ML-TL with a Radio Shack 40-1024 - diyAudio
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Old 10th August 2007, 03:54 AM   #1
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Default ML-TL with a Radio Shack 40-1024

Radio Shack 40-1024
http://support.radioshack.com/suppor...oc15/15880.htm

Hello people,

In a previous post I had up on rebuilding speakers, GM you had mentioned something about building a ML-TL for the Radio Shack 40-1024 8 inch sub. I was wondering how I go about this. I have looked around on http://www.quarter-wave.com/ and where else I could as well as trying to "reverse engineer" designs to relate them to the Thiele/Small parameters of speakers. I think I was successful in a few places, but not others. Hopefully people have some pointers.

The cross sectional are is ~= to 4*Sd. The line length is ~= to 1/8th wavelength. The port length and diameter is ~= to the same calculations used for a vented box (Lv = [1.463 (10^7) (R)^2 / (Fb^2) (Vb)] - (1.463) R).

Is this at all correct?
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Old 10th August 2007, 03:38 PM   #2
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Greets!

'Guilty' as charged! I didn't bother to 'do the (currently proprietary) math' since you apparently wasn't interested and as it turned out it was just as well since I wasn't paying attention, making a suggestion based on using only one driver, so it doubles for two for max gain BW, or ~7.5 ft^3/20 Hz. too big for a pair of dinky 8"ers by today's standards, but then these are 'old school' designs, so weren't optimized for tiny boxes. You can put two in ~3.75 ft^3/20 Hz, it just won't have near as much bottom end, which depending on room acoustics may be plenty, though as I noted, better to have too much and tone it down than be left wanting.

If you can reverse engineer any of my pipe designs as to how they relate to T/S specs, you're a whole lot sharper than me. Anyway, IIRC, you're listing some of MJK's rules-of-thumb as a starting point to finding an acceptable alignment using his software, but won't work well except over a narrow range of Vas, Qts. Anyway, early on I used an Altec nomograph to find a box Vb based solely on the driver's frame size and published Fs (these were huge by today's standards) and later on using Altec's and other's various alignment formulas before T/S became the de facto designer format.

Bottom line, use a T/S program to find whatever alignment you like, then convert this net Vb into whatever pipe shape you want using MJK's software, adjusting it as required with the understanding that as the TL expands into a horn the flattest response is found by sliding the driver down toward the floor and conversely for a TQWT it slides upward, so for best results work it from the desired driver height. That said, many folks claim excellent results using the simple expedient of placing the driver near/at the top of a straight taper MLTL and fine tuning smoothness by moving the vent up the pipe as required, so as always YMMV.

WRT the vent calc, this was my 'stumbling block' (and still is since I can't 'do the math') before his program since I didn't know how to predict the effect of the pipe action on the vent size, length, so again, early on I used an Altec nomograph and later a similar formula for ducted ports as a first approximation and modified as required by measuring impedance.

Bottom line, with the formula it will typically calc a too long vent and if you start with a short one you may need to go to a larger diameter to get the desired in-room Fb, so start with whatever the T/S program calcs for a ~0.3 vent mach to make sure it's big enough and adjust as required in the sim and if still too long, then reduce its area, though IMO don't go below whatever size the box program calcs for a 0.5 mach. Note too that these programs base it on whatever 'Pe' is inputted, so be sure to at least use either the driver's max continuous rating or your amp's peak output power, whichever is higher if low compression is desired or whichever is lower for a bit more driver protection down low at the potential expense of sounding 'congested' if driven hard down around/at Fb (Fp).

GM
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Old 13th August 2007, 02:04 PM   #3
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Sorry I didn't respond back to this quickly. I had a busy weekend with the mother coming down and what not.

I wouldn't mind making a box like this that fit the 8" sub in instead of a fullrange driver:
http://www.quarter-wave.com/Project03/Project03.html

So my best bet for designing this correctly is to take a program like WinISD and calculate out the volume of a ported box and also the port with a mach of ~0.3. Then take this box size and input it into MJKs MathCAD sheets. I would then try to porportion the box to a similar look as the one from Martins website, and play with driver position to get a clean looking frequency response.

I am going to have to read Bob Brines article on why TL's are NOT Bass Reflex Boxes. It sure seems like it with a MLTL.

Thanks for your help GM.
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Old 13th August 2007, 02:48 PM   #4
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Quote:
Originally posted by davidallancole

I am going to have to read Bob Brines article on why TL's are NOT Bass Reflex Boxes. It sure seems like it with a MLTL.
By all means read the article, but the short answer is that that the resonant mode of a MLTL is quarter wave, not Helmholtz. It is a matter of the ratio of length to cross sectional area.

Bob
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Old 13th August 2007, 03:07 PM   #5
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Hi Bob, thanks for responding.

Quote:
By all means read the article, but the short answer is that that the resonant mode of a MLTL is quarter wave, not Helmholtz. It is a matter of the ratio of length to cross sectional area.
Is this ratio of length to cross sectional area something that is commonly known, is it on your website Bob, or is it something that is in MJKs MathCAD sheets that is proprietary?
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Old 13th August 2007, 03:18 PM   #6
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Quote:
Originally posted by davidallancole
Hi Bob, thanks for responding.



Is this ratio of length to cross sectional area something that is commonly known, is it on your website Bob, or is it something that is in MJKs MathCAD sheets that is proprietary?
By now, this should be pretty well known. There is nothing proprietary about this. It is simply a statement of physical fact. MJK's worksheets provide a mathematical representation of the state of nature. Remember that the exact same set of equations describe closed boxes, ported boxes, TL's and horns.

Bob
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Old 13th August 2007, 04:39 PM   #7
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Okay. Thanks Bob.
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Old 16th August 2007, 09:41 PM   #8
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Greets!

Well, we do what we can, when we can. As you can see, I've gotten quite a bit behind myself.

Sounds like a plan, just remember that if you sim for only one driver, then you'll need to double the CSA and number of vents for two drivers for the sim to be accurate (within the limitations of the program of course). Note too that if mounting dual drivers vertically, then in the sim whatever point down the pipe you choose to locate the drivers will be the midpoint between them when laying out the baffle cutouts.

You're welcome! Please post your design and a mini-review for future reference once finalized.

GM
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