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Home » Sponsored » Pi Speakers » Flanking Subs vs Helper Woofers
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| Re: Flanking Subs vs Helper Woofers [message #75414 is a reply to message #75323] |
Fri, 01 February 2013 14:12   |
andy_c
Messages: 7
Registered: February 2013
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Esquire |
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| Wayne Parham wrote on Wed, 23 January 2013 11:57 |
And Welti added a processor box that equalized the response sent to each subwoofer using FIR filters, calling this technique "sound field management". Now Geddes has begun to employ this approach as well.
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Hi Wayne,
The situation regarding Harman and multiple subs has been somewhat confusing. The first multi-sub processor box they made that I'm aware of was the BassQ. As far as I can tell, it used FIR filters and a pretty sophisticated algorithm for computing them, based on measurements at multiple listening positions. But the article by Welti and Devantier describing Sound Field Management (SFM), Low-Frequency Optimization Using Multiple Subwoofers, describes a technique that's different from what BassQ uses. In fact, I was pretty surprised at how crude the method is. For each sub, there is a variable gain and delay, but only a single biquad IIR filter: a cut-only parametric EQ stage. For each subwoofer, the gain, delay, and biquad parameters are configured so as to minimize the variation in frequency response with position of the combined subs, without regard to what the frequency response is. They assume that global EQ will then be used to clean up the response. SFM is used in the ARCOS system (which also implements global EQ). Confusingly, the sub-blending approach of the BassQ looks to be much more sophisticated than what ARCOS is doing. BassQ seems to be computing its filters using the matrix inversion approach described in the above Welti and Devantier article (but not used for SFM).
It looks like what Earl is doing is much more sophisticated than ARCOS regarding the blending of the subs as well, though probably not as complex as the BassQ processing. The strange curves he gets at around 32 minutes into his presentation show that his filters are fairly high-order, though it's hard to tell exactly what the filter order is.
I am also fascinated by the history of the multi-sub approach, but somewhat puzzled by why Harman did not continue with the approach used by the BassQ.
Edit: It may also be that I'm overestimating the complexity of the BassQ, as there hasn't been much information disclosed about it by Harman.
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| Re: Flanking Subs vs Helper Woofers [message #75418 is a reply to message #75414] |
Fri, 01 February 2013 15:35 |
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Wayne Parham
Messages: 18786
Registered: January 2001
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Illuminati (33rd Degree) |
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I would agree (and expect) some differences in details, but I'm not sure how significant those details are. At least, less significant than the main idea, which is to use multiple sound sources spread apart spatially, and to use EQ, where appropriate for even smoother response. I also think that the more sound sources, the less important their positions or individual filters are. The fewer the sources, the more important those things become. It appears both Geddes and Welti would agree on those points.
As for differences, I do think we have seen some erosion of the unique features that set each approach apart. Used to be - around 2005 - Welti was symmetrical and Geddes was asymmetrical. At that time, Geddes clearly made statements that distinguished his process from Welti's. Then Welti began to use EQ to modify subwoofer response, optimized by measurements made from multiple positions. Geddes now also proposes using EQ to modify subwoofer response, and his procedure is also now guided by measurements. These similarities, combined with the agreement on both sides, that once you get to four subs, Welti and Geddes arrangements are pretty much identical with respect to performance, makes me see what is essentially a unified approach. Yes, there are differences in details, but I think we see pretty much an over-arching agreement.
I think "both" approaches are excellent, although as I said, I think "both" are now pretty much one and the same.
And I also think that - unless the main speakers are sitting right on the wall, soffit mounted or constant directivity cornerhorns - that sound sources blended in the 100-200Hz range should be used in addition to the distributed subs blended below 100Hz. The multisub approach is great for smoothing the bass below 100Hz, but it does nothing to address the anomalies from nearest boundaries, most significantly the wall behind the speakers. Flanking subs can (and usually should) be part of the multisub equation, since the flanking subs provide additional bass sound sources. Said another way, you can have two stereo flanking subs and two distributed subs, or three flanking subs in an LCR setup with one distributed sub, for a total of four subs to provide modal smoothing.
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| Re: Flanking Subs vs Helper Woofers [message #75462 is a reply to message #75414] |
Thu, 07 February 2013 11:46 |
andy_c
Messages: 7
Registered: February 2013
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Esquire |
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| andy_c wrote on Fri, 01 February 2013 14:12 | Edit: It may also be that I'm overestimating the complexity of the BassQ, as there hasn't been much information disclosed about it by Harman.
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Just to update this, I found some detailed information about the BassQ in the BassQ thread on AVS Forum. Roger Dressler posted a PDF of the patent application for it. I've attached it below, because I think one needs to be logged in at AVS in order to download it. It turns out that BassQ uses FIR filters and the matrix inversion process described in Welti's paper but not pursued further in that paper.
Also, after watching Earl's video for the umpteenth time, I heard him say that he uses six parametric EQs per sub, which seemed like a lot to me. So I asked Earl about that, and he says he uses up to six parametric EQs per sub, but can usually get by with one or two.
So given the typical implementation of Earl's approach with just one or two parametric EQs per sub, it seems that his approach is very close to what Harman is doing with SFM (which always uses one parametric EQ per sub), while the BassQ is rather different from SFM and Geddes in its use of FIR filters and matrix inversion.
It seems a bit strange to me that JBL introduced the BassQ in 2008, after Welti published AES articles that de-emphasized the approach used by BassQ. It's an interesting history.
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| Re: Flanking Subs vs Helper Woofers [message #75468 is a reply to message #75463] |
Thu, 07 February 2013 19:07 |
andy_c
Messages: 7
Registered: February 2013
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Esquire |
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Yeah, I'll be doing what you call flanking subs out of necessity. I only have room for two subs, one to the left of the left mains and one to the right of the right mains. So I decided to make two sonosubs, with each sub having one up-firing and one down-firing driver with a separate amp and LPF on each of the total of four drivers. Each enclosure will be internally divided into two halves because of having different signals applied to the top and bottom drivers.
I hadn't heard of flanking subs until a couple of weeks ago when I followed one of zheka's links here. I hadn't actually thought of the benefits of subs close to the mains in terms of the 100-200 Hz range before. But my system needs some help there, as I have a 10 dB suckout at about 125 Hz. So I'm going to try crossing at least two of the subs over at 150 Hz or so to get some fill-in.
I had originally considered the need to have the subs and mains close to be a disadvantage, but I hope be pleasantly surprised now. The subs will be behind the mains, as close to the wall as I can get them.
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