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Old 24th January 2010, 06:02 PM   #1
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Default Bass Management - TM with Subwoofer

I searched for a thread on this and did not find one that was up to date and comprehensive regarding this most common setup. I'm guessing that 90 percent of "bass management" questions and answers apply to TM/sub setups on this forum.

The majority of subwoofers utilized in homes are likely just plugged and played. That sufficed for most of us until the last decade or so. The attention to bass management or integration (in the home) gets progressively more painstaking as the DIY bug lures you into the couple of dozen internet rabbit holes regarding the subject.

But I've been unable to find one really good thread applied to this. In fact, Brian Steele, of the diysubwoofer site, mentions recently ...
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Recently, someone pointed out something that was missing from my web pages about subwoofer design - methods and techniques used by people to successfully integrate subwoofers into their audio systems.
The link is here...
The Subwoofer DIY Page - Subwoofer Integration

While this page is informative, it actually does more to point out the bewildering range of approaches to this topic. Everyone (almost) suggests sealed, or ports defeated (stuffed), for optimum midrange accuracy... but without ever really explaining why this is accepted practice.

There is considerable support for the use of active crossovers (for the many of us that still use 2 chan, non HT, gear) in order to high pass the MT 'mains'. Again, the applications are frustratingly anecdotal and diffuse, with pro audio solutions seeming to be the norm. Another designer, whose knowledge I regard highly, suggests that if crossed over (high passed), the mains are better left ported.

In another thread here I pointed out that one highly regarded speaker builder suggested that a sealed 6 or 7 inch TM with sub could never really be expected to match the reference levels capable of a good TMM or MTM design.
Quote:
These levels are somewhat relative, and it's probably best to compare these numbers with the numbers of similar units. I consider the Usher woofer to be an excellent unit and this unit to have overall good distortion. But you probably can conclude that a sealed 7" unit, even a high quality one crossed at 80 Hz to a sub cannot be considered truly full range and won't achieve reference levels.
There's something to be said for MTM's, TMM's, and three ways for dynamic range.
(near the bottom of this page)
http://www.audioheuristics.org/proje...sher_index.htm

And I seem to be noticing a small but growing trend of people kind of throwing up their hands in the face of all of this and simply foregoing these issues by building larger multi-ways.
Quote:
Well its been fairly quiet around here so I thought I spice things up a bit.
This is my 4-way Apollo project which I'm about to begin construction on.

Because I hate to use bass management (routing bass to a subwoofer), these will be full range (-3dB@25Hz), using bi-amping & EQ on the bass driver.
...just one of many such comments I've started to notice.

I suspect that space and placement flexibility considerations (SAF) are often made in choosing smaller mains with subwoofers that can either be hidden or tucked somewhat out of sight. Does this really introduce such a can of worms for the average diy'er that he would well advised to opt for the TMM or MTM (or other such) in most cases?

Is it possible to get some consensus here in one thread on this subject?

Last edited by peace brainerd; 24th January 2010 at 06:30 PM.
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Old 24th January 2010, 06:10 PM   #2
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The best treatment i have seen is in Floyd Toole's new book, and here in Geddes' thread on multiple subwoofers.

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Old 24th January 2010, 07:24 PM   #3
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Timely post Peacebrainerd. I'm thinking about putting together a webpage to summarise all my thoughts on this matter as I too have noticed a lack of clear advice. My new speaker project (Apollo) on another thread follows the principles I mention below.

Basically here's what I think (which admittedly is more of a purist approach):

Most users have little knowledge of crossover design, the physics of sound or room acoustics. The standards and approaches used by equipment manufactures vary widely in regard to crossover frequencies, levels, overload margins, how bass is routed between small/large/subwoofer speakers, different DVD/blu-ray/SACD/DVD-A requirements, etc, etc. Its so complex, with so little information provided to the user, that it usually ends up being a guessing game.

By using full-range main speakers, and all channels set to "large", everything is greatly simplified.
No bass management is necessary, and the subwoofer(s) are used for Low Frequency Effects (LFE) only.

The main speakers should be sealed. The 2nd order rolloff of a sealed enclosure has much less group delay, is easier to EQ and integrates better with the typical room and subwoofer. Properly designed, it shouldnt require any highpass filter in a domestic situation (reasonable power levels).
Being full range, ideally they should be bi-amped, with an actively driven woofer, with EQ for optimising the low-frequency integration with the (unknown) room.

I also prefer sealed subwoofers for their lower group delay, better cone-control, and easier ability to be EQ'd to a flat response (eg by Linkwitz transform). 2 subwoofers is probably the best overall choice from a performance/practicality standpoint.

To summarize, the whole system needs to be simplified, and the best way to achieve that is to use bi-amped full-range main speakers, centre and surrounds big enough that they can be set to "large" (no bass management), 2 x dedicated LFE subwoofers, and sealed enclosures all around.

Last edited by David Gatti; 24th January 2010 at 07:51 PM.
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Old 24th January 2010, 07:40 PM   #4
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Quote:
Originally Posted by David Gatti View Post
The main speakers should be sealed and full range. The 2nd order rolloff of a sealed enclosure has much less group delay and integrates better with a subwoofer. Properly designed, it shouldnt require any highpass filter in a domestic situation (reasonable power levels).

Being full range, ideally they should have an actively driven (power and EQ) woofer for optimising the low-frequency integration with the (unknown) room.

Most users have little knowledge of crossover design, the physics of sound or room acoustics. The standards and approaches used by equipment manufactures vary widely in regard to crossover frequencies, levels, overload margins, how bass is routed between small/large/subwoofer speakers, different DVD/blu-ray/SACD/DVD-A requirements, etc, etc. Its so complex, with so little information provided to the user, that it usually ends up being a guessing game.

By using full-range main speakers, everything is greatly simplified.
No bass management is necessary, and the subwoofer(s) is used for Low Frequency Effects (LFE) only.

I prefer sealed subwoofers for their lower group delay, better cone-control, and easier ability to be EQ'd to a flat response (eg by Linkwitz transform). 2 subwoofers is probably the best overall choice from a performance/practicality standpoint.
Oh how i agree with you except on one single point If you tell any DVD or Blu Ray player that it doesn't have a subwoofer in the setup it'll force the LFE channel to both left & right front speakers. As long as you make them "full range" then no subwoofer is needed at all Obviously i mean make the stereo pair of speakers capable of 20Hz reproduction (full range)

Gotta love sealed boxes for precisely the reasons you mention! Fast, accurate & low group delay. As well as that Linkwitz transform that puts the icing on the cake You just need to move some air...
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Old 24th January 2010, 07:59 PM   #5
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Quote:
Originally Posted by event horizon View Post
Oh how i agree with you except on one single point If you tell any DVD or Blu Ray player that it doesn't have a subwoofer in the setup it'll force the LFE channel to both left & right front speakers. As long as you make them "full range" then no subwoofer is needed at all Obviously i mean make the stereo pair of speakers capable of 20Hz reproduction (full range)
Some do, dome don't (eg my Oppo), and some send a signal reduced 6dB.
See what I mean, it's a dog's breakfast.

But set your mains to large, AND use a subwoofer (for LFE only), and you should be fine no matter what.

Last edited by David Gatti; 24th January 2010 at 08:02 PM.
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Old 24th January 2010, 08:11 PM   #6
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Originally Posted by David Gatti View Post
Some do, dome don't (eg my Oppo), and some send a signal reduced 6dB.
See what I mean, it's a dog's breakfast.

But set your mains to large, AND use a subwoofer (for LFE only), and you should be fine no matter what.
Ah, my apologies as i did query this elsewhere as i was wondering about it. I did get answers but now i'm wondering about there validity

Yeah, i see it can be a total minefield. Looks like i'll need to find out what my film players do, ta for making me take more notice!
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Old 24th January 2010, 10:36 PM   #7
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I hope we can get some input here as well from the minimalists. I rather prefer using 2chan integrated amplifiers. I don't have sub output, multi channels, or options for different speaker settings. I started the thread after agonizing over starting a TM with subwoofer project and bumping into so many of the integration issues. I"m dedicated, for now, to using 2chan integrateds, and just took delivery on a couple of sub plate amps from the NHT sale.

Two questions to start off. Why can't the mains be crossed high at chosen point, say 80hz, with a simple discrete circuit and left at that?

Again, there's no one source or thread that addresses the pros and cons of FMOD preamp outputs vs an 80 dollar Behringer active crossover vs (ideally) designing the main speakers internal crossover to filter sub 80hz (is this possible? or does it introduce too many trade offs in terms of cost, insertion loss, etc?)
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Old 24th January 2010, 10:36 PM   #8
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Quote:
Originally Posted by David Gatti View Post
But set your mains to large, AND use a subwoofer (for LFE only), and you should be fine no matter what.
Except that to get good unpeaky bass response in a room, sitting underneath the mains is not anywhere near an ideal placement for the LF drivers.

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Old 24th January 2010, 11:11 PM   #9
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Quote:
Originally Posted by peace brainerd View Post

Why can't the mains be crossed high at chosen point, say 80hz, with a simple discrete circuit and left at that?

Again, there's no one source or thread that addresses the pros and cons of FMOD preamp outputs vs an 80 dollar Behringer active crossover vs (ideally) designing the main speakers internal crossover to filter sub 80hz (is this possible? or does it introduce too many trade offs in terms of cost, insertion loss, etc?)
Basically, what you've doing is designing a bi-amped full-range speaker, which, as I said, is the best solution. And you have the added benefit of being able to locate the subs anywhere ( nod to Planet10 above )

If you must rolloff the bottom of the mains, you could us a simple line-level capacitor between pre and power amps (6dB/octave) or something like the FMOD (12db/octave). Does your integrated amp have pre-outs and main-in connections? If your sub amp has 24dB/octave lowpass, then use that with the FMOD on the mains which will give you a 4th order Linkwitz-Riley crossover at 80Hz-100Hz.
Quite a neat solution, but with 2 compromises - a poorer group delay (than a shallower-sloped crossover), and you're missing out on the LFE channel (but you could add another sub later).

A Behringer crossover could be used, but being active and more complex it will have some negative effects on sound quality.

A passive highpass filter at 80Hz could be done but would be very expensive and lossy (eg 500uF capacitor, 10mH inductor), and wouldnt function well as it would be operating around the woofer's resonance (where the impedance varies widely). I wouldnt recommend it.

Last edited by David Gatti; 24th January 2010 at 11:21 PM.
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Old 24th January 2010, 11:16 PM   #10
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Quote:
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you could us a simple line-level capacitor between pre and power amps (6dB/octave) or something like the FMOD (12db/octave).
Note that an FMOD is just a PLLXO.

Passive Line-Level Crossover

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