Interesting to note negative resistance measure (shown in light green) in REW chart (from the link provided). What does it mean, or how do you interpret it? Do you know?
Impedance - The most ignored and useful measurement tool
I’m constantly reading about audiophiles diagnosing their speakers or attempting to mod their crossovers with expensive new parts. The one tool I wish they’d all get and rarely do is an impedance graphing tool. These are either nearly free or affordable.
They let you produce impedance charts like Stereophile does, as well as measure capacitors and inductors with ESR/DCR respectively.
In the nearly free variety you can build one and use Room EQ Wizard. In the affordable is the Dayton Audio V3. Either one does an excellent job of measuring a driver, crossover parts and the entire speaker as well. Completely irreplaceable tools.
Diagnosing your speakers with the help of others on the internet is made so much easier when you have one of these. Even if your speakers are fine, measure them and keep the charts handy for when they do go bad, it will make it much easier to understand what is and is not working. Replacing a cap/coil? Measure them and the speaker before and after when you are done to make sure everything came back together properly.
You’d be surprised how many speakers have a woofer or tweeter that has stopped working but the listeners don’t even realize it. These are immediately visible problems in the impedance plot.
Of course, it's just a tool, but when a driver goes bad, or a solder joint fails the impedance charts will go wildly off track. It's up to you to dig in and diagnose further.
Showing 5 responses by lanx0003
@erik_squires @ddlux @richardbrand Thank you. You're correct. The lower chart should represent the phase angle, which varies from -90° to +90°. I prefer Stereophile's presentation, as shown below, where the impedance and phase angle charts are superimposed on the same chart with the left axis ranging from 0 to 20 ohms for impedance and the right axis ranging from -90° to +90° for phase angle, making it easier to interpret. On another note, Stereophile began reporting EPDR (Equivalent Peak Dissipation Resistance) around 2020. As you may know already, EPDR represents the lowest effective resistance, accounting for both impedance dips and reactive phase angles. The exact formulation used by Stereophile remains unknown to me, but thanks to Mr. Jack Oclee-Brown, his derivation appears accurate enough to reproduce the values reported by Stereophile. As shown in the example below for the Wilson Alexia V, winner of Stereophile's 2023 Speaker of the Year award, the nominal impedance is approximately 2.9 ohms, which may seem manageable. However, when factoring in the phase angle, the corresponding effective impedance drops to just 1.15 ohms, indicating that the speaker is actually quite current-hungry at low frequencies.
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@livinon2wheels Got it. Easy to remember. Thank you. |
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:
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.
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