Dual Opposed Subwoofer Theory?

[sloth_kwj]

I read hundreds of posts but I couldn't found exact theory and building guide about DOS.
As I understand, DOS has the advantage of canceling inner vibration but need twice larger volume and use under 80hz.
But how about distance of two driver? Or any other Pros and Cons?

 

[Ilkless]

distance of two driver

There should be no issue here, as the 2 drivers can easily be spaced within 1/4 wavelength of each other due to the large wavelengths in their passband (low frequency = long wavelength), avoiding interference issues.

 

[bigx5murf]

Are you talking about isobaric? I thought the point was to half the necessary volume?

 

[Chromatischism]

• They take less floor space than two separate subs
• They usually cost less than two separate subs
• Yet they provide the power of two subs
• They don't vibrate themselves or the floor much (could be a pro or con)

That's about it.

 

[Scholl]

They don't vibrate themselves or the floor much (could be a pro or con)

I agree with your four points but, out of curiosity, how do you more specifically think the absence of vibration could be a disadvantage ? Maybe less fun because of the absence of floor vibration, or is it something else ?

 

[Juhazi]

Are you talking about isobaric? I thought the point was to half the necessary volume?

No he isn't. Isobaric is totally different concept, sort of serial system.

Dual opposed has most benefits when setup is force-cancelling. This can be done in many ways an one interesting is PPSL - push-pull slot loaded system! It gives also compensation for Bl in/out nonlinearity if other driver is reversed

https://www.diyaudio.com/forums/subwoofers/177905-thread-ppsl-enclosures.html
https://data-bass.ipbhost.com/topic/179-push-pull-slot-loaded-quad-si-ht18s/

 

[sloth_kwj]

• They take less floor space than two separate subs
• They usually cost less than two separate subs
• Yet they provide the power of two subs
• They don't vibrate themselves or the floor much (could be a pro or con)

That's about it.

I'm thinking two DOS for 2.2 setup. Similar to Amphion Basetwo25 or Barefoot stack subwoofer. So only vibration and volume are all of issue?

 

[andymok]

The two driver would have to counter-act themselves I suppose? Wouldn’t that increase their load?

 

[egellings]

My impression of isobaric woofers is that 2 identical drivers are mounted at the ends of a sealed tube just long enough to prevent the rear speaker's cone from hitting the front speaker's magnet. They are wired to move in phase, and this sort of arrangement allows a smaller box size.

 

[Chromatischism]

The OP is not asking about isobaric subs.

 

[egellings]

I think the isobaric part is the constant pressure between the cones of the front and rear drivers, since the air volume never changes there.

 

[Chromatischism]

Now I don't have experience with this, but I think in isobaric, only one driver is powered.

 

[bigx5murf]

Now I don't have experience with this, but I think in isobaric, only one driver is powered.

Wrong, in isobaric both woofers are powered, but they're wired either out of phase, or in phase depending on how they're oriented (face to face = out of phase, facing same direction = in phase). This is also why isobaric increases power handling significantly, and lowers efficiency as well.

 

[Chromatischism]

Wrong, in isobaric both woofers are powered, but they're wired either out of phase, or in phase depending on how they're oriented (face to face = out of phase, facing same direction = in phase). This is also why isobaric increases power handling significantly, and lowers efficiency as well.

Huh, maybe it's the wording. Two sources I checked both said that only one driver is radiating sound. I guess that is because only one is facing outward to the room while the other is inside the box.

At any rate, the OP is asking about dual opposed, not isobaric subwoofers.

 

[Duke]

The dual opposed configuration (woofers on opposite sides of the cabinet to cancel vibrations) was patented, and at least one company went with a three-woofer triangular-opposed configuration to implement the vibration-cancelling concept without violating the patent.

I don't know whether the patent has expired or lapsed or gone undefended, or whether the invention has been licensed. I just remember bumping into it about fifteen years ago when I was researching multiple woofer patents.

I used to build isobaric subwoofers as a hobbyist. I made a direct comparision (using close-miked response curves) between a single-woofer sealed box system and an isobaric system half the size. Both used the same woofer. The published equation indicates a halving of effective Vas for isobaric loading and therefore the frequency responses should have been essentially identical. The actual measured response was very different; the isobaric system had much deeper bass than predicted, so I think the published equation is incomplete.

 

[bigx5murf]

Huh, maybe it's the wording. Two sources I checked both said that only one driver is radiating sound. I guess that is because only one is facing outward to the room while the other is inside the box.

View attachment 104759

At any rate, the OP is asking about dual opposed, not isobaric subwoofers.

Yes, only one radiates, because only one is outside the box. But they are usually wired in series or in parallel depending on the impedance. They are wired with phase that allows both cones to move parallel to each other simultaneously.

 

[sigbergaudio]

I read hundreds of posts but I couldn't found exact theory and building guide about DOS.
As I understand, DOS has the advantage of canceling inner vibration but need twice larger volume and use under 80hz.
But how about distance of two driver? Or any other Pros and Cons?

It doesn't require twice the volume, it requires the same volume PER DRIVER as a regular subwoofer. You will get twice the output in the same box, and less cabinet vibration. Which is obviously practical. The latter is especially useful in small, powerful subwoofers which might otherwise vibrate to the point where they're actually moving around on the floor at high playback levels. It's also claimed that this design will reduce distortion somewhat, but at least based on our measurements that's mostly theoretical (beyond the fact that you have two drivers of course, which require less excursion at the same playback level).

 

[digitalfrost]

I read hundreds of posts but I couldn't found exact theory and building guide about DOS.
As I understand, DOS has the advantage of canceling inner vibration but need twice larger volume and use under 80hz.
But how about distance of two driver? Or any other Pros and Cons?

There is nothing special about them. It's simply a box with two subwoofer chassis, so obviously the box is twice the volume, and the drivers happen to be on opposite sides instead of a single baffle. If possible, I would wire them in parallel (i.e. 8ohm chassis required) instead of serial since this should work better in case there are differences between the drivers.
The distance of the drivers should be of no consequence. Since this is not isobaric they do not need to be coupled via the air volume.

It works very well. I build 2 for my desk to prevent the things on it from rattling with the music.

 

[Juhazi]

WinISD is perhaps the most popular bass box simulator, worth investigating

Dual opposed woofers share same airspace typically. Air volume in the box is an important factor that affects the low end behaviour of drivers when long excursion and air compression/movement happens. This will show in spl response of the unit and maximum spl it can give per frequency. Bass reflex tuning depends on box volume and port length, number of radiators is negligible.

Many dogmas and traditions of loudspeaker's bass performance come from era of drivers with short Xmax and poor power handling, and passive designs without use of signal eq or motional feedback. In this millenium we can bend those rules quite a lot (at least for domestic use) simply and with low cost by DSP and modern driver units, driven by cheap and efficient classD amplifiers.

Dual opposed in my eyes best suited for hifi 3-way speakers, as seen in eg. Vivid Audio Giya. Home theater and PA subwoofers aim for very low freq and high spl capacity, which still means very large cabinets and often some sort of horn to maximize efficiency, then dual opposed is not best giving benefit. I have personal experience with a DIY PPSL dual opposed with two sealed boxes side by side, with 15" woofers. A single Hypex DS2.0 easily drives it to intolerable spl in my HT room!

 

[RealityCheck18]

This thread reminds me of my car audio days back in the 90's...right before my internet addiction started and I had time.

A buddy of mine, he was 21 years old and wanted the highest SPL yada, yada, yada and initialy, a 7 cubic foot box was not a problem. My hormones were under control by then and I realized he would soon tire of something that huge but I'm not the audio nanny. I spec'd a pair of beefy 12" subs and we built a 250 pound box that took up his hatch and back seat--because more is better. The pair of 12's were wired push-pull opposed which gave a few benefits. For him, he wanted to see the fancy sub basket he paid for so it was cone out on one side, magnet side out on the other which garnered cool points (well, it was cool if you were a guy but the girls thought it was ugly) Since audio is about benefits both real and imagined, the push-pull reduced distortion, allowed the sealed subs to run cooler (magnet side out cools better) and prevented his car from vibrating.

Eventually, it took about a month he tired of the magnet side out and the worry about cables, damage etc. and went to conventional opposed. Eventually he obtained a thing called a "girlfriend" and he meekly asked me if we can make the thing smaller. Sure! I cut the box in half to 3.5 cubic feet, slapped both subs in opposed and he was happy.... "for a time" He then wanted the sub to be even smaller, so I told him he could either have a single 12 in a 1.75 cubic foot sealed box or load both of them isobaric in the same box but one of them would be magnet side out and it would lose efficiency--but he can drive both subs with more power AND it would not vibrate...reduce distortion etc.

He ended up with isobaric clamshell magnet side out in a 1.75 cubic foot sealed box. Ya see, when testosterone is involved you need a "reason" to be a wuss....errr, call it "sound quality" Yes! That's it! The Iso sub worked well, did not vibrate (although SPL will shake your car to piece anyway) had poor efficiency but could take the power. Yup, part of the car audio game is WATTS and the Iso could take the watts...for the win! He also gained cool points for magnet side out to show those fancy baskets. He eventually broke up with his girlfriend, she returned his sack (she kept it in her purse) but his thrill of driving a portable subwoofer around was over. Such are the trials and tribulations when you date strippers.... they don't mix well with car audio.

My garage subs are pair dual 15" 8 ohm drivers configured at push-pull slot loaded subwoofers, they generally run ported but can be sealed by design. They do not vibrate, I can impress audiophiles and idiots with my supreme building skills by balancing coins on them! (Ooooh, ahhhh!) They have line arrays with over 80 drivers each placed on them and avoiding vibration with over 200 solder points per speaker is a good idea. Since my woofers are enclosed in a slot, it prevent drunken idiots from damaging them, crap flying around in an active garage hitting them and so on. That slot puts the drivers under pressure as they have a 2.2 to 1 compression ratio which....uhhh...efficiency increase etc. The kids like it because it increases air flow out of the slot which is always cool. It also naturally reduces distortion and the big one--looks unique to people or they tell me I put in one of the subs backwards. The biggest downsides of that configuration is you need to use quiet subs because the magnet side out, if it creates a lot of air turbulance you will hear it--not a good thing. The slot takes up space so the box will be bigger and it must be built correctly to maintain a seal. You also limit the bandwidth of the sub, not an issue if it is a true sub but if you want to go higher than 200Hz--better do the math to find the limts when loading subwoofers/woofers that way. Once you break 300Hz or so, slot loading creates issues so do the math.

Overall, if you use two subs per box--attempt to at least go opposed to gain something. Push-pull is better, it gives additional distortion cancelling but you need quiet drivers, driver protection etc. (not good for children or strippers) I find push-pull slot load to be the best because the drivers are safe, you can put a grill over the slot to hide them and no vibration is a good thing--if you just want subs, not worried about build complexity or size. Isobaric also works but very poor efficiency--unequal cooling of the drivers depending on configuration. However, it is very small and sounds great if you do it right.

Although dual balanced drivers gain cool points--the ultimate in cool is to have a push-pull horn loaded sub--either front loaded or tapped horn--your choice! Since horns by their very nature are heavy, complex, giant beasts--most of them use push-pull when loading them with dual/quad/octal drivers to improve reliability and performance.

Enjoy!