Introduction: Isobaric Subwoofer

Picture of Isobaric Subwoofer

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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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


bhvm (author)2017-07-21

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

sish1 made it! (author)2017-06-18

After a lot of hurdles, completed this on my own including cutting the mdf by hand saw! The result is worth the efforts taken to finish them.

Only problem is the weight and airproofing the cover. I still need to add a cover to hide the mdf.

sish1 made it! (author)2016-12-27

After some time, I got the time to post this image!

wizard124 (author)2016-06-19

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.

sish1 (author)2015-11-12


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

mortezaab (author)2015-01-15

stuffman (author)2009-12-18

Well done, I bet this thing weighs a ton!

toastyboy (author)stuffman2009-12-20


Well at least 40-50kilos I would say.


hack124x768 (author)2009-05-23

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.

toastyboy (author)hack124x7682009-05-25

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

Da Nugesta (author)toastyboy2009-08-16

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

toastyboy (author)Da Nugesta2009-08-16

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


Da Nugesta (author)toastyboy2009-08-16

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! :-)

Da Nugesta (author)Da Nugesta2009-08-16

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 >:-(

toastyboy (author)Da Nugesta2009-08-16

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

hack124x768 (author)toastyboy2009-06-03

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?

toastyboy (author)hack124x7682009-06-03

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.

hack124x768 (author)toastyboy2009-06-04

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.

BMWalsh (author)2009-05-05

Do you get port noise from that ruffled carpet on the inside of the port?

toastyboy (author)BMWalsh2009-05-05

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

BMWalsh (author)toastyboy2009-05-06

Lol, for me, a reasonable volume level is not nearly enough :-P

toastyboy (author)BMWalsh2009-05-25

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

Koil_1 (author)2008-10-31

...That's not to mention being able to get into the cabinet to replace a blown or damaged driver... This is seriously important because if you can't replace the driver you gotta rebuild the whole project. Having worked in this field I've seen how hard it is to open one of these boxes after it's been completely sealed. It's difficult at best and usually renders the project as scrap due to breakage of the building material.

toastyboy (author)Koil_12008-11-01

Yeah, but you can see why they do it. Making the gasket and getting a good seal was very time consuming. I'm glad I did it though. Cheers for your comments

Koil_1 (author)2008-10-31

One could just as easily use a 90° bend available from most places that sell PVC fittings. It's what I do and it works well because of the uniformity of the curve. They usually cost less than $5. Preping the fittings isn't that hard either if you don't mind the cleaning chemicals and glue. Another added bonus to this is the flange on the end that can be used to further extend the port to match the harmonics of larger drivers.

toastyboy (author)Koil_12008-11-01

Good point, I did consider using premade fittings, not sure why I opted to make one now...

-Aj- (author)2008-06-20

OHHH isobaric woofers are sweet hey. well looks cool anyway.<br/><br/> six order bandpass isobaric.haha wooww way to be getting complicated hey :P<br/><br/>ANyway...<br/><br/>best calc tool for speaker enclosures. and is also free to download (freeware)<br/><a rel="nofollow" href="">WinISDPro</a><br/>and here for abit of a tutorial thing<br/><a rel="nofollow" href="">Guide: Using WinISD</a><br/>

manticore (author)2008-06-05

That was a really great idea to mount the speakers like that and make them out of phase. I've been building car audio subwoofer enclosures for myself and friends for a few years now. Right now, I'm making a box for four 6x9s. I'll post it when I'm done. I think I may take your idea with two 6.5"s. They're component speakers, so a bandpass box doesn't really make much sense, but I love the concept. I'll give it a try. I really wish I had a nice router.... Gotta use the Dremmel for now.

toastyboy (author)manticore2008-06-06

Good luck with it. Routers really aren't that expensive anymore, nor are they difficult to use. Good luck if you choose to buy one. Let me know how you get on with the 6.5"s Cheers -Dan

angelwill26 (author)2008-05-27

I sooooo want that router system! lol

toastyboy (author)angelwill262008-05-29

It wasn't expensive, the router was like £35 pounds and the vacuum extract is just my old Dyson - I have a central vac now... see my other instructables!! Thanks for your interest. -Dan

Friggin Smift (author)2008-05-27

"F"ing awesome man I have built some woofers myself and I gotta say your work is top notch

toastyboy (author)Friggin Smift2008-05-27

Wow! Thanks! Glad you enjoyed it. I'm sure anyone could do the same, it's just a case of using good tools, good materials and taking your sweet time. If you aren't happy with something, then do it again... Cheers -Dan

HammyHavoc (author)2008-05-26

I'm extremely impressed (: AND jealous :P Nice one mate

toastyboy (author)HammyHavoc2008-05-27

Thanks for your interest! No need to be jealous, just build yourself one!!! Cheers -Dan

HammyHavoc (author)toastyboy2008-05-27

Well, I think I might spend the next few days making my own, I'll post some pics in a few days to let you know how it turned out! Thanks

toastyboy (author)HammyHavoc2008-05-27

Yeah, good luck, and take your time. That sub took me probably a whole month of evenings after work. So about 120-150 hours. ...but it's worth it for a nice finish and a nice sound

Solitus3989 (author)2008-04-25

Hi toastyboy. there have been alot of 'arguments' above about whether your design is a subwoofer or just a woofer and whether is it isobaric or non-isobaric... i just have one question and a comment. How much did the whole project cost? And my comment is your daughter is very cute and im glad you could involve her. it's a bit of inspiration to me when i tell me little brother that he's too young to help. thanks for the instructable. and i hope your wife doesn't mind the "loudspeaker component designed to reproduce only extremely low bass frequencies" (if i called it a subwoofer/woofer i may be proven wrong in a barrage of comments) too annoying.

toastyboy (author)Solitus39892008-04-25

Hi Solitus,

Regarding the cost, I would say it came in at under £150. This breaks down as:
£80 for the drivers
£20 for the mdf
£10 for the carpet
£10 for the electrics
£10 for fixings, screws, bolts
£10 glues, silicone and fillers
£10 piping bits and bobs

If you had to buy all new it would cost more, but the prices above assume that you don't (say) have to buy a full £10 tub of glue and only use half of it etc...

The time element is far more expensive, I spent probably 100hours on this, even at £10 an hour that's a grand!!

...but I loved doing it.

I'm glad you enjoyed my "enclosure containing a pair of transducers designed to produce (relatively) low frequencies within the bounds of the human audiable spectrum" !! ;-)

Solitus3989 (author)toastyboy2008-04-26

thanks. :-D that would be about $300 USD, but i know we have alot of those things lying around. im thinking of making a sub box for my car, and i am looking for designs. without the need to buy the drivers, this would be a cheap and easy design. :)

toastyboy (author)Solitus39892008-05-01

Yep, about $300 dollars, although stuff is the US is normally $ = £ so you might get it for less.

It does make for a nice small box, and if you have your drivers parameters then you can tailor the design to match.

Good luck!

Punkguyta (author)2008-04-21

Those are woofers not subwoofers....sigh..

Swert (author)Punkguyta2008-04-21

Whats the difference? Are subwoofers just smaller or something?

toastyboy (author)Swert2008-04-21

Yeah, what it the difference then? Mine goes down to about 30Hz, Which is a subsonic freqency, so it's a subwoofer surely? Also, why plural? It's a subwoofer, not a subwoofers. Cheers -Dan

Punkguyta (author)toastyboy2008-04-24

There's 2 subwoofers in there doofus, and my 12" goes to 24hz :D

snagglebeard (author)Punkguyta2008-04-30

The subwoofer is the assembly of said enclosure and the drivers, but I work in a mobile electronics shop(have exp in home theater)and folks in the mobile side of audio often refer to low freq drivers as "subs, subwoofers, woofers," or more simply the dia of the driver. This being said your not 100% wrong but you are, and your 12 will reproduce freq well lower than 24hz, it may not be with much amplitude but the wave will be there. also you won't be able to here it much lower than 20 if that. and to "swaggeringPagan" while I'm at it, the speakers being just out of phase will cause destructive interference, they need to be 180 deg in opposite phase. but considering the fact that your are talking about normal amplifiers and drivers, few amps have variable phase adjustments. In this design you lose considerable bandwidth, the problem with bandpass enclosures they do just what they are called, the allow a certain band to pass, this mens that the driver will only be able to efficiently reproduce freq in that band. you could sacrifice some output for more range, buy doing a standard sealed enclosure. you could obtain the same spl with a more powerful amplifier. On another note there is another enclosure that lacks high spl output characteristics but has the flattest freq curve I've seen. its called an a-periodic or infinant baffle enclosure. a periodic meaning there no freq peak. of course there is a peak but its virtually nonexistent. Its a very simple design concept but I'm not sure of any formulas on creating it. I learned at school that all they are comprised of is a standard sealed enclosure with a hole of undetermined size with and undetermined amount of fiberglass mat or similar material covering the hole. I can't honesty say I've ever heard or seen one, but I've wanted to attempt one, but I've no time. do some research online they are very interesting.

toastyboy (author)snagglebeard2008-05-01

Thanks for your comments Snagglebeard, The speakers are 180degrees out of phase with each other. Regarding the passband of a 6th order sub, it can be as wide or as narrow as you like I tailored mine to match the response of my bookshelf speakers. Thanks -Dan

If you really want to argue semantics, it can be said that the entire unit is a single sub-woofer, and each of the speakers is a low-end driver. The driver itself is not a sub-woofer, it is a speaker that reproduces low-frequencies especially well. One could use a full-range driver as a sub-woofer, so long as it only receives the low-end output of the crossover. Not the best option, but it will work.

Hi FlyingWallaby, Thanks for your post. I think we are all (well nearly all) agreed that this is a subwoofer! :-) It certainly isn't a subwoofers, and it's not a woofer either, or a woofers! Anyway, call it what you want... but I'll call it a subwoofer. Thanks for your comments, glad you enjoyed reading about my (sub)woofer(s) ;-) -Dan

Hey Doofus,
This ASSEMBLY is indeed a subwoofer. It uses dual isobaric mounted woofers capable of subsonic frequencies. As I'm sure you're aware, running two woofers face to face and out of phase halves the necessary air spring volume for the sealed portion of the enclosure.

But then you already knew that didn't you?

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