Isobaric Subwoofer





Introduction: Isobaric Subwoofer

About: I'm fairly mint most of the time I'm a firm advocate of treating people how you would like to be treated yourself. People are always nice to me, so it must work! Try it!

I decided recently that my home cinema system could do with a nice subwoofer, and rather than buy something, I thought I'd build myself one...

NOTE: This is only a short explanation, there is far more info here -

Step 1: Cutting

Construction starts with measuring and cutting the MDF.
My design with its double skinned walls does require quite a few panels - 18 in fact, and to ensure we end up with a rigid and airtight box the cuts must be very accurate.
Ideally for this task you would use a tablesaw - we are looking to achieve accuracy of panels to better than 1mm.
This would take some doing (or at least a lot of plane work and sanding) with a handsaw or a circular saw.
This first stage of marking out and cutting is very important (and a little boring) but it cannot be rushed.
The end result really does depend on how accurate these panels are cut, so measure twice, cut once, take your time etc...
It will pay dividends later.

Step 2: Glue and Screw

Using plenty of glue is key to a good strong assembly, ideally the woodglue will ooze from the joints when the screws are tightened and should form a continuous bead - which is a good indication that enough glue was used. When gluing I tend to use small paintbrush and coat both mating surfaces with glue.
Once assembled though I don't think it hurts to brush just a little more glue into any remaining gaps, I try to get the glue as deep into the gaps as possible using a wallpaper scraper to push it in under pressure

Step 3: Ports

The inner port calculations reveal that the port must be 271mm, this is fairly tight considering that each chamber is just 149.5mm wide plus 36mm for the centre baffle giving a total of 335mm
Assuming we arranged the port so that it was centred on the baffle that would only leave 32mm at each end (less once we fix some wadding/sound-deadening at each end)
It is generally accepted that the ideal is to leave at least one port radius between the opening of the port and any surface. Unfortunately with a straight port located tight in the corner we can't achieve this, so the plan is to bend the port a little.
I suppose it doesn't really matter how you do this as long as some attempt is made to maintain a fairly uniform internal cross section.

Step 4: Gasket

I want this lid to be removable for the purposes of tuning the enclosure, so there needs to be some kind of airtight seal where the lid meets the box, my plan was to route out a semicircular groove using the router and then apply bathroom silicone to the groove to form a gasket - I had no idea how well this was going to work out!!

Step 5: Drivers

The speaker wires come through the baffle and then pop out near to the driver connection tags, the picture below shows the wires connected up. I soldered them to the speaker tags as this is more reliable than crimping connectors on, as always a couple of cable ties and P clips help hold the wire in place, I've also used cable ties to show which wires are +ve and which are -ve.

Step 6: Electrics

There isn't really too much to do electrically, just link up the speakers to some kind of external connector.
I favour the neutrik speakon connectors, they are capable of carrying huge amounts of current, they lock into place very positively and have the added bonus of being 4 pole:
I've used a right angled 4 pole neutrik plug, as the look quite neat, and some 4 core 4mm sq layflat oxygen free copper speaker cable, it is probably the largest cable you could get away with using with this plug, so you do have to be careful with the connections or else you'll run out of space

Step 7: Final Assembly

Then applied plenty of glue to the top of the rectangles. I then placed the base on top and once I was happy it was in the right place I put a few screws in to hold in and then checked that all was well before finally screwing it into place.

Step 8: In Use

Of course I'm pleased with my efforts, I really did put a lot of effort into this, so failure really wasn't an option!
It's been in the lounge for a few months now and is fairly well worn in, and certainly it makes bangs and bumps at all the right times!
A bit of initial messing around with the levels to get it balanced in the mix was required (I use a separate power amp to drive this one) but once that was out of the way it's been plain sailing really.

NOTE: This is only a short explanation, there is far more info here -

Thanks for your interest

2 People Made This Project!


  • Game Life Contest

    Game Life Contest
  • Clocks Contest

    Clocks Contest
  • BBQ Showdown Challenge

    BBQ Showdown Challenge

101 Discussions


11 months ago

You are lucky to have a cute lil girl help you in projects!

Nice instructable. I love that your work is very clean and well thought out. Your in depth replies to most comments means you're a true gentleman and a scholar. Keep up the good work.


2 years ago


Just 1 question, I couldn't see the exact dimensions of the MDFboards can you tell me please??

Do you know anything about the sound output when wired in series or parallel? I built a planar isobaric box for my car (so many benefits... Stackable, protects the drivers, better sound quality...) and when I switched the wiring from 2 ohm to 8 ohm the output volume got much higher. Is that just an amp quirk or a property of these enclosures? Usually amps potential output goes up when you lower the resistance, from what I have read. Very confused here.

9 replies

What you've described is not normally the case.
You see that connecting two 4ohm speakers in series will give you an 8 ohm impedance. Connecting the same two 4 ohm speakers in parallel will give a 2 ohm impedance. Since power is V2 / R reducing the impedance from 8 to 2 ohms will increase the power 4fold.

There is a caveat though.... (isn't there always!!) Most, but not all amps can drive a 2ohm impedance, because 2 ohms does take more current and therefore power some amplifiers will just cut out, especially at high volumes.

If it works for you though, and your amp isn't getting too hot, then you'll probably be ok...

I suspect what has happened in your case is that you have the polarity of one of the drivers wrong... You want the drivers moving in opposite directions, mabybe in your 2ohm (parallel config) you had them moving in the same direction and therefore cancelling each other out?.... If your speakers are set up like mine, then you'll need to connect them out of phase (i.e for a parallel config connect the +ve of one to the -ve of the other and the -ve of one to the +ve of the other) ... for a series config (8 ohm) then connect the two +ve terminals together and then connect the amp to the -ve terminals of the speakers.

Good luck and thanks for the comment

ok so what will i get if i have 8 8ohm speaker wired in parallel (soz i am too thick to work it out!)

8 x 8ohm in parallel will give you 1ohm.
Parallel resistances/impedances are calculated by summing the reciprocals and then finding the reciprocal of that, so
= 1/(1/8 + 1/8 + 1/8 + 1/8 + 1/8 + 1/8 + 1/8 + 1/8)

Which is 1.

1 ohm is a very low impedance to present to an amplifier though, so you may have problems. You might be better off wiring in series parallel to give something more sensible - 4 pairs of speakers wired in series connected in parallel would give a more reasonable 4 ohms


ok thnx soo much you really helped me, the amplifier i need runs 6-16v DC and 4-16ohms i am making a mini 8 driver (18x18mm they re so small) line array, so heres a diagram correct me if i am wrong. A --Speaker1--Speaker3--Speaker5--Speaker7 M | | | | P --Speaker2--Speaker4--Speaker6--Speaker8 Thnx for replying so quickly. i will probably post an instructable when i get the speakers and start to make it! thanks again! :-)

the lines next to 'M' are supposed to show each pair connected but when i posted the comment it got rid of all the unnessesary spaces >:-(

That's right yeah. Take speaker 1 and 2, wire them in series (negative of speaker 1 to positive of speaker 2) Take speaker 3 and 4, wire them in series (negative of speaker 3 to positive of speaker 4) Take speaker 5 and 6, wire them in series (negative of speaker 5 to positive of speaker 6) Take speaker 7 and 8, wire them in series (negative of speaker 7 to positive of speaker 8) Then wire the positives of speaker 1,3,5 and 7 to the amp positive terminal, and the negatives of speaker 2,4,6 and 8 to the amp negative terminal. That little lot will present a load of 4 ohms to the amp Good luck

Hmm. I may have found a fluke then. With the speakers wired in phase or out, I still have more power in 8ohm (tried both polarities as I thought I might have forgotten which way I wired one of them). What caused me to switch in the first place was I noticed my sub volume didn't increase in a very even fashion with my midranges. At a low volume with it balanced, the subs would be overpowering at a high volume. With them balanced at a high volume, the subs would be inaudible at a low volume. I suppose I just have a retarded amp. From what I have heard, sound quality tends to be better at a higher impedance as well. Any thoughts on that?

I guess it is possible that your amp just isn't happy running at two ohms. So long as the sound level is high enough for you running at 8, then great. Another alternative (assuming your amp is a stereo one bridged into mono) would be to drive each speaker from a separate channel of the stereo amp.

It's a 2ohm 300watt rms mono amp. I'm hoping to replace it with a stereo one by a more reputable brand in the future though. My current one is a bazooka, and I'm looking at alpine and pioneer. I've got a pioneer driving my midranges, and it sounds amazing. The problem with running stereo amp, is I would still need to make the signal mono. I doubt the isobaric configuration would perform well with signal only going to one driver. Eg, bass drum on only the left channel.

No not really, turn it up loud enough and it will start to chuff, but not at reasonable listening volumes. Cheers for your comments.

LOL!! Fair play, whatever rocks your boat mate!! Mine would probably not go loud enough for you then.. Cheers for the interest in my project