Impedance - The most ignored and useful measurement tool

@erik_squires +1 "You’d be surprised how many speakers have a woofer or tweeter that has stopped working but the listeners don’t even realize it. “ I will be not!

speakers impedance is good start, but it will not tell us whole story!

whole amp+cable+speakers system need to be characterized on frequency and time domain, incl. distortions etc., using precise measurement microphone in listening room, using fuzzmeasure software or audio analyzer. I wouldn’t be surprised to see large number of setups will show >5% distortions, +/-10dB amplitude deviation in freq. response, and bad resonances! Tests should be done at different power level, starting 50mW. 

speakers impedance is good start, but it will not tell us whole story!

Nor do I ever claim it is.

whole amp+cable+speakers system need to be characterized on frequency and time domain, incl. distortions etc.,

Why? I was only posting this to show how an important tool can help audiophiles diagnose their problems faster, fix broken systems and mod safely.

I never claimed it was a replacement for full speaker analysis, which in the use cases I spoke about is not needed.  When you say need I think you are thinking far beyond the limited situations I was discussing.

So I emailed Mr. John Atkinson regarding what exactly the fomula they have used in Stereophile for reporting the EPDR. He kindly repsond promptly as follows:

I use a formula created by editor Jim Austin. after importing the Impedance magnitude and phase data into an Excel spreadsheet.

Data = R (magnitude) ohms & R (phase) degrees

Calculate Vdiss = 1+4.2 x ABS(R Phase)/90

Then EPDR = R(Magnitude)/Vdiss

For example:

If R = 4.0 ohms and Phase = +45 degrees

VDiss= 3.1 and EPDR = 1.2903 ohms

Here Vdiss represents voltage dissipation. As a further analysis, this linear equation for Vdiss provides two exact boundary values: one at 0 degrees (which is obvious) and the other at 90 (-90) degrees, as derived by Mr. Jack Oclee-Brown, shown below. However, the linear equation overestimates Vdiss values in between, which in turn results in even lower reported effective impedance.

One possible remedy is to fit the data range with an exponential term, as illustrated in the attached figure. Instead of using 1 + 4.2 * abs(phase angle)/90, nonlinear fitting yields 1 + 4.196 * (abs(phase angle)/90)¹.¹⁷⁸, which provides a much closer approximation to Jack’s formulation.

@erik_squires +1 (again) “I was only posting this to show how an important tool can help audiophiles diagnose their problems..

thanks for pointing to the simple tool to check speakers! 

my intension was to emphasize on PA system (amp+cable+spkr) importance, and methods to test it. 
 

@re-lar-kvothe , “ 2 Ohm stable” while better than nothing is really more market speak than of much value. It only suggests that the amp won’t go into oscillation when presented with a 2 Ohm load. It does not suggest how much power and/or at how much distortion will be provided at 2 Ohms. Much more useable would be an actual spec. of  ? Watts into 2 Ohms +/- dB. And in the case of stereo amplifiers with both channels driven.