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Old 4th June 2004, 06:37 PM   #1
gary f is offline gary f  Canada
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Default Measuring speaker impedance

Hi

I need to measure my speakers impedance. Is there a simple method?

I am thinking about sending a tone with 1 volt amplitude and mesure current. I could then calculate impedance for that frequency with Ohm Law. V=Z*I Then, i would repeat for many different frequencies.

Will that work? I have read method about constant voltage or constant current. But the method above is so simple, why do it differently.

Thanks
F
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Old 4th June 2004, 06:45 PM   #2
azira is offline azira  United States
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Download speaker workshop, make the Wallin jig. That's probably just as easy (or easier) as what you want to do. If you have the ability to generate frequencies and measure impedance like you are proposing then this shouldn't be too difficult for you.
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Old 4th June 2004, 06:56 PM   #3
Vikash is offline Vikash  United Kingdom
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You don't even need to make the jig. I think it's quite simple to do without it IIRC. Just flick through the help on SW and it reveals all. But it's dead handy to have the jig if you're planning on using it for multiple projects...
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Old 4th June 2004, 07:03 PM   #4
gary f is offline gary f  Canada
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Thanks, i will check that

But do you think that my method is correct?

F
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Old 4th June 2004, 07:08 PM   #5
gary f is offline gary f  Canada
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Sound Cards

From my experience, soundcards worth nothing for measurements. The output or input amplitude is not flat at all and bandwidth is limited. (I know, it's time i get a decent soundcard)

F
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Old 4th June 2004, 07:17 PM   #6
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The method you propose will work but it has two shortcomings- measuring the current, which can be problematic above a few milliamps, and doing all the calculating. Using a resistor network is far easier as you basically can just run a sinewave tone into the speaker at whatever frequency you wish and instantly read on your DMM the impedance.
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Old 4th June 2004, 07:53 PM   #7
gary f is offline gary f  Canada
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I do all calculating with excel.

I tried yesterday with a 1K resistor in serie with the driver and reading the voltage drop in the driver. I had a few problems:

1- The voltage drop is small in the driver because much of the voltage drop is in the 1K resistor.

2- I had problems with unstable values. At first, the value was large and then, the level dropped slowly during 1 minute before setting to a certain value. Ex: at 1000Hz, At first, the voltage drop across driver was equivalent to 13 Ohm, slowly going down to set at 9.5 Ohm for example. The value of 9.5 seemed plausible but I am worry about the drop.

3- It is possible to calibrate the system using a 10 Ohm resistor, and adjusting gain to read a 0.100 volt drop across driver. But if the sound card has irregular FR, then the calibration is screwed?

F
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Old 4th June 2004, 09:00 PM   #8
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I use a 100 ohm series resistor and a 10 ohm calibrating resistor and calibrate to a 1 volt input. The downside here is that the higher the impedance the greater the deviation from a true reading, but a 100 ohm resistor is a lot more load friendly for the average amp, and high accuracy above 20 ohms matters little, it's where the impedance peaks are that count for cabinet tuning.

I'd never use a soundcard. I use a tone generator going through a 100 watt amp. I suspect your instability problems are related to using a soundcard rather than a power amplifier.
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Old 4th June 2004, 09:11 PM   #9
Vigier is offline Vigier  Netherlands
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Hi,

my first measurements were with a 100 Ohms resistor and a 4 Ohm woofer (3.9 Ohms DC).
Using a simple 50W amplifier , sine-wave generator, frequency meter, and a multi-meter.
Then a table in Excel, with 2 voltage measurements for each frequency: 1 for the voltage over all (ouput of the amplifier) and 1 over the 100 Ohms resistor.

I measured 100 points from 20 Hz to 2 kHz (for a midwoofer).
Be sure to have a FULL battery in the multi-meter!
I thought my meter wasn't precise enough (deviation because of a high frequency), and that's the reason I also measured the voltage overall (to compensate that deviation).

A few month later I bought a "real" measurement-system, DLSA Pro, and when measuring the same woofer again, all measurements turned out to be within 3%, smaller than most capacitors and inductors.
So, when it's only for a few units, this simple (time consuming) approach, is surely good enough.

Grtz, Joris
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Old 4th June 2004, 09:31 PM   #10
Vikash is offline Vikash  United Kingdom
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Quote:
I measured 100 points from 20 Hz to 2 kHz
Rather you than me.

If that's what it takes to do it manually, then I would certainly build the jig if you intend to do it more than once. It's nice to build DIY tools to aid building your DIY speakers

The jig and SW combo can produce insanely accurate results:
http://www.vikash.info/audio/sw_jig/measurements.asp
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