How to measure BL (is it important for TL?)

[azira]

I've got some off brand 8" drivers that I replaced in some 3-ways I had. But the other speakers are crap now and I've been following the TL threads. So I decided to measure up these drivers and throw them into MKs worksheets.

I used speaker workshop to measure them but it doesn't measure BL. I put the numbers into the worksheet anyway and changed BL around to see if it would do anything. It seems to have some pretty noticable effect.

So, how do I measure BL.. or is it actually important for a TL?

[kelticwizard]

BoxModel, freeware by Bullock and White, calculates the BL, among other things, from the other Thiele-Small parameters.

You can download it yourself if you want, or you can give me the following parameters and I will do it for you:
Fs
Vas
Qes
Qms
Re
Sd

PS: If you download it yourself, ignore the TL program. They admitted themselves that it does not predict TL behavior well. The other programs included are very good and accurate, however.

[planet10]

Originally posted by kelticwizard
you can give me the following parameters and I will do it for you:

BL = f(Fs, Vas, Qes, Qms, Re, Sd)

Anyone got a formula for the above handy?

dave

[kelticwizard]

Don't have the formula. I just plug the numbers into BoxModel and out comes the answer.

I'm sure there is a formula out there somewhere.

[Svante]

Well, since

Qes=1/(ws*Cms*Res) and
Cms=Vas/(rho0*c^2 *Sd^2) and
Res=(Bl)^2 / Re

Bl=sqrt((rho0*c^2*Sd^2*Re)/(ws*Vas*Qes))

rho0 = density of air =1.2 kg/m^3
c = velocity of sound = 345 m/s
Sd = Equivalent piston area in m^3
Re = voice coil DC resistance in ohm
ws = 2 *pi * fs
fs = free air resonant frequency of the driver in Hz
Vas = Volume equivalent to the cone suspension
Qes = "electrical" Q value of the driver
and sqrt... meaning the square root of ...

[planet10]

thanx...

saves me digging it up.

dave

[azira]

Originally posted by Svante
Well, since

Qes=1/(ws*Cms*Res) and
Cms=Vas/(rho0*c^2 *Sd^2) and
Res=(Bl)^2 / Re

Bl=sqrt((rho0*c^2*Sd^2*Re)/(ws*Vas*Qes))

rho0 = density of air =1.2 kg/m^3
c = velocity of sound = 345 m/s
Sd = Equivalent piston area in m^3
Re = voice coil DC resistance in ohm
ws = 2 *pi * fs
fs = free air resonant frequency of the driver in Hz
Vas = Volume equivalent to the cone suspension
Qes = "electrical" Q value of the driver
and sqrt... meaning the square root of ...

Thanks Svante..

Interesting, all those parameters are either constants in the mathcad worksheet or values one enters for T/S parameters. I wonder why it doesn't calculate Bl for you...


Here's the parameters I measured:

Fs = 53.5 Hz
Vas = .931 cu-ft
Qts = .692
Qms = 3.291
Qes = .875
Le = .612 mH
Re = 7.6 Ohms

For Sd: speaker is 8" so the cone diameter is really 7" right? So this is 3.5" radius -> 8.89 cm -> .0889 m.
So Sd = .0889^2 * pi = .0248 m^2 right?
--
Danny

[Svante]

Originally posted by azira


Here's the parameters I measured:

Fs = 53.5 Hz
Vas = .931 cu-ft
Qts = .692
Qms = 3.291
Qes = .875
Le = .612 mH
Re = 7.6 Ohms

For Sd: speaker is 8" so the cone diameter is really 7" right? So this is 3.5" radius -> 8.89 cm -> .0889 m.
So Sd = .0889^2 * pi = .0248 m^2 right?
--
Danny

I just randomly selected a 8" Peerless driver, it had a Sd of 235 cm^2 or 0.0235 m^2 so that seems OK. 0.931 cu-ft would be 26.4 litres or 0.0264 m^3. You could measure the diameter with a ruler of course to get the correct area.
So, your Bl should be 9.3 if I type the numbers right on my calculator.

[azira]

Hmm... the units don't seem to line up on the equation, what am I missing?

Bl = N/A or V*S/m or Tesla-meter

[kelticwizard]

Using 214 sq cm as the Sd, (which should give negligible difference from 235 sq cm), BoxModel gives me 7.93 Tm as the answer.

I have found BoxModel to be reasonably close to the published specs in the past.

Is there a mistake here?

By the way, sensitivity comes out as 88.6 dB per watt-which with that high voice coil resistance will be somewhat different from 2.8 volts.