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Old 27th December 2003, 11:44 AM   #1
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Default Best-ever T/S parameter spreadsheet.

Well.... the best one I have ever done anyway.
I have downloaded and played with lots and lots of them, and the simple ones at least, all seem to have some shortcoming. for instance, the majority of them calculate Vas by the put-the-driver-on-a-box method, which I find inconvenient. Not as many have the added mass method. Also, most use the drop in current method of finding resonance and this seems to have all these yucky intermediate steps that make my head spin. The other Fs measurement method that uses a high value resistor to feed the speaker and measures the rise in voltage across the voice coil at resonance seems to be in the minority, and of course there is the error introduced by this method if the speaker impedance goes particularly high.

So... I sat down and wrote my own spreadsheet. It won't set the world on fire but it does just exactly what I need. Feed the speaker from an amplifier through a 1k or whatever resistor. Tell the spreadsheet the voltage coming out of the amplifier, and the voltage across the speaker etc. Any impedance measurement error both at Fs and -3dB frequencies are compensated for.

***********

For an accurate and flat response signal source I burnt a CD with 8 tracks having a linear (not log!) sweeping sinewave of 10-20 Hz, 20-30Hz etc. and each track is 10 minutes long i.e 1 minute per Hz. This means that if the frequency of interest is passed on track 4 at 8 min 30 sec then the frequency was 48.5 Hz. No frequency counter necessary!

On the CD player and looking at the output with a scope, the level varied maybe 1 part in 200 or less across the spectrum. Way better than some toy opamp oscillator. Way better than any soundcard I have too. They're a big disappointment.
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Old 29th December 2003, 10:54 AM   #2
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In the previous post where I said you feed the speaker from an amplifier through a 1k or whatever resistor, it seems to me that this resistor would damp the impedance peak somewhat and so it should be as high a resistance as possible. Of course the higher you make it the less drive you have to the loudspeaker and after a while it gets difficult to measure with any reasonable accuracy.

What I want to know then, is there a way to estimate the damping effect of this feed resistor so that it's effects can be accounted for; so that the calculated result would be as if the resistor was infinitely large?

One other way perhaps would be to use the voltage across the speaker to control a VCA stage so that the voltage across the resistor remains virtually constant. Or maybe use a current-source amp without any resistor.
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Old 29th December 2003, 11:31 AM   #3
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thats it, AC curent source.....

but I think when that you use 10k, its precise enough, with 100 ohms impedance peak of the driver this is only 1% deviation.....
You need an amp with nice high voltage (100Vrms orso ) to drive the thing.... can be just an ordinary opamp circuit with 2 pp high voltage transistors running on +/- 150V...
Is there is someone with an idea of a good/simple lineair ac current source? it is a lot nicer than the high voltage, it could run on a battery....
By the way: I like the spreadsheet circlo!....
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Old 30th December 2003, 04:49 AM   #4
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Default Some real-word stuff

Did some measurements on this el-cheapo 12" driver and the Vas result is a bit odd. The results are as follows:

Source Qes Qms Qts Fs Vas
Manuf. 2.32 4.0 1.46 32 166
Spkr W/S 1.26 2.74 0.86 34.16 141.3
me 1.19 2.84 0.84 34.1 72.1

Spkr W/S=Speaker Workshop. me=me & my spreadsheet.

The Vas calc part of the spreadsheet I grafted off someone else's so I can't be certain of it's accuracy. Can anyone give a second opinion?
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Old 30th December 2003, 11:27 AM   #5
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Hi Circlotron,

What added mass did you use, and what was your fs1 with the added mass?

Formulas I have from Weems are:

Mass of cone M = M1 /(fs/fs1)^2 -1 where M1 is the added mass fs1 is the fs with added mass

Compliance Cms = 1/((6.28 * fs)^2*M)

effective cone area A = 3.1416 * r^2

Vas = Cms*d*C^2*A^2

where d = density of air 0.00129 g/cc and c = speed of sound 34400 cm/sec. He then says that for VAS in litres divide by 1000.

He also measures the diameter of the speaker as being from one edge (including surround) to the start of the surround on the other side (for working out r above).........

I'd be interested to know the results. When I measured my Vifa 10" drivers with SW the manufacturers spec was 130L Vas, but I got 125L with SW.

Tony.
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Old 30th December 2003, 08:25 PM   #6
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Default Try this out

Hi Circlotron,

I modified your formula for Vas. Try this out and see if it is closer to what you would have expected.

Tony.
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Old 31st December 2003, 11:03 AM   #7
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Hi Tony. I have been changing it in the meantime of course! Your one does give better results but there was an error in the cone mass formula. To check, when you double the moving mass the Fs should decreases to root 2/2 of the original.

The formulas below, the upper one is mine, the lower is yours.

Cone mass
=G4/(G6/G7)^2-1
=G4/((G6/G7)^2-1)

Vas
=SQRT(2)*1000*(G18^2)*G20 (my new formula)
=G20*0.00129*(34400^2)*((G19/1000)^2)

Compliance
=1/(((3.14159265*2*Fs)^2)*(G17)) G17=cone mass
=1/ ((3.14159265*2*G6)^2)*G4 G4=added mass

For the compliance I have used cone mass, you have used added mass. What do you think? It is not the exact formula I used but I juggled it around to match the format of yours for easy comparison.

The Vas's give slightly fifferent results, perhaps because of the compliance stuff?
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Old 31st December 2003, 11:37 AM   #8
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Quote:
Originally posted by Circlotron
The formulas below, the upper one is mine, the lower is yours.
Now if I just switch the positions of the first two to make this actually so...

Cone mass
=G4/((G6/G7)^2-1)
=G4/(G6/G7)^2-1
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Old 31st December 2003, 11:58 AM   #9
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Quote:
Originally posted by wintermute
Hi Circlotron,

What added mass did you use, and what was your fs1 with the added mass?

Formulas I have from Weems are:

Mass of cone M = M1 /(fs/fs1)^2 -1 where M1 is the added mass fs1 is the fs with added mass

Compliance Cms = 1/((6.28 * fs)^2*M)

Vas = Cms*d*C^2*A^2

where d = density of air 0.00129 g/cc and c = speed of sound 34400 cm/sec. He then says that for VAS in litres divide by 1000.
OK. I used 21g added mass and the figures I got are on the currently posted spreadsheet.

Compliance Cms = 1/((6.28 * fs)^2*M) You used M1 instead of M.
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Old 31st December 2003, 11:58 AM   #10
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hi

this is a bit of the thread

i was using the woofer tester _ and now its not operating properly _ and a upgradation is long over due

i am planning on the loudspeaker lab (swedish) software + add on unit

how would you rate it , and could you recommend any other such units in the same price range , this is about 400 usd including the mic and add on unit

thanks
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