Cabinet Sizing for Opposing/Facing drivers.

[AllisterMcRae]

Howdy,

Got a bit of a bee in my bonnet around modeling how something like the Sonos sub works, and then finding a lot more information about opposing drivers, becoming interested in that, too.

For all of the discussion out there, I'm still left wondering:

1. What does one do with cabinet sizing when opposing drivers are sharing the same volume?
2. How does facing the drivers together ala SonoS Sub affect the modelling?
   1. Do they still share a volume or each have one of their own?
   2. If they have one of their own, need it only be half the size since one is perhaps getting a doubling of sound output on facing drivers?

It doesn't appear to be isobaric, and it would seem that drivers facing each other would double the sound output of the drivers.

Though also, depending on the spacing between drivers if facing, or the sealed volume if opposing, one could maybe push the drivers a bit harder since the sound waves would keep the compliance of the opposing driver fairly tight. And I'm not quite convinced the size of the hole in the Sonos sub is calculated - thinking as far as to say "it looks like its about the same size as X% of the two facing drivers" would be making too many assumptions without other examples to observe and compare (of which I think I might have seen one and cannot recall what it was)

I was going to link articles - but there is actually so much info out there, I'm kind of surprised I can't find a simple answer. Pretty sure I've been reading everything thoroughly.

Thanks!

[DeZZar]

Here's a series of example results using the SB Acoustics MW16P-8 woofer....just for arguments sake.

Alignment	Volume	f3	Max SPL (at max power)	Max Power (Mechanically limited)
Single Unit, Bass Reflex	22L	40hz	100db	18 Watts
Two units, Bass Reflex	44L	40hz	106db	36 Watts
Two units, Isobaric Loading, Bass reflex	11L	40hz	100db	72 Watts

All of these examples are sharing the same air volume.

In order to achieve the same magnitude response, two woofers require double the volume of a single woofer, take twice as much power and play 6db louder. Isobaric loaded woofers require half the air volume, take more power but sacrifice efficiency to the point where there is nothing gained in terms of SPL.

The sonos sub is essentially two units sharing the same internal air volume but just facing each other. The opposing orientation is all about vibration cancellation - nothing to do with boosting output.

So to specifically answer your questions...

What does one do with cabinet sizing when opposing drivers are sharing the same volume?

You need to double the internal volume to achieve the same magnitude response

How does facing the drivers together ala SonoS Sub affect the modelling?

You would obviously only consider this for woofer playing omnidirectional frequencies - that being said it doesn't in and of itself affect the modelling in terms of the magnitude response, box tuning etc. This is usually all about vibration cancellation.

Do they still share a volume or each have one of their own?

Shared.

If they have one of their own, need it only be half the size since one is perhaps getting a doubling of sound output on facing drivers?

Its not really a question of half as such. If your working with a single unit in its own air space then there are no loading benefits and it just needs to be modelled for an appropriate volume and tuning that gets the magnitude response from the driver that suits your needs.

It doesn't appear to be isobaric, and it would seem that drivers facing each other would double the sound output of the drivers.

Its not iso and no simply having drivers face each other isn't doubling their output.

[AllisterMcRae]

Thanks for the detailed response. That is a full on education on iso tendencies. I appreciate how you pulled it down into the component parts - makes things more clear .

[billfitzmaurice]

Dual opposing drivers has no effect on cabinet panel vibration. It would eliminate the tendency for a sub to 'dance' with a conventional front and rear baffle arrangement.

[JonathanPenner]

I was under the impression that drivers aligned along their moving axis could be considered isobaric, so it seems I was quite incorrect.



If I understand correctly now, in these examples only 1 and 2 are actually isobaric.

1. 0.5x Vb, isobaric
2. 0.5x Vb, isobaric
3. 1x Vb, normal
4. 2x Vb, force-cancelling
5. 2x Vb, normal
6. 2x Vb, force-cancelling

Is that right?

[billfitzmaurice]

That's correct. But as for force cancelling, that doesn't apply to #6. Since the plenum exits are on opposite sides of the box that would tend to cancel lateral forces that might cause it to rock or move, but it would have the same effect with one driver.

[Wolf]

Why would isobaric use 4x the power over a single or twice that of a pair? It should be 36 as well.
Wolf

[DeZZar]

Why would isobaric use 4x the power over a single or twice that of a pair? It should be 36 as well.

Well, yes, each woofer handles "60 watts" and two of them, ISO or not, handles "120 watts". But to me, that's virtually useless surface level information given the subject at hand...

As you know, for a given application/tuning frequency you have to consider what's going on down around the mechanical limits and this is where it gets interesting between these three.

Up at 200Hz, sure the thermal power ratings do as you would expect. But when you take these three alignments and compare them around for an equal magnitude response this is what you get. So I should have stated originally..."given we are interested in bass performance here, this is a comparison of the alignments capabilities @ 50Hz."