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-   -   I need to lowering Qms of the driver, using Resistive braking - one coil shorted (http://www.diyaudio.com/forums/subwoofers/94586-i-need-lowering-qms-driver-using-resistive-braking-one-coil-shorted.html)

christophorus 19th January 2007 07:15 AM

I need to lowering Qms of the driver, using Resistive braking - one coil shorted
 
Hi all,
I have a driver with dual voice coil.
I made a T/S param measure using this methode:
http://sound.westhost.com/tsp.htm
With voice coil in parallel (4+4=8ohm) I got the result, also with single voice coil (4ohm) and second voice coil left open,
but when I shorted the second voice coil, I couldn't get the result,
there was no voltage drop across the resistor to measure Fs, it just at the same level at all frequencies...what was wrong?

I need to lowering Qms of the driver, since it very high, using Resistive braking - one coil shorted as per this article:

"When you run a single voice coil, it has the nominal single-coil Qes of 0.8.
So, with the second coil open, you'd have a Qes of 0.8, and a Qms (unchanged) of 6.5, for a Qts of ~0.7.
Now, when you short the second (unused) coil, it essentially acts as an electric brake.
This will manifest itself by lowering the Qms to the Qes of a single coil: 0.8.
So, you'll have a driver Qes (of the coil you're driving) of 0.8, and a Qms (of the shorted coil) of 0.8, for a combined Qts of ~0.4.
One coil driven, other open, Qts = ~0.7. One coil driven, other shorted, Qts = ~0.4.

Dan Wiggins
President Avatar Audio"

taken from:
http://www.users.bigpond.com/bcollis...onversions.htm

is it wrong to measure single VC with second VC shorted? Should I use the result form single VC with second VC left open and using the theory above to get new Qms value?

thankyou,
Chris.

AKN 19th January 2007 08:40 PM

Hi christophorus,

Do you have any specific reasons for lowering just Qms?

High Qms is a sign of a low loss suspension.
If a manufacturers focus on terminating standing waves in cone a high loss surround is going to "eat" that energy and translate it to heat. Problem is that even wanted energy will be "eaten" to some degree. My personal opinion is that low Qms speakers will sound lifeless especially at low levels.

christophorus 20th January 2007 02:56 AM

Hi Anders,
thank's for your reply, I need lower Qms because it seems too high, here my T/S param result:

#1-With dual voice coil in parallel (4+4=8ohm):
Fs=37,46Hz
Re=6 ohm
Qms=4,06
Qes=1,63
Qts=1,16

#2-With single voice coil (4ohm) and second VC left open:
Fs=35,36Hz
Re=3 ohm
Qms=10,33
Qes=8,28
Qts=4,69

first question:
is it normal Qts result from dual VC in parallel (#1) very different with single VC result (#2)?
second question:
with second VC shorted, it can not be tested for T/S param?
So to get it, I have to use #2 result and change Qms=Qes=8,28 and new Qts become 4,14.

Chris.

AKN 20th January 2007 08:27 AM

Hi,

Quote:

#1-With dual voice coil in parallel (4+4=8ohm):
Fs=37,46Hz
Re=6 ohm
Qms=4,06
Qes=1,63
Qts=1,16


Maybe you just typed wrong, two paralleled 4Ohm coils makes 2Ohm.
Your Re indicates series connection if each coil has 4Ohm impedance.
I would say that your Qms is fine but instead Qes is way off. Shure about the correct phase to coils?

What woofer are you measuring?

Ron E 21st January 2007 12:38 AM

I think your measurements are questionable.

It can be difficult to measure T/S parameters when one coil is shorted because the impedance gets so flat. I think you should measure T/S in series and parallel configurations and if Fs, Qts,Qes,Qms aren''t much the same you have made a mistake.

Assuming they are the same. use the RDO calculation in the article you have to calculate the parameters with a shorted coil.

Svante 21st January 2007 09:03 PM

You don't say how you go about to measure the T/S parameters. If you use an automatic computer program, it might be that it fails to measure the Q values correctly due to the flatness of the impedance curve.

christophorus 22nd January 2007 08:30 AM

Hi Anders,
you're right, it's series.

Hi Ron E,
so it must be measured in parallel or series, how about measure using one coil and the other coil left open?
And the important thing is: which ever measuring methode (parallel or series or 1 coil), the result should be not much different, right?

Hi Svante,
I did T/S param measure using digital multimeter, see my first post above.

Chris.

richie00boy 22nd January 2007 08:53 AM

I think you either measured something wrong, or you have a cheap nasty drive unit.

If you can wait until this evening UK time I can post up my measurement technique which is helpful when you have drivers with small/broad impedance peaks.

AKN 22nd January 2007 07:12 PM

Hi,

This method is less sensitive to measurement accuracy, Rod also rightfully warns that his proposed method is sensitive to input errors, especially high Q drivers.

Regarding your multimeter and generator.
When no speaker connected, do you have same voltage reading at different frequencies, let say one octave above speaker fs, fs and one octave below fs of driver?

I'll ask once again, are you willing to share what driver you are testing or at least a picture of the driver?

christophorus 27th January 2007 07:29 AM

2 Attachment(s)
Hi Richie,
here is picture of my driver, doesn't look too nasty, does it?
Anyway, I'm still waiting your measurement technique, as your promise.

Hi Anders,
When no speaker connected, it has almost the same voltage reading at different frequencies.

Picture #2 is a close up of the driver, as you can see, between spider and chassis, it has a gap (covered with perforated steel grille), how if I sealed it off so no air can in / out from there, can it reduce the Qms?

Chris


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