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Transmission Line Isobaric Subwoofer for home.

post #1 of 41
Thread Starter 
I have a pair of JL12W1 subwoofers that have been laying around. Had them in a car audio application for several years but thought it would be neat to give them new life in my HT.

I already have two powered subs from DefTech in a set of bi polar towers. I think I have positioned them to the best of my ability but still think the bass response could be more uniform and deeper. Reading around it is suggested that multiple subs can yield smoother response overall. Room is 40' x 17.5' by 9'. Kinda large.

Meeting the WAF, I have an area with maximum external dimension of 50cm x 60cm x 89cm towards the rear of the room. This includes about 5cm room around the enclosure for RCA cables on the side(s) that back up to the walls.

The subs are 8 ohm each and will be wired in parallel so I want to keep them in the same box and plan to power them by a BASH 300W RMS plate amp. Technically more power then the subs can handle but.....

So I dreamed up the transmission line type enclosure seen in this post. Thinking about isobaric configuration so that the cross sectional area of the TL can be reasonable and allow me to get close to the 1/4 wave length that is desired.

Note that in the cross section picture, the lower woofer would be open to the room on two sides.

Plan on using 3/4" MDF and making a nice winter project out of it. I have two spreadsheets from websites dedicated to TL Speakers to aid in my line calculations. But nothing yet that I think could simulate the response. I will be measuring the Thiele Small parameters of these subs one weekend when I get some quality time in the lab.

All the boxes I have made in the past were sealed for vehicles. So this will be more of a challenge.

Edited by Gaugster - 11/11/12 at 6:41am
post #2 of 41
I used to be big on Isobarik for the alleged box size savings, but concluded in the end it's an expensive way to halve the efficiency, except at the very low end, and the extra driver uses up a lot of the supposed volume decrease.

It only makes sense if you're super tight on space and will use it for a sub, where the 2X power handling is needed.
post #3 of 41
Also cancels out a fair amount of distortion if mounted push pull, but I've never seen a comparison of that reduction vs the reduced distortion from having twice the SD moving half as far if both drivers were used conventionally.

IMO, the only circumstances I would consider isobarik in non-automobile applications would be cost no object as another incremental performance improvement when all available space for subs is already being utilized, of if i already had a pair of drivers with really subpar motors.
post #4 of 41
Thread Starter 
Thanks. You bring up the points that have led me to consider isobaric. As you can tell from the pic, already got the subs. biggrin.gif

These are 15 yrs old but were in service for about 10 yrs. Back then electronics were still made in Japan and 125W RMS was typical. Thanks to CLASS D Amps and China, we got 500W or 1000W RMS amps for cheap.

I loose 3dB by using isobaric. Since these subs are 8 Ohms each, I get it back when the amp is presented with a 4 ohm with the subs wired in parallel. I just could not make the Sd of two subs work. Cross section area was getting to big for the space available

Two subs, each 4 ohms in a separate box with a seprate amp would be louder but way more expensive.

Edited by Gaugster - 12/9/12 at 8:35am
post #5 of 41
Thread Starter 
Some updates. Took the subs to work and measured a few things.

Fs turns out to be 30Hz instead of 24Hz as the datasheet shows. Measured directly with an impedance analyzer and confirmed using a different test setup consisting of a waveform generator and a multimeter.

Happy Turkey Day!

post #6 of 41
Thread Starter 
O Audio 300W BASH amplifier showed up at my door step today. Should be cutting wood next weekend if all goes according to plan.

post #7 of 41
WOW, old sckool W1's. I'm surprised that the foam surronds are stll good. Back ~1998 when I was in high school i used to have a set of those 10's in my ford ranger. They were great speakers back then. I'm currently building a set of ported 10w7's for my condo. It's nice to see somebody else doing a JL build as it seems almost taboo on this forum to build a box using JL subs. tongue.gif
post #8 of 41
No kidding. I don't even bring up the brand anymore, simply not worth it. That said, nice subs. How those 10W7's working out for you?
Originally Posted by Audiophile1178 View Post

WOW, old sckool W1's. I'm surprised that the foam surronds are stll good. Back ~1998 when I was in high school i used to have a set of those 10's in my ford ranger. They were great speakers back then. I'm currently building a set of ported 10w7's for my condo. It's nice to see somebody else doing a JL build as it seems almost taboo on this forum to build a box using JL subs. tongue.gif
post #9 of 41
Thread Starter 
Lol, This is a great forum but I have read some of the weirdest arguments about the strangest topics on here.

I have to thank the DIY brethren for not immediately telling me to buy a new sub etc.... It is just what I had left over from my pre 2000 car stereo days.
post #10 of 41
Thread Starter 
Got the major cuts done today thanks to a great table saw and any even better buddy of mine....

Jig saw died. Otherwise would a few internal cuts done too.
post #11 of 41
What is the benifit of an Iso trans line? Wouldn't it be better to build 2 ported box or 2 horn?
post #12 of 41
Thread Starter 
I already had two 8 ohm subs. Two separate ported would be louder but i have never been a fan of ported
Boxes due to group delay.

The hope is that this box has tight sound like sealed which is a trait if TL. Isobaric helps cancel unwanted harmonic supposedly.

Didn't do all the math but this type of box would be half the size of a typical horn.

About a 50 / 50 chanes it will work....lol
Edited by Gaugster - 12/31/12 at 8:44am
post #13 of 41
It's awesome you're willing to take a shot. Who knows, it may be shaweeeet! Good luck!!
post #14 of 41
Thread Starter 
N8DOGG - Thanks. All the wood is assembled except for the two side panels. Work continues and this weekend should be fun filled with trials and seeing how if all comes together.
post #15 of 41
Thread Starter 
Got it fired up and integrated into my system (Two DefTech Towers with 12" subs)

No sort of lining or stuffing on the inside as of yet. Still debating if it is needed. Also one side panel is screwed on but no proper seal as of yet.

Happy to report that it did exactly what I was hoping. I.e. bass sound is more clearly defined, thick bass guitars etc... are easier to hear (see) in the sound stage.

AFAIK, this is the only enclosure of its kind so glad that this risk is paying off.

Lots of critical listening ahead!!!!!!!!!!!!

Edited by Gaugster - 12/24/12 at 9:39am
post #16 of 41
Thread Starter 
What can i say? "Weapons Grade Bass" is a good description. IsoTL seems to want to inflate my leather couch when the tracks go low frequency.

Significant other is worried about the sound breaking our home and is having a hard time adjusting to this new component of the HT.

Job well done. smile.gif I can hear air leaks but one side isn't sealed
yet. Will be adding veneer finish after some router and sanding work.
post #17 of 41
Neat-o! I know nothing about this particular alignment, but it looks pretty unique, and I'm glad you're happy with the performance.
post #18 of 41
Thread Starter 
Thanks. Regarding T&H alignments, i need to do a sinwave sweep and determine system resonance vs measure speaker resonance. All other parameters i should have already. I have some alignment table for basic lines for comparison. Backwards but what can i do? Lol.....
post #19 of 41
Thread Starter 
Got some time to plot response curves of the new box. Did two trials, T1 and T2 to check for consistent data.

Autosound 2000 test disc with sin waves from 10Hz - 99Hz in 1Hz steps. AVR at -15dB, O Audio Amp gain at 50% Room is 6330 ft^3 and sub is located along a side wall.

This is only the isoTL subwoofer as the other subs were off. X-Over is at 80Hz so the other speakers in the system are participating somewhat at 80Hz and above.

Original Radio Shack analog SPL meter. Slow reponse, C Weighting. Recorded the raw data and added some generic correction factors found on AVS. So take anything below 32Hz with a grain of salt. wink.gif

Lots of variables of course. Meter was about 1 meter from sub however the microphone was pointing away from the cabinet but was facing the front of the room.

Meter did not register any SPL at 10Hz - 12Hz but started to move the needle at 13Hz.

My purpose was to identify the enclosure resonance and compare it to the measured speaker resonance of 30Hz. I see a small blip around 28Hz but it only occurred in T1. I was thinking that this resonance would be more obvious but perhaps the absence of large peaks is a good thing. biggrin.gif Also I don't have a good way to measure the outputs of the line and woofer separately. On the other had, it looks to have a -3dB point starting at 36Hz, plateauing until 28Hz before dropping if I compare this graph to classic 3rd or 4th order T&S response curves.

Edited by Gaugster - 1/1/13 at 10:21am
post #20 of 41
Thread Starter 
Decided to add a brace to the terminus. It's not even glued in yet but won't be going anywhere anytime soon. smile.gif

Next decision I am struggling with is what type of feet to use on the bottom. Rubber isolation or spikes onto metal disks? Can't see myself paying 50 bucks or more for a SubDude type base plate. Due to the weight, I will probably go with a wide hockey puck style solution. My fronts are spiked into carpet but there it is much more easy to deal with.

Edited by Gaugster - 12/31/12 at 8:57am
post #21 of 41
Do you know if the 12w0-8 is at all capable in a similar set-up? I have a pair laying around.
post #22 of 41
Thread Starter 
Originally Posted by toenail View Post

Do you know if the 12w0-8 is at all capable in a similar set-up? I have a pair laying around.

The specs for the 12W0-8's differ only a little from the 12W1's. So I don't see any reason why not. I hate to knock my own design but I would have went a different route had I more space to work within.

Note that I used measured data for the final transmission line length and that this measured data was significantly different from the generic datasheets.

I don't have any product to sell but would be curious to see other opinions in a different applications.
post #23 of 41
I currently lack proper measuring equipment. If TL's etc are that sensitive to exact specs I'd likely be better off ported or sealed.
post #24 of 41
Thread Starter 
I would think that sealed boxes would be the most forgiving about T&S parameters. I did not intend to imply that TL are more or less sensitive to speaker parameters. Just trying to make the best of what I had available to me.

Ported or other bandpass are probably just as sensitive to changes in speaker parameters. But ported boxes can have stuffing added that can sometimes "fix" any issues. I use the word "fix" extremely loosely.

You can stuff a TL to smooth out response but overall output SPL will be reduced. So long story short, ported boxes can be tuned more easy but tweaking port length/diameter and adding more or less stuffing.

Changing TL length is more of an undertaking so that is why I went the extra mile to confirm Fs of the datasheet.

The book "Bullock on Boxes" that I have shows how to measure Fs etc... with "basic" volt meters. So the process is not completely out of reach for most DIY.

I am lucky to work in the electronics field and have access to measurement equipment that works from DC all the way up to 3GHz. eek.gif
post #25 of 41
Thread Starter 
Been a while since I updated about the ol' isobaric transmission line.

Took is out of service to make a brace for the longer part of the line. Had the terminus brace mounted but never glued. So I mounted and glue both to make them permanent.

My wifey wants to replace my sub with a small fridge / cooler. Ha - I can always find another spot towards the front of my room it need be.

Next plan is to make a plate around the amp large enough to service the internal woofer. Then I can seal off the side for good.

post #26 of 41
That looks AWESOME. Nice build.
post #27 of 41
Thread Starter 
Thx. It has been a fun project.
post #28 of 41
Thread Starter 
Got some veneer dropped off at the homestead today. Any color you want as long as it's black. lol

I am not trying to make this the longest DIY build ever but it will be going on a year pretty soon.... Probably will be snow on the ground when we get around to gluing on the veneer.

post #29 of 41
Thread Starter 
Watching 'Finding Nemo' with my kids. My 5 yr old commented that the bass made the couch 'wiggle' when the water mine field explosion happened. Same comment when the submarine starts to slip off the edge. Knocking on aquarium scene was nutz too!

Sharing is caring!
Edited by Gaugster - 10/5/13 at 11:59am
post #30 of 41
Thread Starter 
So I finally got the motivation up to start using Hornresp in an attempt to simulate my transmission line. Results are kind of encouraging I think. Hard to get a handle on all the parameters however.

I am using quarter space (pi) radiation so far since the sub is placed next to a side wall. Had to take a guess at coil inductance (Le) as I could not find it on the data sheet and never thought to measure it directly. Nd is set up as Isobaric parallel (2p).

For those that wish to help my questions are as follows.
1 - Any idea of Le for a JL Audio 12W1-8?
2 - Speaker parameters are input as for a single 12W1-8. Does setting Nd to 2p make the adjustments? I.e. I used standard Vas as an input but Vas should be half for isobaric.
3 - Using Eg as 2.83V since each speaker is 8 ohms nominal. Does setting Nd to 2p adjust for this too?
4 - Conical, Parabolic and Exponential???? What should I be using? (did not check to see if changing these also changes the output results. Like I said "hard to handle so many parameters biggrin.gif)
5 - Used a path length difference of -25cm but not sure of the correct way to determine it. Anyone have experience with using this feature of Hornresp? Different values do change the shape somewhat.

Here is the schematic. No rear loading whatsoever at this point.

SPL of the line itself not combine with the direct radiator (speaker)

SPL of the combination.

Remainder of outputs.

Edited by Gaugster - 10/14/13 at 9:15pm
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