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Old 31st August 2011, 09:47 AM   #1
ODougbo is offline ODougbo  United States
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Default Isobaric 8 subs

I’ve been building HT subs for many years and this is my favorite design. [/FONT][/COLOR]These subs have super low bass and out perform many store bought boxes.

Two long throw 8” woofers ($30 each) mounted isobaric, using small tunnel. Tunnels sound complicated, but they are pretty easy, simply use square blocks of wood with the same size hole as the woofer, laminated (glue) as needed. In this box I found that a 5” total inside depth works well. [/FONT][/COLOR]Also note; I used 1” thick wood, so bracing not needed.

The speakers are 4ohm, so ran them in series and bridged a 200 watt amp mono to drive them, an option is to run them 4ohm stereo.
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Old 31st August 2011, 01:04 PM   #2
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I was reading a book (the old "The Secrets of Car Audio") the other day that suggested an isobaric design.

It clearly stated that an isobaric alignment will have lower resonance because the air space between the woofers will act as a buffer of sorts.

What size is the box, port length and how much air do you have in the air space between the woofers?
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Old 31st August 2011, 01:39 PM   #3
ODougbo is offline ODougbo  United States
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Default Tunnel

The tunnel is 5” deep inside, with 7” hole cut in the lamented pieces. I did seal the tunnel with rubber spray. I will provide the measurements ASAP. Woofers: MCM Part #: 55-2421

I did make other versions with face to face designs, I think they have a little more output and easier to build, but not much to look at.

Btw, Isobaric has been around a long time, mid 1950s
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Old 31st August 2011, 02:00 PM   #4
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Doesn't face-to-face (clamshell) design effectively require twice the volume as compared to a true isobaric design?

Oh, do you have an frequency response chart?

Thanks for the measurements, I am currently trying to work out a "compact" design around 50 to 66 Liters for portable needs (outdoors use) with a high output to match the "Boominator" design, when a little more 'uupmh' is needed.

Not sure it is doable with only 100 Watt (Tripath TAA4100A based) available though.
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Old 31st August 2011, 02:25 PM   #5
ODougbo is offline ODougbo  United States
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Default face to face, back to back, front to back

I believe that face to face, back to back, front to back is all the same, so the box volume is cut in half. No frequency chart, however many hours of testing and building boxes. I’m a Cabinetmaker by trade, so very light on the engineering, but have cranked out many cabinets. One thing I came up for tuning: semi build the box, to the point where the front plate can be slid up and down, which changes the port’s volume - using a generator and volt meter you can tune the box with certainly and then do the final glue up (works every time, but you need a good number of clamps).

Only a 100 watts? You’re probably going to need more power, however, this design did rock the shop with a old Rotel 60 watt per channel amp and an a small car crossover, set at 50hz
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Old 31st August 2011, 04:56 PM   #6
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Isobaric designs normally half the required box volume, if built like the OPs or as a clamshell or mag-to-mag.

However, if you clamshell or mag-to-mag mount the drivers, wiring one out of phase, you create a much cleaner output as 2nd order harmonics are cancelled by the opposing motion of the drivers-though surprisingly a lot of people don't like that as much as bass with the 2nd order harmonics!

Removing the 2nd order harmonics is very useful for car audio as the sub is normally located behind the listener and the 2nd order harmonics are what "pull your ears back" making the sub locatable-assuming you've got it crossed over low enough for the fundementals not be located (63Hz and down)

Obviously there is crossover into HT if the sub is going to be placed away from the main drivers/screen.
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Old 31st August 2011, 05:17 PM   #7
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Quote:
Originally Posted by PressureFM View Post
Doesn't face-to-face (clamshell) design effectively require twice the volume as compared to a true isobaric design?
No, they are the "same". Face-to-face just gives the smallest coupling volume (althou usually ignored, coupling cavity adds to the volume of the box). A smaller coupling cavity also extends the HF response (shorter distances means that cancelation ripple is pushed upwards)

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Old 31st August 2011, 05:23 PM   #8
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Originally Posted by TheBaronGroog View Post
However, if you clamshell or mag-to-mag mount the drivers, wiring one out of phase, you create a much cleaner output as 2nd order harmonics are cancelled by the opposing motion of the drivers-though surprisingly a lot of people don't like that as much as bass with the 2nd order harmonics!
Not surprising at all. It has pretty much been established that the harmonics of the distortion products should monotonically decrease. If you reduce the 2nd harmonic below the 3rd it can sound unnatural.

Further, Geddes has shown that (measured) distortion needs to get pretty high before it becomes audible.

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Old 31st August 2011, 05:44 PM   #9
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Yup-most won't notice 50% distortion in bass frequencies
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Old 31st August 2011, 06:16 PM   #10
ODougbo is offline ODougbo  United States
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I did try wiring the woofers so they are pushing against each other, I didn’t get much sound output. I found that moving the cones in same directions works the best.

Several years ago I had two pairs of Peerless 12” woofers, clamshell mounted (4 woofers total) in a 6cf cabinet; it worked but the bass was so slow to get going, a lot was lost, before it ever got started.

The 8’s are super quick, they have powerful bass and the sound stays in the room (not louder/deeper in the adjacent room). Typically the bass is strong without a lot of cone movement, but warning, they will bottom out!

Oh btw, I use the bass limiter setting in a Lexicon processor so I don’t over drive them (explosions are the worst). The old Lexicon DC’s and MC’s are perfect for DIY, with built in crossovers and sub sonic limiter.
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