition of a transient is a HF signal the term "HF" itself is only relative.

When radio people talk about transients would that only be the upper range of the radio frequencies then? In such case that would exclude the term transient to be used in the audio spectrum.

Good room behaviour is essential for good localization, even for bassinstruments. With excessive standing waves the impression will be that the HF parts of the instruments is placed somewhere "deep" into the soundstage and the bass looses it's definition and is pulled closer "into the room".

With good speaker/room bahaviour the impression is that all the energy from the instrument is placed at the same spot in the soundstage.


/Peter
 
janneman said:



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.

Jan Didden

If a transient do not excite room modes (resonances) how come acousticians and engineers use us pulses and gun shots to examine complex systems frequency and time behaviour?

Would you not consider a 25us pulse or a gun shot being a transient signal?


/Peter
 
Re: response smoothness

When we discuss about response smoothness I assume this means FR smoothness at a single point (spatially averaged perhaps). However, Welti's paper concentrates on getting the most similar response for many listeners (least deviation from the avg response). With the best configuration the FR may or may not be the smoothest. For the setup with least seat-to-seat variation a global EQ may be then successfully used to flatten the response.
 
janneman said:



A transient/short duration signal is by definition an hf phenomenon and, as explained by Mr Fourier, will be routed by the xover to the mid/hf driver. If you for instance Spice a xover and feed it with a drum kick type signal, the lf component appears at the lf output of the xover while the transient or attack component appears at the mid/hf output. So it is difficult to see how messing around with the subs would change localisation etc which is depending on the transient part.

Jan Didden
Well, you have to model the whole path including the time differences and delayed resonance of room modes, and sum all together as in reality by the ear. Then you will see how very different the result is from the original signal.;) Even better, model the energy storage characteristics of the drivers. It's amazing how we actually can still know what we are listening to just looking at the final output.:D
 
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soongsc said:

Well, you have to model the whole path including the time differences and delayed resonance of room modes, and sum all together as in reality by the ear. [snip]


I thought we agreed that the localization relied on the hf/transient parts? I thought you stated somewhere that messing with the subs would mess up the localisation?
My point is that it is not necessarily so because the localization clues (coming out of the mid/hf drivers) are not messed up when we mess with the subs. But maybe I didn't understand you correctly.

Jan Didden
 
janneman said:



I thought we agreed that the localization relied on the hf/transient parts? I thought you stated somewhere that messing with the subs would mess up the localisation?
My point is that it is not necessarily so because the localization clues (coming out of the mid/hf drivers) are not messed up when we mess with the subs. But maybe I didn't understand you correctly.

Jan Didden
Well, if you have an instrument with frequency span in the bass and subs, you will hear that instrument moving around, and sometimes having aparent localization sometimes not depending on the notes played. This was what I was trying to point out. It seems not many people have experienced this so maybe that's why most don't understand.

One person here where I live held an open house gathering and we tried different speakers in a room of 4Mx4M or so. One larger speaker was placed in the room, and the whole sound stage was messed up. Then I brought in one of my little speakers, reproduction of a play with people stomping around on a wooden stage was very well presented. The fact was the little speakers just slightly excited the room modes because it did not have high low frequency SPL capability.

A simple test anyone with multiple subs can do is aske someone to play signals through the subs and you can listen to see if you can identify which sub is being played. Do this at different volumes, and you will probably notice that below a certain volume level, the sub becomes identifieable.
 
Pan said:

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.

I believe that Dr Geddes does have a right to take this thread personally because his name is in the thread title. Also, I think that Markus is still unhappy with "Oh, I read some papers that addressed all these points. I think they appeared in the same journal describing practical methods to achieve cold fusion and world peace. I assure you they have all the answers. Oh, no, you can't read them because they were written in Sanskrit, and I don't remember where I read them." I certainly would have been.

Well it's exactly what's going with your sub set up and it is very effective as can easily be shown with measurements.

No, mode cancellation is NOT how Geddes' approach works. For mode cancellation to work, the two subwoofers have to be in phase on opposite sides of and equidistant to a node, like a node of one of the primary axial modes. Geddes' approach seems to rely on driving the modes in different phases.

No, typically you push the problems up above where the subs is active. Also the lowest axial modes is typically those who stand out the most and are most annoying.

Toole's book shows how mode cancellation and node placement can result in the remaining mode(s) being accentuated, just as Geddes describes, like the tangential mode on page 224.

Yes it does as can be shown (and has been shown)

Not necessarily, again, page 224.

Nope, no-one suggest dealing with axial modes in only one direction. Ideally you try to adress all axial modes in the sub range in several dimensions and the more subs you have the more you can cancell and smooth out room response by doing this. The oblique and tangential modes are typically much lower in level than axial modes and those are also taken down in level by multiple sources at different locations for low bass output. No-one is focusing on only one axial mode, that would be a mistake since there are three (or more) dimensions in a room and several room resonances that stand out.

You cannot deal with all axial modes with two or four subwoofers, since just as you note, there are three dimensions. Mode cancellation only addresses the odd-order axial modes for one axis, not the even-order modes. Node placement of a subwoofer will only deal with the modes for which the node is relevant. Again, page 224 and 227 in Toole's book show how prominent the even-order modes can be with midwall or corner placement of two or four subwoofers. With quarter-room placement of four subwoofers, the tangential mode becomes quite prominent. Because of the symmetric approach of subwoofer placement in Welti's recommended setups, even-order modes and tangential modes may be accentuated far beyond what you'd expect. Again, take a look at page 224 and 227. Welti's approach relies on significant equalization, as noted in his Harman white paper.

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?
 
In response to soongsc, post #177.

Exactly, there's masking effec at play when the room modes are allowed to build up to 10-20dB peaks above nominal. This can have a huge effect on the focus and realism in the soundstage.

Multiple subs, directional bass, bass absorbing tools or walls, DSP and EQ are some of the tool to improve fidelity.


/Peter
 
soongsc said:

Well, if you have an instrument with frequency span in the bass and subs, you will hear that instrument moving around, and sometimes having aparent localization sometimes not depending on the notes played. This was what I was trying to point out. It seems not many people have experienced this so maybe that's why most don't understand.

True, clean output in the bass frequencies by a subwoofer should be extremely difficult to localize precisely due to the size of the wavelengths involved. Problems with subwoofers making the location obvious are often related to rattling/buzzing of any loose household fixtures or distortion from the subwoofer itself. Perhaps you have some extraordinary powers of hearing or else an extremely large head?

(edit) I'm sorry, I should amen this to note that I'm talking about typical listening rooms, especially in the context of listening to music. Issues with modes and frequencies below the transition region (200-300 Hz) will exist, whether they're generated by subwoofer(s) or loudspeaker(s). Soongsc, are you arguing that you can precisely localize frequencies below 80 Hz and therefore hate subwoofers? Is your point that a single loudspeaker can generate a time-coherent transient response in a listening room that the listener will perceive as such? The large head suggestion was simply a joke, referring to the wavelengths involved. Sorry for any offense.

One person here where I live held an open house gathering and we tried different speakers in a room of 4Mx4M or so. One larger speaker was placed in the room, and the whole sound stage was messed up. Then I brought in one of my little speakers, reproduction of a play with people stomping around on a wooden stage was very well presented. The fact was the little speakers just slightly excited the room modes because it did not have high low frequency SPL capability. [/QUOTE]


How does one speaker generate a soundstage? I would have thought that one speaker represented a sound source. Anecdotal "evidence" can be highly problematic...