"Complete Wavelength TL?"

[Jezz-the-Fezz]

I'm currently playing around with a couple of rather ancient 12" x 6" elliptical drivers of unknown origin which (I was assured by my late father) are rated to thump all the way down to 20Hz. I was thinking of using them as bass units in an active set up with either one or four JX53's per channel.

Most of the TL designs that I've found are 1/4 wavelenghth, and seem to be quasi - bass reflex in their operation. I was wondering whether anyone had experimented with a full wavelength TL, and if so what were the findings? (I realise we're looking at forty-odd feet of internal accoustics here - don't tell the missus how big it'll be!).

Cheers,

Jezz

[hilbren]

It won't work (as you want it to work)

For example your speaker F res. @ 30 Hz
Normal your tl would be about 2.5 meters long, you want to build one of 10 meters!!!
First of all the "resonant" frequency will be lower as where you want it to be:

For 2.5 meters 1/4 wavelength is 30 Hz
For 10 meters that’s 7.5 hz

The first harmonic is at 15Hz, the second is at 22.5Hz, 3rd is 30Hz

first of all it wil take a lot of energy to get the 30Hz to resonate (only because this is the 3rd harmonic) so your Q will probably be very high.
Your speaker will sound like more than a BR as ever before.

(My theory)


on the other side, for pulse response your speaker would act as an closed sub and thats always good

[Jezz-the-Fezz]

Cheers for that Hilbren; one of the factors (not!) in the equation is, for example the F res of the chassis. I know it's a tall order, but I was trying to eradicate as many of the variables as possible.
Although I'm aware of Martin Kings TL formulae (http://www.quarter-wave.com/), it does rely on knowing a few(!) parameters of the driver in question. My idea (although undoubtedly impractical given the amount of required 180 degree turns in the circuit) minimises the effective internal volume whilst keeping the realisable colourisation above the operating frequency of the unit (eg width of line: 60cm = 500Hz/depth 1cm = 30KHz; length 10 x 100cm x 1cm = 1litre TL volume, far less than the required "infinate baffle" or sealed box volume required leaving more than a little room to play with). I'm hoping that (if any) the resultant 360 degree emissions will not cause blurring, whilst giving the driver a moderate internal pressure to work against. When operating at lower levels, I would expect the driver to realise more bass due to the lesser restriction on excursion as seen in some BR & IB enclosures, wheras higher levels might see a little augmentation from the TL port, effecting a more linear dynamic response. Since I'm frying my brain on this, I'd be really grateful for your take on the idea (disregarding the practical limitations of the 180 degree turns involved!)

Cheers, Jezza

[hilbren]

An other factor (and this is a little offtpic) why is everybody always trying to push the bass output at lower frequencies with a principle that’s based on resonance?

BR or TL are both systems that only "amplifies" waveforms based on a sinus... so all the other details fall away... why???

[pinkmouse]

Does your driver have any info/serial numbers on it? Could you post a pic? I have a suspicion it might be a Kef B139...

[Jezz-the-Fezz]

Hi Pinkmouse,

I think we'll be very lucky if it's a Kef - from what I recall, my father got them as end of run stock (more than thirty years ago) the original shipment bound for an upmarket "radiogram" or something of the like. It's certainly unusual, having a coaxial tweeter strapped across the front (removable via nuts & bolts thank Christ!) and a square chassis (13 1/2" x 8"). Fixing centres are 6 3/4" x 10" (just the four), and the front of the chassis sports a cork gasket - it's obviously designed to fit behind the baffle.

[Jezz-the-Fezz]

Although a little fuzzy, this image shows the unmarked coil; the "condenser" being hidden behind the riveted terminals. I've got a feeling it's a metal film capacitor - you can just see the red painted end but not much more, so the coaxial make-up was obviously a factory job rather than post production.

[Jezz-the-Fezz]

The only numbers on the back appear on two circular stickers about the size of a two pence coin (both having "MADE IN GT. BRITAIN" in standard print). One sticker has the numbers 92390 and DD15, the other 4272. My dad had fitted them with IB enclosures: 10" x 24 1/2" x 8" top & 13 1/2" bottom (slanted baffle) 10mm chip with a 10mm ply baffle. Allowing for the 10mm thickness I make that just a tad short of 36 litres.

[Jezz-the-Fezz]

Although I can't find any evidence on the units, I know they're rated at 8 Ohms; my father fed them via the classic JLH Class A's at 27V; I've still got them & I'm scratching my chin as to whether or not I should keep them on duty for the bass section, whilst using a tube amp for the JX53's (comments welcolme!). So, if anyone can shed further light on the origins of these units I'd love to hear. For my part, I simply want to find a way to get the most from these elderly units without resorting to megawatts & equalisation.

[jmateus]

Hello everyone

Even though I can't pinpoint the origin of this woofer, one thing
is for sure, it's NOT a KEF woofer, a B139...
Being an old driver, made in Great Britain, I tend to think it will
be an ELAC driver, those woofers were sold a lot in GB in the 50
and 60 even though it was a german company. Excelent ones
by the way.
However I really can't swear about that.
But KEF B139 is not for sure, I still own 2 of those, they were saved from the rage of experimenting with diy amplifiers....