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Old 10th December 2007, 07:28 PM   #1
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Default measuring speaker impedance

Hello, I have read an article in the ESP Audio website regarding measuring TS parameters of drivers. Rod Elliot says in this article that the amplifier, frequency generator, and associated equipment MUST be very accurate to get good readings, e.g. " the anplifier and frequency gen. must put out a very consistent voltage accross the audio spectrum." I am using a PC based generator from REW program, to a extermal sound card, to a Onkyo reciever for amp, the meter I am using is a Fluke true RMS meter, also capable of frequency measurement.
When I set everthing up as in per the pics on his website article, I tested at a starting voltage of lets say 420mv at ten hz and by the time I increased the frequency to say 400hz, I was now reading 420mv, is this within specs to be considered OK for measuring TS parameters? This is my first time doing this so I don't know. If when Rod said amp MUST put out linear voltage, did he mean NO fluctuation in voltage at all when frequency is increased??
Thanks,
Vince
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Old 10th December 2007, 08:58 PM   #2
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I think you made a typo. Were both readings 420mV?

Anyway, your measurements will be as good as your soundcard. The Onkyo & the Fluke are both good enough although just to be sure, check the specs on your Fluke. The true RMS may only be valid over a limited frequency range.

Also the frequency reading will degrade if the signal is too small . I seem to remember my 8060 cr@ps out at 200mVrms or so.
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Old 10th December 2007, 11:55 PM   #3
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Yes, you are correct, I did make a typo. There is about a 20mv differential between about 20hz and 400hz. I was considering purchasing a function generator, only about $200, I just want to get this accurate so I don't have to wonder. I will check my meter for what you had mentioned above. I was wondering about the sound card as you had mentioned. I do trust that my Onkyo reciever more than my sound card!
Thanks,
Vince
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Old 11th December 2007, 06:09 AM   #4
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Default I stand corrected

My little Fluke blew me away again. It can measure frequency from 12Hz to 20KHz with a 20mVrms input. The AC volts rms scale is 0.2% from 45Hz to 10KHz!

Back to the thread.
With a 20mV differential we're talking less than 0.5dB here. Probably a too-small cap on the output of the soundcard (the 10Hz reading is less than the 400Hz reading right?). You can track it down by repeating the measurement at the soundcard output but I'm not sure it's worth the effort.

Anyway, it's probably not as bad up at 40-80Hz where most of the T-S action is. I'd say you're good to go.
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Old 11th December 2007, 10:44 AM   #5
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Yes, you are correct again. The 20mv difference is pretty much seen in the 10hz to 30-40hz region and then "slowly" grows from there.
Thanks again,
Vince
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