Geddes multiple subwoofer method - 3 subs vice 4

I agree with Geddes and this he is one of the few out there with the right idea.  

ATC has always agreed with "don't insert another filter in a very audible place" idea when adding a sub.   Our efforts on the pro side with multiple subs have worked every time in solving room bass issues, especially avoiding bass build up on the back wall (behind your listening position) and nulls in the space that you cannot resolve due to console placement (which would parallel a sofa position in a home listening room).   So we follow the plan that multiple, smaller, lower SPL level subs located on different walls (if at all possible) seems to give us the most even bass in the listening space.  

What appears to be more important than high pass filtering the mains (which inserts a very audible dip in response at 80-125 and gets worse with distance of mains to sub) is adding delay to the mains.  This part ATC does not agree on inserting a DSP across the entire audio band to accomplish this mains delay, as you can "hear" many of these DSP platforms.  This is why they stick with an analog input to not force you to listen to a permanent unchangeable DAC. The DSP effect is certainly as audible as cable differences, or differences between DACS, CD players or other digital sources.  

The reality is that in many cases the negative sonic effect of the DSP platform may be less than the negatives of not using delay at all or filtering the mains and creating phase issue.

Brad

“The reality is that in many cases the negative sonic effect of the DSP platform may be less than the negatives of not using delay at all or filtering the mains and creating phase issue.”

I agree with this statement. Any negative of high-passing or time alignment is greatly out weighed by the positives.   

Dear @lonemountain  :  I don't know if you know the Harman Subwoofer White Papers that are a scientific modeling used to avoid standing waves and those null points, it's truly extensive and here Harman Inernational ( JBL between other audio items. ) conclusions:

" 74 CONCLUSIONS • How many subwoofers are enough? Four subwoofers are enough to get  the best results of any configuration tried. Two subwoofers is very nearly tried. Two subwoofers is very nearly as good and has very good low as good and has very good low frequency support as well "

That's at room/system seat position.

Top speaker manufacturers with passive designs use the passive crossover ( high-pass, band-pass, low-pass and the like ) to blend " perfectly those speaker used drivers and not only the crossover but time alignment and several other issues.

There are several ways to implement the high-pass filter when using a pair of self powered true subwoofers with out " problem ".

In my case ( audo system ) the high-pass was implemented at ampifiers input doing only a change in the input value capacitor that comes/came by amplifier design and blended almost " perfectly " using the subwoofers own low-pass filters , phase and volume. 

Never is an easy task to blend the mains with subs but it can do with a little of patience . Btw, my subs are ( acoustic suspension. No ported/reflex. ) positioned in front of the main speakers and facing each to the other not looking to the seat position as the main speakers but side to side ( I don't know how to explain it. ).

R.

Earl Geddes investigated both symmetric and asymmetric placement of multiple subwoofers and concluded that asymmetric placement following a few basic guidelines was superior. Nothing against Todd Welti’s study of symmetrical placements, but there are other options.

As for whether or not to highpass filter the mains, the short answer is, it depends. If the main speakers and/or their amplifiers are being taxed, then it makes sense. In @audioquest4life’s situation, his main speakers have approximately sixteen times the cone area of a 6.5" midwoofer, and probably at least twice the excursion. Those interested can do the math.

Ideally any highpass filter in the main speakers’ signal path is not audible to the point of being detrimental. I have customers who have tried it both ways and use a protective highpass filter, and customers who have tried it both ways and do not.

When I design main speakers with subwoofers in mind, I tend to give them sufficient excursion-limited headroom that a protective highpass filter is optional instead of mandatory in most applications.

Duke

Well I’m glad to see that several on this thread, including @audiokinesis are taking up the issue of distortion and dynamic range of the mains being something worth considering when adding subs. :)

Too often that behavior gets overlooked for one reason or another.