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Old 12th November 2005, 03:04 PM   #1
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Exclamation I need help calculating missing Thiele/Small params.. (plz help me! negative qms??)

I'm projetting my car audio system.. .
I have a 32cm (12') Kenwood subwoofer (KFC-W112) but searching for its T/S params I found only few... (I asked Kenwood too.. but they say to have no infos )

Here what I know about the speaker..
(I think important params are there or may be calculated.. but I need help doing it... I'm searching for formulas, too ! )

Pmax = 400W peak, 130 rms (per coil?)

Fs: 27Hz
Vas: 159.08 litres
Qts: 0.37
Diameter: 30.48 cm
Sens: 93 db 1W - 1m

(I ask you cause using some formulas, I found a negative Qms.. is it possible? )

Thank you in advance..
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Old 13th November 2005, 09:25 PM   #2
kea is offline kea  New Zealand
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what do you need to know?
What are your intentions? For calculating the cabinet size you only need what you already have.
I have a little program that tells you the size for given parameters. You are welcome to have that... (it is freeware).
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Old 14th November 2005, 07:04 PM   #3
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I have some software (WinISD is one of these) and I was trying to project a 4th order bandpass enclosure. I inserted my params in this software and it automatically calculates missing ones..
It give me a negative qms (or qes.. I do not remember now). Is it possible? I've done some projects but seeing this strange thing, before buying wood, I would lik to know if this is possible or if there is an error in data i have (I fond it in a strange site... the only one on the INet telling something about my subwoofer). Or you are saying qes and qms are not required for this calculations? (the software tells me about impedence, cone excursion, amplifier load and much more.. not need other Thiele Small Params?)

Thank you very much for your help.
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Old 14th November 2005, 07:04 PM   #4
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Another question / details...

1) Years ago the suspension was changed (old was broken).. Do you know how much this can change T/S params. (When i buyd the sub used it was in a bass reflex enclosure that I simulated with T/S params i have with WinSD. The result was a good sub but in the reality it sound sucked.. I have doubts)

2) I have just buyed a multimeter that can measure inductance and capacitance and freq too... with this could I measure some T/S params? (I think Le surely... and making some circuits may be other parameters too.. I reading about this...)

Thanks again.
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Old 14th November 2005, 07:37 PM   #5
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Use a computer and this program to generate the sine waves.
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Old 14th November 2005, 09:25 PM   #6
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Thank you so much!
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Old 15th January 2006, 03:12 PM   #7
kea is offline kea  New Zealand
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Default negative Qt

Yes you can get a negative Qt or Qe etc. if you have the wrong parameter somewhere. This means your result is definitely wrong!! The Q must always be positive.
I wrote a little Excel program to calculate all the parameters of a driver if you have the parameters: mass, suspension, magnetic field strength. This is usually not the case but you can easily see how your parameters change if you change the suspension for example. Usually it is not too critical to change the suspension however because this is not a linear function. That means that if your suspension is relatively high (soft) then changing this by up to lets say 50% does not change a lot in sound. If your suspension is rather stiff then you will have more change in sound however.
I am happy to send you a copy of the program if you like.

Bandpasses are quite critical by the way, I tried a few ones and it was difficult to make them sound good.

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