Opposite Drivers Sub

[Spaiz]

Hi,
I'm looking for an opinion or an educated answer to a question that bugs me: I've seen recently very often the design of subs with two opposite drivers - I'm thinking BR configurated. Pros are obviously that vibrations to the cabinet should cancel out.

I was searching infos on how 2 drivers act on a bigger enclosure. Basically, all the answers I found were: double up the volume for a single driver, you'll get +6 dB output. Somebody even suggests to put a divider in the enclosure, thus forming 2 fully separated enclosures.

Somehow, in my mind there's the notion that with double the surface of the drivers and a bigger enclosure, the bass extension should be somewhat lower. The "double the volume" theory instead would suggest identical low extension.

What I'm I missing? What's the correct theorical approach?

Thanks

 

[zelgall]

I've never understood the "cancel out vibrations" thought. It would seem that the vibrations are from the pressurization and rarification of the air in the cabinet. Two drivers will still create the same pressure unless they are working in oppposite phase which also makes no sense.
I saw a write up about a sub with a passive radiator, although they had a marketing name for it, and they claimed that the PR would cancel vibrations. I can only assume that it would be through the generous application of magic.

 

[just a guy]

Hi,
I'm looking for an opinion or an educated answer to a question that bugs me: I've seen recently very often the design of subs with two opposite drivers - I'm thinking BR configurated. Pros are obviously that vibrations to the cabinet should cancel out.

I was searching infos on how 2 drivers act on a bigger enclosure. Basically, all the answers I found were: double up the volume for a single driver, you'll get +6 dB output. Somebody even suggests to put a divider in the enclosure, thus forming 2 fully separated enclosures.

Somehow, in my mind there's the notion that with double the surface of the drivers and a bigger enclosure, the bass extension should be somewhat lower. The "double the volume" theory instead would suggest identical low extension.

What I'm I missing? What's the correct theorical approach?

Thanks

Double the drivers and double the cab volume (and port area) and you get the same thing as two separate cabs, so everything is identical to a single cab times two (including extension).

I've never understood the "cancel out vibrations" thought. It would seem that the vibrations are from the pressurization and rarification of the air in the cabinet. Two drivers will still create the same pressure unless they are working in oppposite phase which also makes no sense.
I saw a write up about a sub with a passive radiator, although they had a marketing name for it, and they claimed that the PR would cancel vibrations. I can only assume that it would be through the generous application of magic.

The vibrations that are cancelled are from the mass of the cones. These newer large drivers can have moving mass upwards of 1/2 kg (some are almost 2 lbs). Move that back and forth quickly and all kinds of stuff can happen, sometimes the cab can even "walk" itself across the floor. Put two drivers in opposed fashion and this cone motion force cancels out.

Also if you put them close enough together and not face to face (or magnet to magnet - so one is pushing and one is pulling) you can also cancel 2nd order distortion due to unsymmetrical stroke. This is not a tremendous benefit though, as 2nd order distortion is not objectionable like odd order distortion, and it can actually sound quite pleasant and full. It's not accurate but it's not unpleasant either.

 

[Spaiz]

the two examples that come to mind are kef's speakers and barefoot pro monitors. they both got opposite sub drivers.

Double the drivers and double the cab volume (and port area) and you get the same thing as two separate cabs, so everything is identical to a single cab times two (including extension).

so should they go lower? there's double the moving surface and double the volume, isn't it already a recipe for deeper bass? What about tuning the enclosure differently, ie giving it only one port? I'm still baffled (sorry for the pun).

 

[just a guy]

No, two cabs will not go lower than 1 cab (assuming they are all the same and no attempt is made to figure out the increased baffle area of two cabs or the increased radiation resistance of two closely coupled ports, neither of which will make much difference in acoustically small cabs). The sentence you quoted clearly says it will be identical including extension, in other words, not tuned lower.

If you only give it one port ideally it should be twice as large in cross sectional area (giving the same tuning), otherwise it will have velocity issues.

You could make the port LONGER and that will tune lower (in effect trading some or all of the 6 db gain of the dual cab for lower tuning), but you can do that with any cab so it's got nothing to do with this subject per se.

 

[djk]

" It's not accurate but it's not unpleasant either."

Ever heard a servo sub?

They sound much cleaner than a regular subwoofer.

The PPSL designs sound like a servo sub.

 

[zobsky]

"
The PPSL designs sound like a servo sub.

Because of what ?
1. the slot,
2. the cancellation of even order harmonics,
3. the cancellation the mechanical vibrations

The reason I ask is I was pondering a Tangband 6 x 9 version of this T-TQWT design since I have the drivers lying around ( http://www.diyaudio.com/forums/subw...1f-8-x-12-box-reccomendation.html#post2812188 ) and was wondering if merely reversing one of the drivers mechanically would yield significant benefits?

Thanks

 

[badman]

Because of what ?
1. the slot,
2. the cancellation of even order harmonics,
3. the cancellation the mechanical vibrations

Replace "slot" with "manifold" and then, yep, those are the key reasons (the manifold acts as a low-pass filter and mass-load)

 

[just a guy]

Because of what ?
1. the slot,

The slot will cause a spike in spl at a frequency related to it's dimensions. The spike may not be as bad as a sim predicts, but that's what the slot does. This can be anywhere between benign or problematic. The only way the slot could possibly sound better than slotless is if you actually need that spike, high inductance drivers will have a falling response so the spike could be beneficial to keep response flat as frequency increases.

2. the cancellation of even order harmonics,

The cancellation of even order distortion is the only audible benefit and it may or may not be worth the trouble - more on that later.

3. the cancellation the mechanical vibrations

The mechanical vibrations we are referring to here don't have a sound. The pressures in the box remain the same so what you are cancelling is the mechanical force of heavy cones moving. This can stop the box from physically rocking or "walking" but it won't sound different apart from the even order harmonics cancellation (except for the imperceptible loss of spl from the forces lost to rocking the box).

There are probably 3 main factors that will determine if you prefer the cancellation of even order harmonics.

1. How much even order distortion is there in the first place. This type of driver loading cancels out distortion due to non linear stroke in the forward stroke direction vs the backward stroke direction. This type of distortion can be due to the suspension, the Bl or the inductance.

Some drivers made by manufacturers that use a Klippel during the driver design phase are very linear and this type of driver won't benefit nearly as much from stroke non linearity cancellation as a cheap garbage driver with loads of even order distortion.

This type of linear driver is typically very expensive but measures very well on a Klippel and is very symmetrical throughout it's stroke. See the B&C 18TBW100 Klippel results as an example - http://circuitcellar.com/wp-content...kasonTBench_BCSpeakerReprintedwPermission.pdf

2. Whether or not you like the lean and clean sound of low even order distortion. This is not a given, in fact the fuller and very likely psychoacoustically louder sound afforded by 2nd order distortion may actually be preferable.

3. How much odd order distortion is present. If you take away all the pleasant sounding even order distortion and leave large amounts of odd order distortion the sound can be quite bad. If the subwoofer has large amounts of odd order distortion of course.

We all choose our favorite compromises. DJK very famously likes PPSL, whereas PPSL (and Servo for that matter) are things I would never even consider. PPSL requires a good amount of box volume for the loading chamber (compared to slotless) and Servo is expensive. And the only benefit to either is cancellation of even order distortion.

On the other hand there are two other approaches that are far more to my liking. Doubling up subs (both drivers and enclosures) and/or horn loading provide the benefits of lowering even order distortion (for a given driver, tuning and spl level) as well as lowering odd order distortion as well as a bunch of other benefits including the ability to get a lot louder. These two options are quite a bit larger and more expensive than the PPSL or servo options but they also have a lot more benefits.

 

[zobsky]

Well put, @just_a_guy