Transmission Line Isobaric Subwoofer for home.

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

Thanks. You bring up the points that have led me to consider isobaric. As you can tell from the pic, already got the subs.

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

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!

O Audio 300W BASH amplifier showed up at my door step today. Should be cutting wood next weekend if all goes according to plan.

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.

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.

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

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.

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

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. I can hear air leaks but one side isn't sealed
yet. Will be adding veneer finish after some router and sanding work.

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.....

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.

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. 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

Decided to add a brace to the terminus. It's not even glued in yet but won't be going anywhere anytime soon.

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

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.

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.