4 ohm TL = narrow pipe?

[bwbass]

I'm new to speaker building but have been considering a simple straight sonotube TL sub as a weekend project. I have been using Martin King's worksheets to explore driver options, and just came upon something that looks odd to me...

I plugged in the parameters of the Dayton Quattro 8" woofer and arrived at a pipe diameter of 6.48", as the So/Sd formula puts Re in the numerator. Does this mean that lower impedance (and thus lower resistance) drivers always result in TL pipes with smaller cross sections? It seems like if this were the case people would always use these drivers to make the enclosures smaller, so I suspect I've misunderstood something...

Any help?

[bwbass]

Perhaps some more specifics would help....

T/S Specs for PE #295-545 8" Quatro Subwoofer:

Fs = 36Hz
Sd = .021m^3
Qts = 0.40
Bl = 10.37
Re = 3.9 ohms

Pipe tuning: 35Hz
So = Sd (no taper)

From MJK's Classic Alignment Table:

L (effective) = 96 in
Dz = 34.001
Dr = .095

So/Sd = (1.21 kg/m^3)(342m/s)(.021m^2)(0.095)(34.001)(3.9 ohms)/(10.37N/amp)^2

So/Sd = 109.4707 / 107.5369 = 1.01798

In english measure, I get 33 in^2 for So, which seems unusually small for an 8" driver. It also seems odd to me that if Re doubles, So doubles. Is it possible that the MJK's calculated tables (Dr and Dz) values assume an 8 ohm driver?

[BillFitzmaurice]

I can't address the accuracy of the tables you're using but I can tell you that Z does not affect the Vp of a TL. Perhaps a visit to MJKs site and a personal query to him would be in order to clear up the confusion.

[Timn8ter]

Originally posted by BillFitzmaurice
I can't address the accuracy of the tables you're using but I can tell you that Z does not affect the Vp of a TL. Perhaps a visit to MJKs site and a personal query to him would be in order to clear up the confusion.

I've used Martin's worksheet with drivers of various impedance and don't believe there is an assumption of an 8 ohm driver. I agree with Bill to not get too dedicated to the use of the alignment tables. It's only a starting point. I've built a tube similar to what you're proposing using 8" Sonotube. Try shortening the length and increasing So and SL.

[bwbass]

I've sent an email to MJK - perhaps he'll pop in on this thread. I'm probably missing something obvious...

It's good to know others have tried this kind of design successfully.

Timn8ter - what size driver did you use for your TL? I'd think 7" would be as large as you could go in a sonotube unless you mounted it to a box. And since you're local, what is a good source for sonotube? All I've seen at hardware stores is the cheaper 48" cardboard varieties.

[MJK]

Hi Brian,

I got your e-mail and will answer it here in this thread.

Is it possible that the MJK's calculated tables (Dr and Dz) values assume an 8 ohm driver?

No the alignment tables can be used for any driver resisitance. My first set of alignment tables did make an assumption that the driver was 8 ohms. George Augspurger was kind enough to take an interest and one of his recommendations was to make the alignment tables a function of driver resistance. It is the ratio of Re to BL^2 that is included in the calculation of the area. This driver has a very low Re and a decent BL.

I have to admit that when I saw your calculations for cross-sectional area I was a little surprised. I have never seen a driver generate a cross-sectional area this small. So I double checked your numbers and got essentially the same results. Very interesting, I put the driver T/S parameters and the alignment table results into one of my worksheets and calculated the response for an open end TL with the driver at the closed end. I also left the line unstuffed for the last two sections. The results are attached in the following pdf.

This driver looks like a great candidate for a compact TL. I recommend that you put the parameters into my MathCad worksheets and further optimize the geometry and the stuffing distribution. This might be a real winner!

[bwbass]

Thanks for the quick answers! It's great to have THE expert on the subject around!

Great result, too, though it wasn't the one I was looking for. Not a great candidate for a simple sonotube design, though perhaps I'll try making an extended baffle ring of MDF to mount this 8" driver in a 7" tube, if I can find one. That would make the cross-sectional area about 16% above the calculated value.... how much of a problem would that be?

It will be more interesting to try optimizing a tapered folded line and see if I could make an even smaller box. That'll be project #2, I think, when the sonotube is done. I'm working this out with the MathCAD worksheets now - I'll post results when I have something interesting.

[MJK]

Hi Brian,

It's great to have THE expert on the subject around!

I don't claim to be an expert on anything, more like a guy with too much free time on his hands and a strong interest in acoustics and engineering.

That would make the cross-sectional area about 16% above the calculated value.... how much of a problem would that be?

This is actually a good thing. Increasing the area will help the bass response. Every design I have ever done I have been squeezing the area smaller to keep the enclosure from being huge. This driver allows a small area, adding a bit more will only make the design better.

It will be more interesting to try optimizing a tapered folded line and see if I could make an even smaller box.

My thoughts exactly, this is a very interesting project and I would love to hear how it turns out.

Keep me posted,

[bwbass]

After I posted last night I worked on the MathCad sheets way too long, and saw the improved bass performance for myself. In fact, I'm considering an alignment with an 8" sonotube of the same length, with a .75 lb/ft^3 stuffing density in the first two line segments. This yeilds a what looks like a nice flat bass plateau from about 40 - 100Hz, with an F3 near 35-36Hz. It would also allow the use of the easier-to-find 8" tube which should have an OD a hair larger than the 8.25" flange of the driver, so it won't look so silly mounted.

It seems lilke the trade-off is a slightly less flat impedance curve, as well as somewhat degraded transient response. Still better than a sealed or bass-reflex box, though. Also, the response above 120Hz or so is pretty ugly, but in a subwoofer application this shouldn't be a problem.

A question about the worksheet, though: The line segments have subscripts from 0 to 4, so when I have stuffing densities in segments 0 and 1 and the rest empty, is that the first half of the line after the driver or the first 2/5?

[MJK]

Hi Brian,

It is the first 2/5 ths of the line, There are five equal sections in the "TL Open End" worksheet numbered 0, 1, 2, 3, and 4.

Hope that helps,