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Old 13th June 2006, 02:32 PM   #1
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Default Or maybe a TL

I have a 15" musical instrument speaker that I want to use for a sub. I took a shot at measuring the params. I didn't quite do it right so I tried to estimate some of what I needed until I have a chance to remeasure. I am a little skeptical of the Qms and the Vas is a total guess as I haven't made a test box yet but here are the estimated params.

Fs: 47
Vas: 8'^3
Qms: 16
Qes: 0.51
Qts: 0.5
Re: 5.45
Eff. Dia.: 14"

An EBS ported design gets me down into the 30Hz range or slightly below but I worry about low xmax in that design. A bass horn is most likely to be optimal for this driver but I began to wonder about a TL. Would this driver be suitable for a TL? It seems like construction would be much easier than the horn.

mike
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Old 14th June 2006, 05:50 PM   #2
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Default Xmax is very important.

I run 4 Lab-12 drivers in my T/Ls. When I did a sine sweep from 40 to 0 hz, the drivers were moving at a pronounced rate of travel. I'd venture to say they were moving 1/2" peak to peak by the time the sweep hit 15hz. I did this on minimal volume settings. The test began with 2.75 volts, but the EQ certainly kicked in to add an additional 14 db gain at 20 hz. So, the voltage is bound to higher down low.

If you use a PA/instrument speaker, then plan on using a high-pass filter to protect it. Set the filter for 35hz at a minimum.

John
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Old 15th June 2006, 02:59 AM   #3
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I should point out that my intention is obviously not to build the be all and end all with this one. I just want to put something onhand to use and improve on the little 8"er that I am using now. It is OK but I thought that I might get a little cleaner and more powerful bass (even if not much lower) with the big 12"er. I know that it can kick heineken in a horn but was looking for simpler options.

mike
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Old 17th June 2006, 03:27 AM   #4
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Default Re: Or maybe a TL

Quote:
Originally posted by mashaffer
.............but I began to wonder about a TL. Would this driver be suitable for a TL?
Greets!

You tell me , 32 Hz end loaded TL:

L = 102"
L x W (CSA) = 363"^2

CSA may go up or down depending on actual Vas or vice versa, and to keep acoustic gain high, a golden or acoustic ratio is required due to the relatively large CSA. Since this is for 'sub' duty, I assumed having the first notch around 100 Hz was high enough, and of course a high order XO is required to minimize the ripple's ability to comb filter with the mains.

For my records, what make/model driver is this?

GM
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Old 18th June 2006, 03:35 AM   #5
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I believe it is a Jensen. It is out of a Wurlitzer console organ.

See here for some numbers...Magnet Size Thread

And here for some pics...
A bit further on in same post

mike
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Old 19th June 2006, 01:36 AM   #6
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Today I found a freeware TL modeling program (I know... worth what I paid for it but it gives me a starting point) and I played around with it using my initial measurement/estimate of TS parameters and a preliminary design looks promising. In fact the results are good enough that I am somewhat skeptical. The resulting cutoff of about 28Hz is about 2/3 or 3/4 of an octave below fs which seems slightly (but not wildly) optimistic.

Given the params in the picture attached I came up with the following.

Line Length: 1.75m
Back Chamber volume: 250 (anything from 150 to 250 modeled well)
Pipe cross section: 2000cm^2 no taper
No fill.

Assuming that my estimated Xmax of 3mm is correct up to 8watts can be input below cutoff before exceeding Xmax. This equals about 106dB in the pass band (will low pass at about 60Hz). Above cutoff up to 40Watts can be accepted for about 114dB. If we can get close to that it should be very satisfactory in my 14x14ft room. Once built I can see if stuffing is of any real value. In simulation it caused significant droop in the low bass output but maybe just a trace to make sure that I don't get any high frequency noise out of the pipe.

I would like to build it into an old armoire that I have on hand. It is about 5'x4'x2' with an open shelf on the bottom and doors on the top. The plan would be to build the enclosure out of MDF inserted in the shell of the armoire with the doors solidly glued on (hinges removed). I would like to place the speaker on the lower right with the pipe going up and over to the left and fireing forward from the bottom left. The lower half of the armoire (below the doors) would be enclosed by grill cloth.

It just seems like this would be an fairly easy thing to experiment with compared to a horn loaded system and simulations indicate a bass reflex box would have to be over 20 cubic feet with four 3" diameter ports in order to get non-negative port lengths and similar fs.

Before building anything I plan to do a more careful measurement of the TS parameters to be sure of my starting point.

One thing I noticed in playing around is that reducing the Vas by half had very little effect on the simulation results. Is that to be expected?

mike
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Old 19th June 2006, 05:27 AM   #7
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Greets!

Before we go any further, this looks like BoxPlot, which is a very basic T/S box calculator and the demo is a very early version with some bugs, so not to be trusted. If it's a TL designer, then where can I DL it to review since there's no way a ~69" long straight TL is going to have a 28 Hz Fp.

No, net Vb is in lockstep with Vas for a given alignment. Notice that with a 4 ft^3 Vas, the baseline efficiency is lower and the response is now underdamped since the pipe's net Vb is too large:

GM
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Old 20th June 2006, 01:52 AM   #8
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Sorry for the confusion. The pic is indeed boxplot. I used it to do the BR modeling and it was convenient to use the speaker window from that program to show the TS parameters. I actually used the DosBox (MSDOS based) TL program shown below to do the TL modeling.

mike

Opps, forgot the pic first time...
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Old 20th June 2006, 02:19 AM   #9
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Actually I just realized that I was changing Qms not Vas. Sheeze!

Anyway my confidence in this program is not too good because I put your length and pipe area into it and got wildly different results. Though the base output level was also around 100dB/watt I got a horrendous rolloff at about 55Hz and down 10dB by 30Hz!

This is the input screen for that run.

mike
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Old 20th June 2006, 12:47 PM   #10
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I played with the sim a little more this morning. First I corrected the speed of sound which they had at 300m/s (probably to account for stuffing which I wasn't using) and I get about the same results that you did if I double the cross section of the pipe to the 1800-2000cm^2 range. Sorry for the metrics but this program is all metric so I have to convert.

When I use the smaller pipe I get quite a bit of low frequency loss. Any specific idea of what could cause the difference in the results? What happens in your simulation if you increase the pipe cross section to 2000cm^2?

Thanks for your patience.

mike
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