Instructables

Isobaric Subwoofer

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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 - http://www.yourmissus.com/subwoofer/
 
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Step 1: Cutting

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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

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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
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stuffman4 years ago
Well done, I bet this thing weighs a ton!
toastyboy (author)  stuffman4 years ago
 hehe!

Well at least 40-50kilos I would say.

Cheers
Dan
hack124x7685 years ago
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)  hack124x7685 years ago
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!)
toastyboy (author)  Da Nugesta4 years ago
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

Dan
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 >:-(
toastyboy (author)  Da Nugesta4 years ago
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?
toastyboy (author)  hack124x7685 years ago
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.
BMWalsh5 years ago
Do you get port noise from that ruffled carpet on the inside of the port?
toastyboy (author)  BMWalsh5 years ago
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, for me, a reasonable volume level is not nearly enough :-P
toastyboy (author)  BMWalsh5 years ago
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_15 years ago
...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_15 years ago
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_15 years ago
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_15 years ago
Good point, I did consider using premade fittings, not sure why I opted to make one now...
-Aj-6 years ago
OHHH isobaric woofers are sweet hey. well looks cool anyway.<br/><br/>..do 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="http://www.linearteam.dk/default.aspx?pageid=winisdpro">WinISDPro</a><br/>and here for abit of a tutorial thing<br/><a rel="nofollow" href="http://www.the12volt.com/installbay/forum_posts.asp~TID~74558~PN~1">Guide: Using WinISD</a><br/>
manticore6 years ago
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)  manticore6 years ago
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
angelwill266 years ago
I sooooo want that router system! lol
toastyboy (author)  angelwill266 years ago
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
"F"ing awesome man I have built some woofers myself and I gotta say your work is top notch
toastyboy (author)  Friggin Smift6 years ago
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
HammyHavoc6 years ago
I'm extremely impressed (: AND jealous :P Nice one mate
toastyboy (author)  HammyHavoc6 years ago
Thanks for your interest! No need to be jealous, just build yourself one!!! Cheers -Dan
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)  HammyHavoc6 years ago
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
Solitus39896 years ago
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)  Solitus39896 years ago
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" !! ;-)
-Dan
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)  Solitus39896 years ago
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!
Punkguyta6 years ago
Those are woofers not subwoofers....sigh..
Swert Punkguyta6 years ago
Whats the difference? Are subwoofers just smaller or something?
toastyboy (author)  Swert6 years ago
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
There's 2 subwoofers in there doofus, and my 12" goes to 24hz :D
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.
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