Sub: 1 large or 2 small drivers ?

[Bigun]

I've read discourses on this question in several threads but couldn't find a consolidated discussion and clearly most sub's seem to be based on a single large driver. It's a question I am still asking myself I thought you wouldn't mind me stirring it up with a new thread ...

Which is best ? two 'medium' sized drivers verses one 'large' driver (e.g. SDX7 x 2 vs SDX10/15) so in essence the total cone area is about the same between the two options. The goal is something that has the sound quality to match a Full Range Fostex main. What I think I've learned so far is:

single (large) driver
> bragging rights !
> easier build
> easier load on the amp compared to two drivers in parallel
> smaller box volume
> lower f3 and f10 in sealed box
> lots of driver choices out there

two (smaller) drivers
> cancels out the driver-box vibrations
> cone excursion is reduced for same overall output (SPL)
> push-pull option cancels out cone movement non-linearities
> smaller driver diameter = more flexible in box dimensions
> smaller drivers can often support higher XO frequencies

[planet10]

A good set of criteria... to my mind the advantages iven by vibration cancelation should be given a lot of weight... that a smaller driver tends to go higher i feel is very important for mating with a FR.

Boc size, excursion, efficiency are all tied to Hoffman's Iron law, there is prbably not a whole lot of difference between 1 driver & 2. And as to amps & impedance, most plate amps seem optimized for 4 ohms. I also take advantages of a push-push woofer to be able to use both channels of an amplifier to drive 1 box.

dave

[Bigun]

So does it come down to

[larger cone = lower fs] vs [two small cones = vibration cancellation]

In a dedicated sub where vibrations don't affect other drivers would this favour the lower fs of the larger cone ?

real world example:

SDX10: fs = 26Hz, cost $135
2 of SDX7: fs = 34Hz, cost $192

(the Sd of the SDX10 is still larger than 2 of SDX7 but it's the nearest match I had available)

[GM]

For < 80 Hz BW there's no 'replacement for displacement', so it's all about driver swept volume (Vd) and having enough low distortion power handling (Pe) to use it all. As you increase the XO point and especially if you decrease its slope order, then of course its HF BW increasingly comes into play as does its placement WRT the mains.

WRT driver Fs, as a general rule there's no such thing as too low an Fs, just a point of diminishing returns, but with basic vented or sealed cab loading you want it to be at least as low as the lowest note it's expected to reproduce at any significant output with ~0.707*Fs being the practical lower limit.

GM

[Bigun]

Using the same drivers here's the data:

SDX10: Vd = 330sq.cm x 18.4mm = 607.2 cu.cm
two SDX7: Vd = 2 x 128 sq.cm x 11.1mm = 284 cu.cm

On this basis the SDX10 'smokes' the double SDX7's (not literally I hope !)

SDX10: 0.707 fs = 18Hz
SDX7: 0.707 fs = 24Hz

Both are pretty low, probably low enough.

Let me add something related to vibrations. Let me assume I can at least characterize this issue by inertia of the driver movement which is a combination of the effective mass (Mms) and it's speed, which when we compare drivers working at the same frequency varies with sqrt of the displacement.

SDX10: vibration 'factor' = 112g x sqrt(18.4mm) = 480
single SDX7: vibration 'factor' = 21.9g x sqrt(11.1mm) = 73
two SDX7: vibration 'factor' = zero

Well, the vibrations for the larger cone are significantly higher than the smaller cone.

[GM]

Yeah, unless pipe organ symphonies and/or special effects is required, then around 27 Hz is a good choice. I'm running 20 Hz Fs tuned to 16 Hz for pipe organs, but it's not low enough for some movies or special effects recordings. As long as I stay in this house though, there's no point in going any lower and/or louder than what my decades old subs can put out as I made a mess of it shaking it to pieces some years ago with a pair of ServoDrive Contrabass 14 Hz sub kits/500 W.

Anyway, I can make a sub stand still, so in this comparison the SDX10 is the 'no-brainer' choice for < 80 Hz if there's a spot in-room where it will blend well with the mains without calling attention to itself. This to me is far more important than any of the other criteria listed, so multiple subs that can be placed around the room and even one up high to deal with the floor-ceiling modes as Dr. Geddes recommends is the way to go.

GM

[Bigun]

And to control the LF extension with the sealed box is simply a case of choosing the volume. If I want a more gradual fall off in response I go with a larger box - is this a case of BIB ?

[GM]

Depends on the room's gain curve, i.e. a perfect room will have a 12 dB/octave rise below the first axial mode, so a T/S max flat alignment is the ideal match. Most rooms are somewhat lossy, so a <0.707 Qtc is usually required, though for a super lossy room such as mine, a critically damped 0.5 Qtc is the best choice overall IMO if there's enough space available.

GM

[Bigun]

Originally posted by GM
a critically damped 0.5 Qtc is the best choice overall IMO if there's enough space available.
GM

That's a 127litre box (sealed). It's not small, but quite acceptable for my application. Just have to add a Bash300 and I'm away

[GM]

Just so long as you don't overdrive it. 0.5 Qtc doesn't protect the driver much.

GM