Transmission Line drivers

Ok, after researching enclosure benefits, I think I will definately
go with a TL sub to supplement my Acoustat 2+2's which already
have some fairly decent bass. I have several (8) KEF B139 SP1044
drivers that I've considered but they are definately older design
compared to todays woofers (see: http://www.diyaudio.com/forums/showthread.php?threadid=91810).

I also have several (4) of the
carbon fiber woofers used in the Pioneer HPM-100's with the
treated cloth surrounds. Some (4) Realistic Mach One Woofers
and finally some (2) JBL 2242HPL's.
Has anybody any experience with any of thes drivers using
them in as larger of numbers of each particular driver for TL
enclosures? I am wondering which of the drivers would work
best in TL enclosure to go with the Acoustats?
Is there any criteria used to determine whether a particular
driver will work good in TL? Are drivers intended for sealed
or ported reflex enclosures usually best?
I plan on going with a fairly long line (10-16? ft.) to really
go deep!
 
just some suggestions...

I believe that Calvin gave a design for a pair of 18 inch Beyma drivers in an ultra tiny Ripole design perfect mate for an Acoustat. Also if you have a standard 8 foot ceiling you should try bracing your 2+2's to the ceiling with a couple of turnbulkles and a small wooden plate (to protect your ceiling). You wedge the turnbuckles between the top of the speakers and the wooden plate at the ceiling. Make it tight and your bass will increase big time. You might also consider a hardwood brace a foot long on the inside of your Acoustat frames in the middle of each vertical side piece. This is where the four panels meet in the frame. Reason? Well I have seen ceiling braced Acoustat 2+2 frames crack in half due to the increased bass energy and since the top of the speaker is effectively clamped the only place the frame can bend is in the middle. I did not believe this could happen untill I saw a pair that had done so.
 
We used to use a pair of single B139 TLs with acoustats... they work fine. With 4 per side you will have a BIG box, but it should sound awesome.

Lengthening the line will not give you more bass, it will actually produce less bass.

Scottmoose & i worked up an optimum TL for the B139 using MJK for all those still fortunate enuff to have some around. I have CADed up lines for single and double woofer lines, i could do 1 for 4 or you could build 2 double lines. The other possibility if this is too big would be to use 4 per side in a push-push, push-pull TL. With the flat face and skeletal basket the B139 would be ideal for this. That would allow you to put 4 in the same size box as 1, and get the swept volume of a pair with a maximum amount of isolation from time smear back thru rhe cone as well as a reduction in 2nd order distortion (given it is an older woofer this might well be the biggest bonus)

Here is a straight version... it could be folded into a rectangular box.

PP-B139-TL.gif


http://www.t-linespeakers.org/projects/vProjects.html

dave
 

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GM,
I don't remember where I saw it, but I recall reading
somewhere that the only difference between the 2242H
and 2242HPL had something to do with a cover over the
magnet or something like that. I'll look around and try to
find that again. As far as the Rat Shack woofers and the
Pioneers I have nothing and would have to come up with
some measurement stuff. The only thing I know about the
Realistic Mach One's is that they were sealed design and
the Pioneer HPM100's were a bass reflex.
 
As far as using the KEF's, I think that I understand that
each time drivers are double up, it lowers the bass extension
by 1/2 octave? If that being the case I would probably want
to have all 4 drivers in contact with air rather than cones
facing each other, right? But I could still have a pair mounted
with magnet side facing out to get the distortion cancellation?
 
Greets!

OK, 2242H specs it is then.

Pretty much any driver is suitable for TL loading, though obviously the ideal F3, Fp will vary based on its T/S specs.

WRT blending one to the Acoustats, my understanding is that it has a ~ Q = 1, though highly damped in its usable passband, so ideally we want a critically damped Qp = 0.5 sub system, ergo ideally a driver with a 0.5 Qts or a lower Qts driver tuned up to a 0.5 Qp, which means a steeper roll off below Fp. Higher Qts drivers can be used in over-damped TL alignments, but wouldn't be my first choice since it puts more stress on the driver.

Bottom line, like any other alignment you should use the right driver for the app, so 'best' is a floating target, ergo to use a very long line requires a suitably low Fs. For 14 - 16 ft, it would be ~3390"/168 or 192" = 20.18 - 17.65 Hz, so neither of the drivers we have specs on 'fill the bill' as is and I seriously doubt the others do either.

The 2242 with its high Vas, low Qts would be tuned to 125 Hz and still be ~8.7 ft^3, but I imagine its efficiency is much > the Acoustat's, so using a filter ahead of the sub amp to raise its Qts to 0.5 allows a 35 Hz Fp, though in a hefty ~17.7 ft^3 cab.

This leaves us with the KEFs as the 'best' choice. While Dave's right about extra drivers not lowering the cab alignment, it does allow us to take that extra efficiency and extend the BW with it, so four drivers in a dual bipole layout looks good at a glance tuned to ~17.7 Hz/~16 ft long TL, but it's going to be around 780+ L. or dual ~390 L bipole TLs if this is too big for whatever reason, and of course isobaric will cut these sizes to only ~195+/97.5+ L with power requirements accordingly and EQ will be required to flatten it in-room.

Note that a ~17.7 Hz Fp will mean a huge 3rd harmonic dip around 70 Hz, so if you need a higher XO point, then you may want to consider a higher tuned TL since to damp it out requires so much stuffing that it will be over-damped down to at least the driver's Qts.

A better choice for over-damped drivers (Qts < 0.5) is the TQWT (aka TQWP) since its reverse taper loads the driver more, allowing a shorter pipe for a given Fp and and smoother response overall. That, or use some form of EQ to raise its effective qts to 0.5 or higher since down low room gain increases at up to 12 dB/octave, which requires either a 0.707 Qp or Qts driver.

GM
 
coloradosound said:
Can you model a cab for 4, but all exposed to air. (2 facing
in and 2 facing out? I think I will go with at least 1 fold to keep
height under about 5-6 feet.

For 4 you just increase the line cross-section of the double driver box by a factor of 2. Then fold the top over and you end up with a rectangular box.

dave