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Old 28th June 2010, 12:37 AM   #1
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Default A mystery and a choice...

First the mystery:

I'm using two 12" DVC drivers in a sealed enclosure as my car audio subwoofer. The driver's parameters (measured with a WT3) are approx Fs:18.8 Hz, Qts:.457, Vas 3.767, Re: 1 Ohms (coils in parallel).

The box is approximately 2.8 cu.ft., and stuffed. The coils for each driver are set in parallel, and the drivers are wired in series. This should result in a system with an Fb around 32~36 Hz, and Qtc around 0.74~0.83, and impedance should be 2 ohms.

However...

...the WT3 is saying that the Fb is 33 Hz and the Qtc is 0.96(!).

I thought additional series impedance may be contributing to the problem. I checked and it came up to 0.2 ohms. I ran new cabling and redid the connections, and managed to bring this down to 0.08 ohms. Qtc dropped a bit, but it's still quite a bit higher than I expected. The box itself is not one of best pieces of work - looks ok, but a little too much vibration for my liking. Still, why the higher than expected Qtc?


Now the choice...

I want to upgrade the subwoofer. Fb is a bit too low for car audio use, and the overall efficiency is a bit too low for my liking (I'm using a class D amp that can feed the drivers 250W each). I'm therefore looking to replace the drivers and rebuild the box.

Two drivers have caught my attention: the $139 Trio12 (http://www.creativesound.ca/pdf/CSS-...ata-051009.pdf) and the $125 SSA DCON (Sound Solutions Audio - DCON Series Subwoofers).

The Trio12 looks like a pretty decent driver and looks like it might work well as a car audio sub in a small sealed box. Efficiency at 85dB/1W/1M isn't a whole lot greater than my current drivers however.

The DCON12 is a lot more sensitive (89dB/1W/1M), and it's lower Q (0.33 vs. 0.43) suggests that I can actually use a small vented box instead if I want even more output, without having to worry about a significant hump in the passband. It also has dual spiders and it's cheaper than the Trio12. However, it doesn't have an XBL^2 motor and its rated Xmax is just a little lower (16mm vs 20mm). Tough choice and I can't seem to make up my mind. Any suggestions?
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Old 28th June 2010, 02:03 AM   #2
infinia is offline infinia  United States
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Yes the higher Qtc is a mystery... just for fun It would be interesting to wire the DVC in series first remeasure and then wire the drivers in parallel and recheck Qtc. I think your idea to wire the drivers in series is good BUT the individual driver coils in parallel Re ~1ohm is too low for the WT3 to measure accurately as this is the area of 4 wire measurements now.
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Old 28th June 2010, 02:24 AM   #3
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Quote:
Originally Posted by infinia View Post
Yes the higher Qtc is a mystery... just for fun It would be interesting to wire the DVC in series first remeasure and then wire the drivers in parallel and recheck Qtc. I think your idea to wire the drivers in series is good BUT the individual driver coils in parallel Re ~1ohm is too low for the WT3 to measure accurately as this is the area of 4 wire measurements now.
That thought had struck me as well. However, there was little variation in the driver's measured t/s parameters (apart from the obvious ones) when I varied between series and parallel connections - certainly not enough to account for the significant increase in Qtc. The drivers are actually now wired in parallel, with their coils wired in series, to give the same 2 ohm load. I measured little change in Qtc, apart from that expected when I decreased the additional series impedance from 0.2 ohms to 0.08 ohms (instead of using speaker cable, I used the thickest car audio power cable I could find that I could fit in the crimp connectors. Replacing the speaker terminal on the subwoofer box might drop the additional series impedance even lower, but I think I'm at the point of diminishing returns for effort ).
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Old 28th June 2010, 03:45 AM   #4
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Try removing the stuffing and check again. You might be choking off the airspace.
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Old 28th June 2010, 07:10 AM   #5
djk is offline djk
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Qtc value changes due to stuffing are suspect below 50hz.

When the waves are slower, there is less friction to cause the loss of energy to mimic a lower Qtc.
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Old 28th June 2010, 12:05 PM   #6
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Quote:
Originally Posted by Tommythecat View Post
Try removing the stuffing and check again. You might be choking off the airspace.
This should raise Fb to a higher value. However, the readings from the WT3 indicate that Fb is lower than that for an unstuffed box.
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Old 28th June 2010, 12:07 PM   #7
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Quote:
Originally Posted by djk View Post
Qtc value changes due to stuffing are suspect below 50hz.

When the waves are slower, there is less friction to cause the loss of energy to mimic a lower Qtc.
Two things to note here however is that (1) Fb is lower than what would be expected with an unstuffed box of the same volume, and (2) Qtc is higher than what would be expected with an unstuffed box of the same volume.

It's all very strange. I think I'm going to try to take all of the stuffing out, to see what the unstuffed box measures like.
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