Transmissionline: F.A.S.T. System with Fostex FE83 + 8' Bass RFT L2911

[jockel77]

I've built a FAST System with Fostex FE 83 fullrange driver and 8' Bass driver from RFT Typ L2911 (these bass drivers produced in German Democratic Republic have been a good buy in the past).

I know need some advice to improve the enclosure design and the crossover.
For a first try I'm using the bass driver in a 20 Liter closed cabinet.
Which gives good bass but not lower than 60 Hz I guess. Bassreflex design for this chassis doesn't give good results as I tried out.
So I'd like to go for Transmissionline Design.
Maybe someone could calculate a Transmissionline with chamber + line for me ?

Here are the specs of the bass driver

F-Range: 40 Hz – 9 KHz, 4 Ohm
Qt: 0,57
VAS.: 23 Liter
fc: 50 Hz
efficency: 88 dN

For crossover I took 6db filter
Bass 4,8 mH
FE 83 (in small closed cabinet) 50µF

Maybe a 12db filter should give better results ?
Who could give me advice what to improve here.
Unfortunately the FE83 plays 3 db louder than the bass driver

[jockel77]

I'v attached a plan for a Transmissionline enclosure which I'd like to choose for the described F.A.S.T. System with Fostex FE 83 and RFT L2911.

This design was used for a 10' driver, Coral Beta 10 in the eighties.
It was tuned to 40 Hz.
For the L2911 the breadth of the cabinet should be reduced.
I wonder if the chamber behind the bass driver should also be reduced, because I read that there is a relationship between sd of the driver and the linke wideness.

The FE83 should get an extra enclosure (closed) with around 1 liter, placed above the enclosure for the bass driver. (datasheet of FE 83 is attached)

[GM]

Greets!

These appear to be a measured pair. If so, then the published specs will calculate a ~35% too small a cab for one and a ~75% too large a cab for the other. This could explain why it didn't do well in vented, which isn't as tolerant of mis-tuning as a TL.

Qms: zwischen 4,32 u. 4,61
Qes: zwischen 0,57 u. 0,62
Qts: zwischen 0,50 u. 0,54
Zmax: zwischen 30,6 und 31,6
Rdc: 3,6
Fs: zwischen 45,1 und 48,8
Vas: zwischen 20,4 und 36,99

Of course this assumes yours fall within these, so Caveat Emptor if you don't measure yours. FWIW, I always try to err on the large side since it's got more tuning options, especially if some form of TL, but if the extra size is a room space/WAF/whatever problem, then I guess go with the published or even the average of them to get the theoretical best trade-off between size, performance: 27.47 L Vas, 47.49 Hz Fs, 0.52 Qts.

Regardless, they all calculate a 36-37 Hz Fb T/S max flat vented alignment, so this is the lowest it should be tuned IME and ideally ~69-88-90 Hz depending on the specs used for a max flat impedance alignment (Fs/Qts).

You read wrong information then, 'Sd' per se isn't used in speaker cab design, only in calculating its peak SPL capability. MJK does use it as a convenient way to define a cab design's cross sectional area (CSA = width x depth) in his MathCAD software and decades ago David Weems published a rule-of-thumb TL design routine based on Sd and a driver's Fs that by and large yielded mediocre results at best, but to amateur DIYers back then it was better than nothing.

Today, MJK's CLASSIC TL and/or Rick Schultz's ALPHA TL design routines for T/S specs are what you want to use.

WRT XO advice, there's not enough information to comment beyond the fact that it's 'best' to have ~one octave of ~flat response on either side of the XO point/slope order to ensure it's attenuated enough to be out of hearing range, so let's say the HF is ~flat to 150 Hz before rolling off, then 4th order = 300 Hz, 3rd/424 Hz, 2nd/600 Hz, 1st/2400 Hz.

From this we see that without response measurements of both drivers, especially the (mid-bass) woofer's, XO design is a real crapshoot to get right enough for the DIYer even if done by ear if it's not at least 2nd order or in our acute hearing BW centered at ~2 kHz.

GM

[jockel77]

Thank you Greets! for your detailed answer to my transmission line project questions.
Unfortunately, I am not able to check the paramaters of the bass driver.
That's why I would trust on the rightness of the values I got from the manufacturers datasheet (qts 0,57, VAS 23 Liter, fs 50 Hz).

I'm not familar with mathsheet calculations from Martin J. King.
Maybe, you could give me some help, what to change on the design I posted.

• How many liters should the chamber behind the bass driver be ?
• The wideness of the inside enclosure should be around 220 mm.
• Is there anything more to change/improve on the line dimensions ?
• How many liters should the small closed enclosure for the fullrange Fostex FE 83 driver have ?

Concerning X-over I would like to go for 12 db filters.
X-over frequenzy around 200 Hz.