Geddes multiple subwoofer method - 3 subs vice 4

“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.

There is no right one way to setting up a subwoofer system, but there are some common denominators. The first is as @erik_squires described, a complete two way crossover is mandatory preferably digital because this is the only way to accurately time align the individual drivers. @james633 With a 64 bit floating point processor running at 24/192, DSP is totally invisible. There are only sonic advantages and no disadvantages as long as good DACs are being used.

The most important concept that Geddes advances is the use of multiple subs also championed by @audiokinesis. Two is the minimum, I use 4 as does audiokinesis.

@audioquest4life It is not the size of the woofer that counts, it is the frequency range it operates in. The higher a woofer goes the more important is the use of a high pass filter. Many large woofers are running up to 350 or even 500 Hz. This is well into the midrange. Middle C is 256 Hz. Bass under 100 Hz will cause doppler distortion of everything else carried by the driver. The is also no such thing as too much head room. You also have to use a filter if you plan on time aligning the drivers which IMHO is mandatory. 

Another important concept is running low bass with up to 20 dB of gain. This gives you the visceral character of a live performance. If done right it does not make the bass warm or boomy it just increases the vibratory sensation of the music. My favorite curve increases bass at 4-5 dB/oct from 100 Hz down. 20 Hz is hot by 10 dB. Some rooms may require more.

The room is key. I am very fortunate as my listening room was designed as such and there is no back wall. The room opens to the rest of the floor which is acting as one big diffusor. So in my situation I am listening just as much to the drivers as I am to the room whereas most people are listening more to the room. 

Another important factor that was mentioned was room treatments for bass do not work. It is a total waste of money which could have been spent on another subwoofer or a better crossover. 

@erik_squires I have no idea in this day and age why someone would use a ported loudspeaker. We have loads of powerful amplifiers and digital signal processing can give you any reasonable frequency response you require. Subwoofers in particular should always be sealed as port noise is ubiquitous with the amount of air moved by a subwoofer driver. The effect of lowering many types of distortion including IM, Harmonic and Doppler and increasing headroom is just as important as adding low bass when using subwoofers with two way crossovers. Good for you. Now try getting this into the heads of most subwoofer users who are being sold a bunch of malarkey by the manufacturers.