Is it ok to put 2 subwoofers in a single sealed enclosure?

Is it ok to put 2 subwoofers (wired in series) in a single sealed enclosure or should they be seperated?
My worry is that the single sealed enclosure may cause too much pressure rather than adding more pop to the sound.

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If these are the Alpines you posted about in Music, the reccomended sealed box size from the manufacturer is between 0.6 and 1.25 cu/ft. It is listed under the Hands-on Research tab. At that size, you can just double the size and put a divider 1/2 way to make 2 chambers. Easier than trying to calc everything out.

As for the different sealed box sizes... smaller boxes generally reduce system Q (Qts) while raising the resonant frequency of the box (Fb), meaning that bass is tighter controlled but not as deep. Conversely, larger boxes raise Qts and lower Fb, ie, they deepen the bass but can make it sound a bit muddy.

Running some calcs, for a box size of 0.6 cu/ft, Fb will be 55 Hz with a Qts of 1.1. For a box size of 1.25 cu/ft, Fb will be 45 Hz with a Qts of 0.9. In this case, the opposite of what I said above is the case. If I were you, I'd use the larger box size (doubled with a divider between) if you can make it fit where you want.

For fill, try some Polyfill. This is synthetic stuffing you put in pillows and can be found at craft stores and, more easily, at Walmart in the craft section. Fill the box (after installing everything else) but don't crush it in there. The purpose of fill is to reduce reflections inside the box, making the box seem slightly larger.

Regarding wiring in series, you will reduce the potential volume if you do that, making them go from 4 ohm to 8 ohm. The amp you use will deliver less power due to the higher impedance. Use either a stereo amp at 4 ohms/channel and wire as reccomended or a mono amp rated for 2 ohms and wire the subs in parallel (pos. to pos., neg. to neg.). The bonus of this is that IF a sub blows, you will still have at least one working and it won't blow the amp. Note: for a given solid state amp impedance rating, a higher speaker rated impedance is safe, lower rated speaker impedance runs the risk of pulling too much current through the amp which could do damage.

I do appreciate the crossover examination from my other questions but this was more of a practicality question for others who might find this. It was the issue of how the movement of 2 speakers would effect each other when fighting to change air pressure or when helping each other by moving together that I was trying to answer.

I was wondering about the fill. I have seen on very LARGE applications of subwoofers where they do not use a 'fill' but instead will line the outer surfaces in carpet-like material then place what looks like a set of cubby holes in the back. I've wondered if this method applies to smaller speakers.

As to the wiring in series. I chose to do this to use the crossover on my amp. While it does increase my impedance it also keeps me from drawing too much as I am building this with a car amp and desktop psu. I tested the power limits of running them seperate in stereo and individually with the crossover but the series-wired dual connected to the crossover gave the most volume before causing power supply to give out.
Thanks for the clarification.

Take #2...

The way a standard audio driver (what most people mistakenly call a speaker, just to clarify - a speaker is the total system of driver(s), enclosure, crossover, etc.) works is that the surface moves back and forth in a linear fashion, moving the air in front and behind it causing pressure waves in that air that radiate outward from those surfaces. As the surface moves in one direction, let's say forward, it will create an area of higher pressure in front, while causing an area of lower pressure behind it. As long as these "waves" are kept separate enough, the sound will be a reaonable representation of the driver movements. If the waves are too close together, they can suffer from cancellation (due to incorrect phase addition - 0 deg. phase plus 180 deg phase results in cancellation), reducing volume, frequency and/or other components of sound.

To answer your question, let's look at some scenarios.
1. A standard rectangular box with both drivers located on the front. Both drivers should be wired to be in the same phase as the other. So, as one moves outward so should the other.
2. A standard rectangular box with one driver on the front and one driver on the back. The drivers should be wired 180 deg. out of phase so that as one moves forward the other moves backward. If you were to look at it from above, both drivers are actually moving in the same direction at the same time.
3. A tubular enclosure with a driver at each end. This works the same as #2.

What it all boils down to is that the drivers mounted to the enclosure should move in the same direction at the same time, regardless of box position.
You may find references to having the midrange driver wired "out of phase" in order to enhance or smooth the frequency response of the system, but this will only become an issue if you hear a gross discrepancy when you are testing the system. The only way to find out if that is true is to switch the leads around on that particular driver, leaving the others as they were.

Re: wiring. Gotchya! Good way to experiment with your setup's limits.

Re: fill. Large subs need substantial boxes in order to keep internal box vibration to a minimum. This helps direct all (most) of the low frequency energy where it is supposed to go. The "carpet like" material acts as a damper for the box wall while the "cubby hole" areas also act as damper and a stiffener to the wall. If the "cubby" is not consistently placed throughout the box, they are most likely there to prevent the box from resonating at different frequencies. This particular option is a much longer discussion than what I have already put forth, so if you want more info, post back.

Hope this helps clear up some questions.

No. That's perfect. Exactly the answer I needed. I was all hung up on the idea of the air pressure inside a sealed environment causing issue. I had felt that if they both went outward at the same time they would be pulling against each other and reducing the volume or causing the driver to work too hard. It had never occurred to me that it may not be the inside that mattered but how they reacted with the air outside. I hadn't even thought there could be a difference in mounting them to different sides of something.
fordf150man5 years ago
make sure the sealed enclosure is big enough you can find that cubic feet measurements on the internet
Re-design6 years ago
If you design your enclosure to the correct dimensions, and you phase them the same way it will work fine and there will be plenty of airspace to let them do their work.
Andale_The_Great (author)  Re-design6 years ago
I don't know what you mean by "phase". At a guess I would say I think you mean the polarity?
I had thought of the polarity. I imagined that with them connected properly [+>->+>-] that it would cause the speakers to each push out (or pull in) at the same time, causing more intense pressure changes.
Conversely I imagined that doing opposing polarity [+>->->+] would cause the speakers to do the opposite, one pushing out at the same time the other was pulled in, creating a lack of change in the pressure of the enclosure.
The second one seems safest but causes no pressure changes at all. I think this would cause the cones to have more a more fluid, strengthened movement. (Like two rowers in a boat facing the same way allows for a greater push when strokes are matched up.) The first seemed like it might cause the speakers to fight against each other, reducing movement of the cone. (This being more like two rowers facing each other rowing towards the other then apart at the same time, hence going nowhere.)
You want the cones moving in the same direction or they will cancel out each other almost creating no sound.

"wired in series" - doesn't sound too good.

Andale_The_Great (author)  lemonie6 years ago
there are often reasons for choosing to put speakers in series or parallel. For example:
My car amp is made to run 4 to 8 ohm drivers.
My car radio is made to run 4 ohm but can handle up to 8 ohm at seriously reduced volume.
My home theater I currently run is an Onkyo receiver that requires 6 ohms or more. Since the system is a 6.1 but I wanted to do it with the speakers I already owned then many of my speakers were too low impedance. I combined two speakers for my center channel which contain a midrange and a tweeter and come out to 3.5 ohms, making them 7 in series allows them to be used on my Onkyo safely. I did the same with some another midrange/tweeter speaker set and a pair of subs that are really just mids with a passive lowpass filter.