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B&C 18TBW100-4 Fs measures higher than spec
B&C 18TBW100-4 Fs measures higher than spec
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Old 19th November 2019, 08:20 PM   #1
tommus is offline tommus  United States
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Default B&C 18TBW100-4 Fs measures higher than spec

I'm building a reflex subwoofer with a B&C 18TBW100-4. Using Dayton DATS I measured the Fs in free-air. The driver is placed cone up on a small platform (about the size of the magnet) on a tripod stand about 5 ft off the ground. Driver was broken in for about 16 hours with a 20-60Hz sweep.

Fs on the spec sheet is 33Hz.
Fs measured by me is 43Hz.

Is this in the range of normal variation?
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Old 19th November 2019, 09:18 PM   #2
Brian Steele is offline Brian Steele  Grenada
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That's a pretty big difference.

I recommend trying to determine Fs by an independent method to see it closely matches the DATS measurement, e.g. add a small resistor between the driver and the amp, and then drive with a tone generator and measure the voltage across the resistor while changing the frequency being generated by the tone generator. The voltage across the resistor will be at a minimum at Fs.
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Old 19th November 2019, 09:37 PM   #3
ErnieM is offline ErnieM  United States
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Those are very stiff drivers. I believe they'll work perfectly at normal prosound levels compared to the levels DATS measures at(is it version 1,v2,or v3?) . Brian has good advice as always.
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Old 20th November 2019, 08:11 AM   #4
chris661 is offline chris661  United Kingdom
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Well, you've blocked off all the magnet vents for a start. What level was your sweep at?

If you want to break in a PA driver, you'll need to hammer it.

Chris
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Old 20th November 2019, 12:36 PM   #5
Brian Steele is offline Brian Steele  Grenada
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Yeah, if the magnet vents were blocked, that will likely change Fs.
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Old 20th November 2019, 03:54 PM   #6
tonskulus is offline tonskulus  Finland
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Suspensions will get looser and Fs will decrease over the time and (heavy) use.

As example, I measured some RCF L15P540's.

Fs (almost brand new): 58Hz
Fs (old, used a lot): 44Hz

Fs (manufacturer specs): 50Hz
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Old 20th November 2019, 05:05 PM   #7
tommus is offline tommus  United States
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So after another overnight break in the Fs has dropped to 39Hz. I'm not hammering it, the amp is only 100W and the driver looks to be moving to somewhere around Xmax. I'm using DATS v2 for the test and whatever the drive level is, it's definitely small signal. BTW, the pole vent is not covered.

I gonna go with Ernie here and just assume that this driver is going to fall into spec.
I'll test it again after I pound it with some dubstep!
Thanks.
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Old 20th November 2019, 05:47 PM   #8
weltersys is offline weltersys  United States
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Quote:
Originally Posted by tommus View Post
Driver was broken in for about 16 hours with a 20-60Hz sweep.
Tommus,

As well as the observations about blocking the vents "stiffening up" the suspension, and to expand on Chris' suggestion to "hammer " for break in, it probably won't be "broken in" without being pushed past Xmax, towards Xlim.
Xlim for the B&C 18TBW100-4 is 57 mm peak to peak, about 2.25 inches- it takes a lot of force to reach that. I did a push up on a 18TBW100-4 to see what it took to reach the excursion limit (my hands measure about 87 pounds doing a push up on the bathroom scale), though never measured Fs "before and after".

On a new B&C 8NDL64-8 using the method Brian Steele mentioned in post #2, measured the Fs at 87 Hz, 7 Hz higher than the spec sheet rating of 80 Hz.

I then “broke in” the 8" speaker by pushing the cone in and out by hand to around Xlim (as far as the suspension would allow) several times, then repeated the test, the Fs dropped to 78 Hz.

I had used around 4V for the two initial tests, after a few minutes decided to try 1V (”small signal”) and read around 82 Hz, 4 Hz higher.

Bringing the voltage back up to 4V, the Fs dropped to 80Hz, as specified.

Art

Last edited by weltersys; 20th November 2019 at 05:52 PM.
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