TL and the derivatives using two midbass drivers in a D'Appolito style configuration

[Puggie]

I'm looking to build a Mass Loaded Transmission line design, using Morel MW-114s drivers and probably the MDT-44 tweet (but not yet decided). My current design is looking along the lines of:

34-36" tall (unfolded)
1-2Sd (53-106cm^2) crossection
with a port of 1" radius X 5 or 6" length (shorter port with higher Sd).

Here is a link to the Morel driver Data if anyone wants a go at modeling them:

http://www.morelusa.com/midranges/mw...ifications.htm

I've put this into the MJK worksheet and the response looks promising especially for such a small driver (I'm going for very tall thin cabinets without the effort and cost of line arrays) it does get a bit lumpy higher up (6-800Hz) but I presume these standing waves can be dealt with by strategic stuffing.

Now if I was to fit 2 drivers in a D'appolito style layout (MTM with low X-over) would I just need to double the Sd of the line to take into account the now doubled Sd of the drivers, or would I need to compensate for cabinet volume and tuning? How would the positioning of the drivers and their interactions affect the standing waves?

If it would work I'm liking the idea of two midbass drivers as it will retain the sleek look of the cabinet and allow a bit more output.


If anyone has any better ideas for a design please let me know!

[Greg B]

Sounds like a solid plan. I'm not quite curious enough to stuff the numbers into MJK's sheets to 'see', but that's about the approach I would take with those drivers.

AFA using dual drivers, there's a PDF somewhere on Martin's site that explains what to change. Very roughly, doubling line cross sectional area and port area should be sufficient.

Using two drivers spaced along the line will usually smooth things IME.

If it were me, I'd also consider making one of the drivers rear or up firing, rather than going MTM. MTM should work fine though.

Those drivers have shelved up response above 1kHz, which is about where the baffle step will be. It's possible that a simple inductor to start a rolloff at that point, plus the natural rolloff at about 3.2k, will be sufficient for a lowpass crossover.

GB

[Puggie]

I'm currently running the 114s mid and MDT-12 tweet in a very small 2 way (2l ported to about 85-90Hz) and using a very simple 1st order Xover (0.4mH on the bass and 4.7uF on the tweet) it seems to work quite well, I'll be experimenting with higher order slopes in the future. I will be upgrading to the MDT-44 tweet which will lose the nasty resonant peak at 1K which can be heard.

I like simple, it gives me less options to screw up

[Greg B]

You may want to try a series notch impedance filter on the tweeter in conjunction with the same basic crossover you're using now. The morel tweeters I have sound much more relaxed with one.

My first guess on the choke would have been ~1.35 mH for a single driver, or .68mH for dual (MTM), but just a WAG.

Sounds like a fun little project.

[ERICSPEEd]

Hello.

I'm working on a MTM TL using MJK's MadCad Worksheets myself (thou not a ML).

Link to MJK's .pdf on use two drivers

You'll need to double the Sd, adjust the Re (double or halve), double the Vas.

I haven't begun calculating the line with the sections worksheet (TL open end till I find a good alignment) so I can't help with the woofer placement issue. And the Two Driver pdf makes no mention. I'm going to try calculating that by running the worksheet with the woofer (treated as a single woofer using the two woofer parameters in two different places and "see" if i can "sum" the results (maybe in my head).

[Puggie]

Yeah I've found and read MJK's bit on twin drivers for use in the worksheets. I'll have a play with that, I'm also looking at Audax drivers too as a set have appeared in the classifieds nice and cheap .

[GM]

Greets!

This driver has too low a Vas to be a really good candidate for an ML-TL. Anyway, using your Vb, I calc'd that based on published specs 2.5 ohms series R is required to get a semi-long pipe and flatten the response:

L = 28.03"
CSA (w x d) = 21.1"^2
Z0 = 9.875" (bipole preferred, or make this the tweeter position of a MTM)
Zp = bottom exit
rp = 1"
Lp = 4"

Minimal stuffing and space up to suit from a spiked massive base.

GM

[Puggie]

Thanks for that, I've had a change of plan now I've picked up 4 Audax AP100Z0 drivers and I'm looking at using the Morel MDT-20 tweet which I have.

I've decided to go with a 4th order LR filter on the woofers (wired in series to show 12Ohm) and 2nd order Butterworth on the tweets, they will be crossed over about 2 to 2.5kHz.

I'm completely undecided on the enclosure at the moment though, tall and thin so either TL or ML-TL/QWT I'm aiming for 35-40" tall 5.5-6" wide and upto about 7" deep (all external measurements assuming 18mm MDF or Birch ply) which gives about 22l I think.

I may still try you suggestion with the Morels, I currently have a mid and tweet in 2l enclosures tuned to 85-90Hz and despite having very little bass their mid and top end sounds lovely, very nice imaging for near field monitors.