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?
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?
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.
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.
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 ...
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 ...
Svante said:
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
azira said:
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.
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.
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.
Oops. Apparently the Sd matters more than I thought. Plugging 235 sq cm into Box Model yields 8.71 Tm.
That Peerless 8" is really a 217 mm, which is somewhat larger than 8". A normal 8 incher is usually between 215 and 220 sq cm.
The normal way a cone is measured is to go from the edge of the diaphragm on one side to midway into the surround on the other side. There are different ways of measuring it, but that is the way of audio writer David Weems, and it usually yields results similar to the published specs.
That Peerless 8" is really a 217 mm, which is somewhat larger than 8". A normal 8 incher is usually between 215 and 220 sq cm.
The normal way a cone is measured is to go from the edge of the diaphragm on one side to midway into the surround on the other side. There are different ways of measuring it, but that is the way of audio writer David Weems, and it usually yields results similar to the published specs.
Svante:
I don't think it makes that much difference, but I was given to understand that it is not mid surround to mid surround, but dege of diaphragm to mid surround.
I think both answers would be close.
Using either method, I think Azira will get a Bl number which will give good results in Martin King's TL models.
I don't think it makes that much difference, but I was given to understand that it is not mid surround to mid surround, but dege of diaphragm to mid surround.
I think both answers would be close.
Using either method, I think Azira will get a Bl number which will give good results in Martin King's TL models.
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