Multiple Small Subs - Geddes Approach

[Pan]

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

[Pan]

Originally posted by 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.

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

[breez]

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.

[soongsc]

Originally posted by Pan
...

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

This is what I am constantly striving for. The lower in frequency, the more difficult.

[soongsc]

Originally posted by janneman



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.

[janneman]

Originally posted by soongsc

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

[soongsc]

Originally posted by janneman



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.

[youngho]

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.

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?

[Pan]

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

[youngho]

Originally posted by soongsc

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...