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Flanking Subs vs Helper Woofers [message #74649] Thu, 29 November 2012 11:57 Go to next message
zheka is currently offline  zheka
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Wayne,

Have you ever considered a crossover option for your 2-ways to connect a helper woofer? The enclosure would have a set of terminals to feed the helper woofer box from. Is this even theoretically possible?

In general, would you agree that true helper woofer implementation is a cleaner approach than the flanking subs one? My reasoning is based on the fact that flanking subs deal with summed LF signal where as helper woofers would only get the signal intended for a given channel. After all, at the frequency we are discussing, some spacial queues can be expected.

Thank you
Re: Flanking Subs vs Helper Woofers [message #74653 is a reply to message #74649] Thu, 29 November 2012 12:58 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
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Registered: January 2001
Illuminati (33rd Degree)

Flanking subs do not get a summed signal - They get the same signal as the main speaker they are flanking, with a low-pass filter applied. They're stereo subs. That's one of the key distinctions between flanking subs and more distant distributed subs in a multisub configuration. Flanking subs are closer than distributed subs, and they are sent a low-passed copy of the same signal as the mains they're flanking.

You could use a speaker level passive filter, but the problem is most subs are 10dB less efficient than the mains, at least the ones we're talking about here. So that makes a separate amplifier the most attractive option, mostly for matching SPL.

In my terminolgy, helper woofers and flanking subs are the same thing. Both are woofers that augment another woofer in an overlapping band. The low-passed woofer in a 2.5-way is a helper woofer, but its disadvantage is that it cannot be positioned independently of the main woofer. That's a pretty significant disadvantage, in my opinion, because it limits placement options and prevents the owner from setting them up in a way that mitigates self-interference from nearest boundaries. That's the whole idea of the flanking sub approach - To use a physically separate helper woofer positioned between the mains and the nearest boundaries to smooth self-interference notches and higher frequency room modes.

A multisub configuration should include flanking subs and distributed subs. A typical arrangement has two flanking subs and two distributed subs. Flanking subs smooth the upper modal region and self-interference notches in the 80-160Hz range. Again, flanking subs aren't summed - They are sent a low-passed version of the signal going to the main speaker they're flanking. Distributed subs smooth the lower-frequency range, below 80Hz. At those low frequencies, localization isn't as much an issue, so the signal to the distant distributed subs is summed. I actually think flanking subs are more important than the distributed subs, because response anomalies in the 80-160Hz octave are more noticable than deeper bass modes. But the best response can only be achieved using both.
Re: Flanking Subs vs Helper Woofers [message #74656 is a reply to message #74653] Thu, 29 November 2012 14:04 Go to previous messageGo to next message
zheka is currently offline  zheka
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Interesting. I never thought of it this way because I am dealing with multichannel HT so bass management is a given. If I used subs for the individual channels only, they would never get the LFE or LF content from other channels. Seems a little wasteful to me. Besides I would not know how to implement it even if I wanted to try. I guess if the receiver has line level outputs this could work. Still I'd probably want at least one powerful sub upfront that would be getting summed LF signal.

The (apparently wrong) way I had it done in my system was by setting the LPF on the receiver high, say 120-150Hz, setting the mains to "full", and adjusting LPF on the distant subs only to values under 80Hz. This indeed helped with the interference notches all the way to 200Hz or so.

How powerful would the flanking subs have to be in your scenario? Can they be limited to, say, 60-250Hz range?
Re: Flanking Subs vs Helper Woofers [message #74657 is a reply to message #74656] Thu, 29 November 2012 14:39 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 17583
Registered: January 2001
Illuminati (33rd Degree)

You're right that home theater sound processors don't have any provisions that make flanking subs or distributed multisubs any easier. Honestly, the underlying philosophies and technologies most home theater processors are based on was developed before multisubs and flanking subs became popular. This whole approach is less than a decade old, and its common acceptance is just a few years old. It's still in its infancy, relatively speaking.

The way to do a flanking/multisub approach is to set the mains to "large" or "full" so they aren't high-passed. We want the mains and subs blended, overlapping in the modal region. Use an external low-pass filter and amplifier for each flanking sub. The low-pass filter can be an inline crossover or it can be the built-in crossover of a plate amp. Distributed subs are a little easier, because they can just be driven by the sound processor's built-in summed subwoofer output.

Low-pass frequency for flanking subs should be between 80Hz and 150Hz, and use second-order or third-order for best results. Fourth-order can be used, but it doesn't sound as good to me - It doesn't blend as well. The higher the slope, the higher the the crossover frequency. So for example, second-order at 90Hz or 100Hz often works well, third-order is better around 125Hz and fourth-order usually has to be set higher still. Set the amplitude for SPL matching the mains in the overlap band.

Re: Flanking Subs vs Helper Woofers [message #74658 is a reply to message #74657] Thu, 29 November 2012 14:58 Go to previous messageGo to next message
zheka is currently offline  zheka
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If flanking subs are designed to address boundary interactions and room modes only in the 80Hz-200Hz region, can "mid-bass" bass-reflex modules with high efficiency pro-drivers be used instead of full fledged subwoofers designed to go much lower?
The sub channel signal would still go through "conventional" multisub.

What do you think?
Re: Flanking Subs vs Helper Woofers [message #74659 is a reply to message #74658] Thu, 29 November 2012 15:32 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 17583
Registered: January 2001
Illuminati (33rd Degree)

I think that's a fine approach. What you are suggesting essentially is to use high-efficiency woofers both for midwoofers and for helper woofers. Blend them together in the modal region, but low-pass the helper woofer to prevent it from developing any output above the Schroeder frequency. Roll off the helper woofer gracefully in the transition region.

The π constant directivity cornerhorn is designed this way. The midhorn runs low enough that it blends with the bass bin between 100Hz and 250Hz.

With your four π speakers, for example, you could add a second 2226H in a separate box and run it with a speaker level passive crossover, just a coil or maybe a coil/cap and Zobel. This solves the problem of matching sensitivites of the two subsystems, and therefore removes the requirement of separate amps. What you would have is a 2.5-way loudspeaker with a detached helper woofer, placed in a flanking sub configuration.

The trade-off, of course, is that you either have to sacrifice bass extension or you have to make the helper woofer box really big. But for the midbass blending - what the flanking sub approach is designed to address - you don't really need the extension anyway. So you could run the same size box, and use the more distant subs for LF extension.

However, this yields another consideration, and that is when flanking subs are run deep, they provide modal smoothing down low as well as up high. They provide additonal bass sound sources used in conjunction with other more distant subs. We want distribution of sound sources in the deep bass range, and the flanking subs can provide that if they're generating deep bass output. If not, you're back down to a small number of sound sources in the deepest bass range.

As with all things, it comes down to a matter of competing priorities and trade-offs. I think if I didn't notice problems in the deep bass, I'd probably be happy with high-efficiency helper woofers and one really deep sub. Likewise, I am also happy in many rooms with just a pair of deep-reaching flanking subs, for much the same reason. Some rooms have better modal behavior than others, and particularly homes with framed drywall construction have some damping that smoothes the lowest modes a little bit. Other rooms, like those with brick, stucco or concrete walls, really benefit from a full compliment of four subs, two (deep running) flanking subs and two distant subs.

Re: Flanking Subs vs Helper Woofers [message #74660 is a reply to message #74659] Thu, 29 November 2012 15:36 Go to previous messageGo to next message
zheka is currently offline  zheka
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Registered: June 2012
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That's a lot to think about.
Thank you very much!
Re: Flanking Subs vs Helper Woofers [message #75070 is a reply to message #74657] Fri, 28 December 2012 12:18 Go to previous messageGo to next message
dheflin44 is currently offline  dheflin44
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Wayne Parham wrote on Thu, 29 November 2012 14:39

The distant subs are a little easier, because they can just be driven by the sound processor's built-in summed subwoofer output, with crossover set somewhere between 50Hz and 80Hz, depending on how far away the subs are placed.


Would it be better to run the mains, flanking subs, and distant subs from the same "large" L/R signals (with appropriate low-pass cut-offs on the flanking and distant subs) in order to get both mid and low frequency blending? Is there any low-bass content I would miss out on if I didn't use the LFE channel at all?

Thanks,
Darrell
Re: Flanking Subs vs Helper Woofers [message #75074 is a reply to message #75070] Fri, 28 December 2012 13:02 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 17583
Registered: January 2001
Illuminati (33rd Degree)

Flanking subs and distributed multisubs are different animals. They have a similar purpose and correspondingly similar execution but have some specific differences because of the frequency ranges where they are intended to be most effective. Flanking subs are stereo subs that are relatively close to the mains and each is sent a low-passed copy of the main speaker they're flanking. Multisubs are placed further away, distributed throughout the room, and each is sent an LF signal that is summed from all channels. They can optionally incorporate decorrelation filters to increase their ability to smooth the LF modes, but they would still be sent a single, summed LF signal.

Flanking subs are stereo subs, each blended with the main speaker they're flanking. They're run high enough they would be localizable if they weren't physically close. But they're just far enough away from the mains in all three planes to provide smoothing of the response anomalies caused by self-interference from nearest boundaries. Their main purpose is to reduce the notches created from the reflection off the wall behind the speakers and from the floor. They also smooth the higher frequency room modes, between about 80Hz and the Schroeder frequency, around 200Hz to 250Hz.

Distributed multisubs are mono subs, each sent a low-passed signal that is summed from all channels. They are placed further away, and would be localizable if they weren't low-passed at a relatively low frequency, usually no higher than 80Hz to 100Hz. Use the LFE signal to drive the distributed subs. They are there to smooth room modes below 80Hz.
Re: Flanking Subs vs Helper Woofers [message #75078 is a reply to message #75074] Fri, 28 December 2012 15:31 Go to previous messageGo to previous message
zheka is currently offline  zheka
Messages: 80
Registered: June 2012
Location: Chicago burbs
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Quote:
Multisubs are placed further away, distributed throughout the room, and each is sent an LF signal that is summed from all channels. They can optionally incorporate decorrelation filters to increase their ability to smooth the LF modes, but they would still be sent a single, summed LF signal


Can you explain what the decorrelation filters are, how they work, how effective can they be in multisub and what kind of processors are used to make them?

Thank you very much.
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