electrical math whizzes pleeze help!

[speekergeek]

I lugged all my speakers to work so i could measure their TS parameters and i thinkn i may have messed up. 1st of all i forgot the resistor, so i just measured the voltage across the speaker itself. that helped me find Fs. but when i tried to find impedence at resonance i was SOL.

reistance at DC is 5.2 ohms
Fs 49.5 hz

5 volts in at resonance (49.5 hz) and i got .4 volts on multimeter.

added 30 grams and Fs was 29.5 hz


I tried ohms law and it didn't give me the right answer, couldn't be.

5.2 ohms at 5 volts is .96 amps 5.2/5= .96

.96 amps all through the circuit, am i right there?

now at resonance the voltmeter said .4v. so i would guess that the next step would be E over I gets R. so .4v divided by .96A equals .41 ohms? That cant be right!

someone please slap some sense into me and show me the path into righteousness. do i need that darn resistor?

thanks for any help.

[Centauri]

I think you will need that resistor......

As DC resistance is 5.2ohm, then your .96 amps will be with 5V DC, not ac signal - therefore this calculation is meaningless.....

Your multimeter will have to be a digital type with true RMS ac voltage ranges with decent frequency response - quite a few don't cut it. Remember you are measuring AC voltages here.

If you say you had 5V in and measured .4V across speaker, then there must be quite a lot of resistance in line, or your measuring techniques not up to scratch.

Recommend downloading Speaker Workshop (free) and making up the Wallin jig linked to from Speaker Workshop site.

Cheers

[roddyama]

You also didn't say if you made your measurements with the speaker in the box or in free air. You will want to make the measurements both ways so you can calculate Mms and Vas.

[speekergeek]

In my radio shack speaker building book it has a method where you add 6 nickels (5 grams x 6 = 30 grams) instead of building test boxes. I am unable to build test boxes right now so this must suffice. I guess i will just go to radio shack and shell out the big bucks for a 10 ohm and a 1k ohm resistor. damn the luck. thanks anyways.

[Matttcattt]

have you looked at http://sound.westhost.com/tsp.htm ? it is a guide to measuring thiele/small parameters of a loudspeaker by Rod Elliott.

[Ron E]

The constant current way to measure T/S parameters is found at:

http://www.epanorama.net/documents/a...arameters.html

There is a calculator that helps with the calculations at www.kbapps.com

It seems the method at sound.westhost is another way of measuring parameters using something close to constant voltage. The 5 ohm resistor is used to sense the current.

Both methods have issues with cheap multimeters because the frequency response is often not flat - mine both roll off below 40Hz and above 1kHz.

[speekergeek]

I will be doing it again in a few hours with the new resistors i bought. My work has expensive equipment that is calibrated regularly so i hope their stuff is adequate. i do want to get my own multimeter though, this friday is my last day at work. my enlistment is finally up.

[speekergeek]

YAY! it seems to have worked!