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15th March 2005, 11:44 PM  #1 
diyAudio Member
Join Date: Mar 2005
Location: Southeast

Calculating Bl
Does anyone know how to calculate Bl? I need the equation for a little project I'm doing.
Thanks, Andy 
15th March 2005, 11:48 PM  #2 
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Join Date: Aug 2004
Location: Behind you

B * l?
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16th March 2005, 12:29 AM  #3 
diyAudio Member

like that would do him any good....
If you have Fs Qts Qes Qms Vas Mms and Re then WinISD can calculate BL, I dont know the equation but I bet its right!
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16th March 2005, 02:12 AM  #4 
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Join Date: Mar 2005
Location: Southeast

I want to calculate Bl myself. I also want to see the equation so I can see how the various t/s parameters interact with one another in regard to Bl.

16th March 2005, 02:14 AM  #5 
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email the linearteam guys! they wrote the proigram, theyll know how it works!
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16th March 2005, 03:07 AM  #6 
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Join Date: Jun 2002
Location: USA, MN

Look for a program called DPC  there are many text files included with the program that are very informative.
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16th March 2005, 03:29 AM  #7 
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Join Date: Jul 2003
Location: San Diego, CA

Unless you have a snazzy Adire Audio XBL^2 driver, BL is going to vary with displacement. You can measure actual BL for any given driver if you have a currentlimiting bench supply and a force gauge handy. Apply 1 Amp to your driver (unless it’s a tweeter!) and measure the force in Newtons. This force IS your BL since the units of BL are Tesla*Meters which is equivalent to Newtons/Amp, and in our case Amps = 1. A currentlimiting supply set to 1 amp is nice since you get your 1 amp regardless of the voice coil resistance changing with time as it heats. Obviously for smaller drivers you could use less current, but you will need a more sensitive force gauge. Caveat: This only works at the resting (zero displacement) position of the diaphragm and the diaphragm must not be allowed to move during the test. If you wish to measure BL at other displacements, you will need to measure the change in force since the suspension will be applying a constant restoring force at that displacement.
Casey Walsh 
16th March 2005, 01:16 PM  #8 
diyAudio Member
Join Date: Mar 2005
Location: Southeast

Okay, it not that easy, I understand.
I'm using a Dayton DVC in a resistively damped application (power to one coil, a resistor on the other coil for tuned damping). I have been calculating the exact value of the resistor I need to tune my 50L sealed cabinet to precisely Q=.707. The only variable I'm having trouble with is Bl which varies with 1 coil vs. 2 coil operation. It also varies with the size of the damping resistor. Since the value of the damping resistor is inversely proportional to the SQUARE of Bl, it becomes very important to have the correct value. I have a decent approximation, but if I had the formula, I could run the numbers with various resistors and be more precise. Thanks, Andy 
16th March 2005, 02:39 PM  #9 
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Join Date: Apr 2002
Location: Chatham, England

Wouldn't it be easier just to use a wire wound pot and adjust it until you got the required Q?
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16th March 2005, 03:41 PM  #10  
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Join Date: Jun 2002
Location: USA, MN

Quote:
Note that Qes = 2*pi*Fs*Mt*Re/(Bl)^2 rearrange and get Bl=sqrt(2*pi*Fs*Mt*Re/Qes) For simplicity sake, use the Qes for the parallel connected case, as Bl is the same for parallel and one coil driven..... Then go through the math as below..... If you need more help, feel free to ask. You might need an equation for Mt now (Hint you can make one from: Vas = Sd^2*rho*C^2/((2*pi*Fs)^2*Mt)  all quantities in SI units....) cheers. ======================================= Snipped from Adire's White paper  where they got things right except for how to spell my last name "The math behind RDO is fairly simple; the best explanation/set of equations we've seen comes from Ron Ennega, and was posted at Brian Steele's DIYSubwoofer site: First you need the original Rms: Rms = 2 * pi * Fs * Mt / Qms Next calculate: Rms'=Bl^2 /(Re + R) where R is the shorting resistor Rmt = Rms + Rms' The new Qms is: Qms' = 2 * pi * Fs * Mt / Rmt Qes does not change: Qts = 1/(1/Qes+1/Qms) "
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