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Is there easy way to measure speaker impedance

SIY

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You need to measure the amplitude of the voltage & current and the angle between them at the same instant if the signal is varying over time. If the signal is unchanging over time in all characteristics, then you could measure voltage, and then current, and after getting a beer, catch the phase angle. For something like an audio signal, you have to snap them all at once.
For slow moving audio signals, it’s trivial and routinely done.
 

NTK

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[Edit] The method shown in this post is incorrect. The problem is when Z_spkr is complex, we have | Z_spkr | + | R | ≥ | Z_spkr + R | (triangle inequality for vector sum). The updated method is shown in post #24.

This will get you the impedance magnitude, but not the phase.

Impedance Measurement.jpg


For each frequency, you measure (with your DMM) the voltages across the speaker and a sense resistor (R_sense), and the voltage across the sense resistor alone. Using the voltage divider formula:

z_spkr_measurement.png


This will give you the speaker impedance magnitude at 1 frequency. You'll need to step through multiple frequencies in the range of frequencies you are interested in to plot the impedance magnitude curve. You can use a R_sense with a value about the same as the nominal impedance of the speaker.
 
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NTK

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The method shown in post #22 was incorrect. An updated method is shown here.


Impedance Measurement.jpg


Speker Impedance.png


The v_1 and v_2 we measure with a DMM are the magnitudes, and the relationship between the measured voltage and impedance components is given by the last equation shown above. Since v_1 depends on 2 values, Z_a and Z_b, we need more than 1 measurements with different values of R's to determine Z_a and Z_b.

A Mathematica notebook showing the calculations is attached. This method, however, is unable to determine the sign of the reactive component (i.e. whether it leads or lags). The "simulated impedance " is a series combination of a 6 R resistor, a 0.7 mH inductor, and a 250 uF capacitor.
 

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egellings

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The method shown in post #22 was incorrect. An updated method is shown here.


View attachment 296181

View attachment 296188

The v_1 and v_2 we measure with a DMM are the magnitudes, and the relationship between the measured voltage and impedance components is given by the last equation shown above. Since v_1 depends on 2 values, Z_a and Z_b, we need more than 1 measurements with different values of R's to determine Z_a and Z_b.

A Mathematica notebook showing the calculations is attached. This method, however, is unable to determine the sign of the reactive component (i.e. whether it leads or lags). The "simulated impedance " is a series combination of a 6 R resistor, a 0.7 mH inductor, and a 250 uF capacitor.
Math looks good (correct) to me.
 
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BeerBear

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I'm also considering making a rig like this and I have a few questions.

In the example implementation they connected the grounds of the L and R line input terminals, which might just be redundant, but IDK. Is there an advantage to doing that? Or some possible disadvantage?

Does the output impedance of the headphone output affect the accuracy of the measurement? Let's say, if it's 1 Ohm vs 50 Ohm.

The REW guide says the resistor must be non-inductive. How much worse would it be with an inductive one?
 

r042wal

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Sure. It is actually fairly simple and doesn't need fancy equipment at all. Here is a quick tutorial from Audio Precision on the setup: https://www.ap.com/technical-librar...edance-using-apx-derived-measurement-results/

When measuring speaker impedance, does resistance have a factor? Is it more accurate to have a set of Kelvin probes to offset the resistance the test probes create?

The reason I ask, I am going to recap some xovers and have a nice BK LCR with Kelvin clips to dial my replacement parts in. Thanks for your advice.
 

pedrob

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As was mentioned previously, I also have a DATS V3 Computer Based Audio Component Test System, and it measures impedance over the audible spectrum. Resistance is the impedance at any one chosen frequency and doesn't mean anything. Before I start, I like to fit a Zobel Network across the speaker's terminals. I prefer the latest version, which has been named an Improved Zobel Network. It uses 2 resistors and 2 capacitors. Forget the recommended formulas, as they are complex and don't work. Instead, use resistor and capacitor substitution boxes to determine the values yourself. After a value is altered, click the Impedance Sweep to see the change. The four values interact, so it's somewhat time-consuming, but well worth the effort. Once done, you can use the DATS to measure the exact value of the substitution boxes. Try to get the components as close as possible to the determined values. I prefer to make up the values by adding components in parallel, and forget about in series. Capacitor values add up, so don't go over the desired value. Resistors are the opposite, so start high and keep adding values until the measured value drops to the desired value.
 

JimA84

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There,are really a couple answers depending upon what you intend to do with the information.

If you want to build a simple analog electric crossover network or roughly estimate the load on your amplifier it's not too hard.

If you want to reinvent the Theory of Relativity or convince people to pay a hundred thousand dollars for your supposedly phase and time aligned speaker systems, the answer is more complex.

In the early 1980s I analyzed the data from a digital system designed to measure the impedance and phase angle of Navy sonar transducers to determine when an array was sufficiently degraded to need a scheduled shipyard refurbishment.

This also raises the issue of high order analog crossovers.

With broader range drivers and the availability of digital crossovers at affordable prices it is likely not worth pursuing very high order electric crossovers.

In addition actually measuring the effective impedance of speaker systems around the crossover frequency is problematic at best unless you want to use a basic second order crossover.

This is kind of like estimating the definite integral of a function using numerical methods as opposed to the actual antiderivative.
 
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grogi.giant

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If you want to build a simple analog electric crossover network or roughly estimate the load on your amplifier it's not too hard.

If you want to reinvent the Theory of Relativity or convince people to pay a hundred thousand dollars for your supposedly phase and time aligned speaker systems, the answer is more complex.

Tony Stark would know already, so it's safe to assume that when someone is asking here, it is the first case ;)
 

fpitas

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pedrob

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If you want to reinvent the Theory of Relativity or convince people to pay a hundred thousand dollars for your supposedly phase and time aligned speaker systems, the answer is more complex.
Utter rubbish. Denon and most other home theatre stems do phase correction (or alignment if that's your point of view) when you add speaker distances. Annoyingly, Denon and probably Marantz only uses one significant digit when making the calculations to lighten the processor load, so you'll need to alter the distances accordingly. No real hassle. Just multiply the distances by 343 and divide by 300.
 
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grogi.giant

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Utter rubbish. Denon and most other home theatre stems do phase correction (or alignment if that's your point of view) when you add speaker distances. Annoyingly, Denon and probably Marantz only uses one significant digit when making the calculations to lighten the processor load, so you'll need to alter the distances accordingly. No real hassle. Just multiply the distances by 343 and divide by 300.

No, they don't. Setting distances simply adjust delays, but there is no more advanced phase correction done. Not in "most other home theatre systems"
 

pedrob

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Thanks for the laugh. You made my day. LOL!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 

egellings

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Thanks for the laugh. You made my day. LOL!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Don't need so many bangs; one will do.
 
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