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Old 4th August 2003, 02:46 AM   #1
Street is offline Street  Canada
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Default Transmission line subwoofer question

I've decided to build a transmission line subwoofer, but I have a few questions about driver selection and pipe design before I start ordering parts.

I liked the idea of using a sonotube to build a TL subwoofer however after further reasurch I decided that the pipe should have a tapered design. My ceiling is just under 8 feet so I'm assuming I don't want to build the tube much longer than 7 feet, or so. I'm planning on building it from MDF with 8-16 sides and finishing it with veneer. The math on the building dimensions is pretty simple and I think I have the carpentry skills and tools to cut the pieces acuratly (16 sides will be tricky).

I have been using Martin King's Mathcad worksheets for modeling driver and tube combinations.

Ok, onto the barrage of questions. (I think some of my confusion stems from differences between TL fullrange and subwoofer design.)

I have been using Martin King's MathCAD worksheets for modeling driver and tube combinations.

Ok, onto the barrage of questions. (I think some of my confusion stems from differences between TL full range and subwoofer design.)

How large of a taper ratio (cross-section area at driver to cross-section area at opening) is acceptable? I have seen suggestions at 2:1 and 3:1. Are there any negatives to increasing this to say 8:1 as an example?

I have seen recommendations that the cross-section area at the driver be as close to Sd as possible. Are there negative aspects to increasing this dimension, and if so what are they? Also, is there any rules of thumb for what the cross-section area at the opening should be and what are the consequences of deviating form this?

Should tube volume be taken directly into consideration during design or just indirectly through line length and cross-section areas at the driver and the opening?

My thought on suitable driver parameters are high Qts, low VAS, and fs suitable to the line length accounting for line taper and stuffing density. Any objections/corrections to this?

I'm still not sure if I want to use a single woofer, probably a 10", or go with a push push configuration. It will be in a 14"x12"x8" room so one woofer would probably be enough, but the push push definitely has its drawing features.

Well, I think thatís it for now. Thanks in advance for the help.

-Street
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Old 4th August 2003, 11:49 PM   #2
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Problem with subwoofer freqs is line length to tune low. This one is for a 15" driver and line is ~120" long to get ~30Hz tune.

Click the image to open in full size.

Pete
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Old 5th August 2003, 01:07 AM   #3
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Default Re: Transmission line subwoofer question

Some notes:

1) It's possible to make a tapered design with nested Sonotubes. See last month's AudioXpress.

2) For a TL, you want a low Qts, no more than .4 or so.

3) For the taper, Augspurger recommends a ratio up to 1:4. Martin King's charts to to 10.

4) In that small room, I would think one 10" woofer in a small box would be sufficient to rattle the walls.

5) Ten days ago, I didn't know my Vas from a hole in the ground. You can find an enormous amount of info on the web -- and here. Google and search.

6) Doubt everything I say, and most of what I write.
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Old 5th August 2003, 03:37 AM   #4
Street is offline Street  Canada
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Ok I think I have found answers to many of my questions and have come up with a few new ones too.

I think I will likely use a high taper ratio of 7:1 to 10:1. After doing some modeling in mathcad I donít see any problems with such ratios.

X-section area has no correlation to Sd, but is based on the Vas. What is the relationship between this? Or should I just tweak x-sections in the mathcad worksheet until Iím happy with the result (what I have been doing so far)?

I think Iím pretty close with desired driver parameters with 0.5<Qts<1.0, low to medium Vas and appropriate fs. Dave I think I recall reading somewhere that a Qts no more than 0.4 is more a guidline for full range TLs and that subwoofers do beter with higher Qts. I am far from certain on this though. Any input would be appreciated.

Martin advises that the first acoustic impedance peak should be at or below the driver resonance ( http://www.diyaudio.com/forums/showt...469#post155469 ). How much below the driver resonance is acceptable? Also, what phase characteristics should I be paying attention to when playing with the mathcad worksheets?

I have modeled a few Lambda drivers which look very appealing. How large of an effect does the faraday ring have on the sound quality of the driver? I was looking to spend about what the LE motor versions (no faraday ring) cost however if the improvement with a single faraday ring is substantial I might be able to scrape together the cash.

-Street
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Old 5th August 2003, 08:02 AM   #5
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Quote:
Originally posted by Street
I think Iím pretty close with desired driver parameters with 0.5<Qts<1.0, low to medium Vas and appropriate fs. Dave I think I recall reading somewhere that a Qts no more than 0.4 is more a guidline for full range TLs and that subwoofers do beter with higher Qts. I am far from certain on this though.
I don't know where the Qts <0.4 comes from ... i'm always on the lookout for drivers with Q higher than this for TLs.

Quote:
How large of an effect does the faraday ring have on the sound quality of the driver?
Quite a bit.

dave
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Old 5th August 2003, 12:13 PM   #6
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This is from memory, so you need to model it to confirm:

Peerless 850146
So=4*Sd
Sm=0.5*Sd
L=120"

I don't have a feel for Martin's tables, since I've used the worksheets enough to plung right in. Augspurger's table work better with larger drivers, since his F3 will always be higher than Fs. You don't have to have Fp>Fs. I never do with small (8" and less) drivers.

Bob
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Old 5th August 2003, 10:02 PM   #7
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Quote:
Originally posted by planet10


I don't know where the Qts <0.4 comes from ... i'm always on the lookout for drivers with Q higher than this for TLs.



Quite a bit.

dave
I read it on the web, so it must be true. :-)
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Old 6th August 2003, 12:46 AM   #8
Street is offline Street  Canada
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Quote:
This is from memory, so you need to model it to confirm:

Peerless 850146
So=4*Sd
Sm=0.5*Sd
L=120"
I modeled this driver using your dimensions. The frequency response looks decent, however, the first peak in the acoustic impedance is around 12 Hz or so. Should this not be matched to the free air resonance of the driver? In this case 22.6 Hz from the peerless spec sheet. Is this difference acceptable? I have been trying to keep these matched as closely as possible, normally within one or two Hz. If a larger difference does not pose any problems it would give me more freedom when modeling.

-Street
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Old 6th August 2003, 04:44 AM   #9
GM is offline GM  United States
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>How large of a taper ratio (cross-section area at driver to cross-section area at opening) is acceptable? I have seen suggestions at 2:1 and 3:1. Are there any negatives to increasing this to say 8:1 as an example?

====

Taper ratio is a function of how long the pipe can be and how much boost you want. I'm not much of a TL fan, but some drivers just won't work in an otherwise vented alignment, so when all else fails.......

====
>I have seen recommendations that the cross-section area at the driver be as close to Sd as possible. Are there negative aspects to increasing this dimension, and if so what are they? Also, is there any rules of thumb for what the cross-section area at the opening should be and what are the consequences of deviating form this?

>Should tube volume be taken directly into consideration during design or just indirectly through line length and cross-section areas at the driver and the opening?

>My thought on suitable driver parameters are high Qts, low VAS, and fs suitable to the line length accounting for line taper and stuffing density. Any objections/corrections to this?

====

Drivers with low Vas for the intended BW don't do well in vented designs once you move away from a golden ratio cab due to insufficient area around the driver, causing peaking at Fb that no reasonable amount of stuffing will damp.

A driver wants to 'feel' a certain amount of acoustic mass to perform properly, so Vb for the intended BW is where I start. I look at what the T/S max flat is for a starting point, with 0.707 to 1.414x Fs being the BW of max performance. This pretty much dictates a Qt around 0.4 as the ideal, just like a reflex or ML-TL design. IMO, low Q drivers are better off in Daline type pipes, while high Q drivers need larger, heavily stuffed pipes to acoustically lower their apparent Qt.

Once the Vb/Fp is chosen, the taper increases with decreasing pipe length. A pipe generally has the smoothest response when the driver is at some point down its side, and the greater the taper, the further down/up the pipe it must be towards the big end. If it's on the end, then the taper should terminate in a very small terminus. This is a good way to get virtually IB response in a smaller package.

Anyway, unless you're dead set on a TL, for the 850146, a minimally stuffed ML-TL looks pretty good and isn't large (by my standards anyway). Punch these numbers into the MLTQWT worksheet to view it:

L = 50.63"

S0 = Sd*2.0326

SL = Sd*2.0326

density = 0.25lbs/ft^3

rport = 1.88" (minimum)

Lport = 8" (of course this is adjustable, I just chose an alignment that normally works well in a typical room)

X0 = L*0.4367

Of course for pounding LF you'll either need a true subwoofer driver or two of these as part of the mains, or double S0/SL/vents and do a bipole sub.

HTH,

GM
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Old 6th August 2003, 06:01 AM   #10
Street is offline Street  Canada
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What does BW stand for?

-Street
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