Help please, designing a TL sub

[ShinOBIWAN]

Greetings all,

A couple of months ago I visited the CRTprojectors showrooms and they had on display a PMC XB3 transmission line subwoofer. To be honest I've heard many subwoofers and owned nearly as many, but never have I heard such fantastic bass. Really can't compare it to anything I've heard before.

I've done a little research and the XB3 uses the Volt R3853 driver in a cabinet featuring a 4m long TL.

I've drawn up a rough idea of what the TL would probably look like:



Does the look something like?

I've also done some preliminary models using Martins' mathcad worksheets.

I'm having a little trouble understanding the units used to represent the open and closed ends. What does Sd mean?

[stee-b]

Hello Shinobiwan - Nice to see a fellow avforumer here!
Sd is as far as I know the driver radiating area.
Good Luck with the project, I've been observing your other projects and am in awe with how they've turned out!
Regards Steve

[azira]

Sd is surface area of the driver. But it's measured from a diameter that goes from half-way through the surround to halfway through the surround on the other side.

The Sl and So are also areas that represent the CSA (cross sectional area) of the closed end of the line and the open end of the line. Since the taper ratio has a lot to do with the characteristics, it's easier to take note of it in the worksheets if Sl and So are represented as a taper ratio scaled by Sd.

Units would be square inches since those are Sd's units.
--
Danny

[ShinOBIWAN]

Originally posted by stee-b
Hello Shinobiwan - Nice to see a fellow avforumer here!
Sd is as far as I know the driver radiating area.
Good Luck with the project, I've been observing your other projects and am in awe with how they've turned out!
Regards Steve

Thanks chap, I've enjoyed sharing them with the good people over there.

Thanks for the heads up on the Sd, its all just clicked into place since its a T/S parameter. Here was me thinking it was some unit the MJK had made up for the TL sheets.

[ShinOBIWAN]

Originally posted by azira
Sd is surface area of the driver. But it's measured from a diameter that goes from half-way through the surround to halfway through the surround on the other side.

The Sl and So are also areas that represent the CSA (cross sectional area) of the closed end of the line and the open end of the line. Since the taper ratio has a lot to do with the characteristics, it's easier to take note of it in the worksheets if Sl and So are represented as a taper ratio scaled by Sd.

Units would be square inches since those are Sd's units.
--
Danny

Woah Danny, slow down mate!

This is my first TL so I'm struggling to understand the concepts at the moment. I do understand the closed end is the driver end and the open end is the port. The CSA is pretty obvious but Sl and Sd?

Could you elaborate please.

Also what should I be looking for with graphs outputted by MJK's worksheet? At the moment I'm just comparing them to default settings with the fostex driver, which seems like a less than scientific method

Thanks!

[cjd]

Sl = area of closed end as a function of Sd
So = area of open end as a function of Sd

Sd = area of driver, we've already established.

So, if Sl = 2 and Sd = 600cm2, Sl = 1200cm2.

Your TL concept is probably nowhere near big enough and will likely work best with a taper (but may not).

I modeled out what it would take to put my 15inch drivers into a TL (I have an "IB", manifold mounted opposed 15's with ~3500ft3 enclosure) and ended up with a box size of about 1.75m by 2.25m and .5m deep for each driver. I think Sl was 7 and So was 3. The only thing I gained over the IB response was power handling, so I never went for the TL enclosures. It also needed a LOT of stuffing. In the end, cost of materials to build the enclosures could have had me another two of the drivers I'm using, which would have done the same thing for my powerhandling. . .

Take some more time reading Martins work, then read it again - he goes through exactly what to look for in the results in great detail, with pictures and everything. Step by step for designing - no use duplicating that work!

C

[BillFitzmaurice]

Before going too far along with the project you should invesitgate the work that George Augsperger, one of probably the four or five top loudspeaker experts in the world, has done on TLs. For one thing he's found that using Sd as the basis for line cross-section is not the best alternative, and that a percentage of Vas is a better way to go about it.

Augsperger's calcs were used by Joe D'Appolito, another of that group of top designers, to create the Thor TL for Seas. If you give me your driver specs I can give you the required calcs for your TL, which also will work a lot better if there is no chamber behind the driver and the line is a constant taper. By the way, a 4 meter line will work down to about 22 Hz. If you don't need to go that low you can shorten the line accordingly.

Not to criticise MJK, but his designs are generally not true TLs, but rather TL/reflex hybrids. Not that there's anything wrong with hybrids; I just designed a TL/Line Array hybrid myself. But you should be aware of that fact.

[cheesehead]

cabinet featuring a 4m long TL.

Hmm... a TL is a quarter-wave resonator, and their site says it's an "effective" 4 m , so if we believe them, the TL is tuned to 16 m wavelength or 21 Hz (speed of sound = 342 m/s).

"Effective" 4 m might mean that it's port-loaded (mass-loaded) or that they're stuffing the (Dick Cheney word ) out of it. The total internal volume looks like it's around 250 - 300 L (350 L external volume), from the web site

H 1040mm 40.94"
W 432mm 17.01"
D 790mm 31.10"

Taking the T/S parameters of the driver & really skimping on the cross section at 2 x Sd = .1468 m^2 (you know you're working big when the best units are meters ) would give a length of about 2 m or 80 inches. (Similarly, a "4 m" line would give a cross section of about 1 x Sd with this internal volume. And I'm assuming that the line isn't tapered...)

Try these parameters:

Length=90
zdriver=6
zport=84
S0=SL=2xSd
rport=4
Lport=10

and stuff the first 2/3 or so with 0.5 lb/ft^3 gets close to a port frequency of 21 Hz.

Don't know if it will sound good, you'd have to get some feedback from some of the pros here who've actually built more than 1 TL

mk

[planet10]

Originally posted by BillFitzmaurice
Before going too far along with the project you should invesitgate the work that George Augsperger, one of probably the four or five top loudspeaker experts in the world, has done on TLs. For one thing he's found that using Sd as the basis for line cross-section is not the best alternative, and that a percentage of Vas is a better way to go about it.

Augspurger's work is parallel to Martin's and generates essentially the same results. Martin has made his work much more accessible then George.

Martin has used Sd as a convential that came up from classic TL design. You can actually choose any random number and it has no affect on the actual generated model.

The elegance of the Mass Loaded TL & TQWT have given this approach a lot of press, but it is also covered in Augspurger's papers (he calls it a restricted terminus).

Joe's work on the Thor seems a bit of a disservice to TL design -- he used George's (somewhat skimpy) tables to generate a design and then ignored it because the box was too big... the one set of measured results i have seen for the Thor put F3 at not much below 100 Hz.

I have a more detailed essay on my website about TL design and how important the parallel work of King & Augspurger is (amounst other things).

http://www.t-linespeakers.org/readme1st.html

dave

[ShinOBIWAN]

Originally posted by BillFitzmaurice
If you give me your driver specs I can give you the required calcs for your TL, which also will work a lot better if there is no chamber behind the driver and the line is a constant taper.

Very gracious of you to offer and much appreciated.

Here are the T/S parameters for the Volt RV3853

Fs: 32 Hz
Re 7 ohms
Qa: 3.25
Qe: 0.44
Qt: 0.39
Vas: 172 litres
Xmax: 5.5 mm
Sd: 734 cm2
Vd: 600 cm3
Le: 1.2 mH

A big thanks to everyone else who replied. Clearly I have a lot of food for thought and will post back here again after I have a more solid foundation TL theory.