Measuring Impedance

Measurement Methods

Audio Interface Hardware

Direct Impedance Method

Shunt R Impedance Method

    Theory

    Setup

    Calibration

    Method

Impedance Meter

Impedance Plot

Measurement Methods

This document describes how to measure impedance using AudioTools and iAudioInterface2. Impedance differs from resistance in that impedance may change with frequency.

We have two different methods for measuring impedance, Direct, and Shunt R. Pick the method that best suits your application.

Direct Method vs Shunt R Method

Direct is the quickest and most foolproof method, and the easiest to set up and use, but should only be used for lower impedance values. It works well in the 1 - 100 Ohm range. Direct requires only one simple cable. Use it for single voice coil measurements, or for a number of 70 V loudspeakers in parallel.

Shunt R is very accurate in the 1 - 10k ohm range, but requires a special cable and a more complex setup. Calibration invloves more steps, but it will give excellent results for single 70 V loudspeakers, for example.

Once you have chosen a method to use, follow the instructions below.

Audio Interface Hardware

This document assumes the use of the iAudioInterface2 for the audio input and output. This hardware is required to use the Direct Method.

When using the Shunt R method, it should be possible to apply this method to any 2-channel iOS audio interface, although care must be taken to avoid clipping, to match the input sensitivity to the output level, to match the right and left channel input gains, and to keep the input and output gain settings unchanged throughout the setup and measurement process. Substitute the left input channel for the line input Tip, and the right input channel for the line input Ring in the Shunt R Setup discussion below. Also  make sure that the generator ground is in common with the input ground.

Direct Method

The Direct Method uses an internal loopback to measure the output load impedance.

Direct Method Setup

To use the direct method, you need a single, special cable. This cable is a 1/4" TRS plug with only the tip and ring connected. Typically you will bring these out to alligator clips to connect to the load that you want to measure.

direct cable

Plug Wiring

Direct Method Calibration

Open the Setup screen from the Impedance Meter or the Impedance Sweep screen.

Leave the test leads open, not connected to anything.

Turn on the Generator, and let the level stabilize. Tap Set Maximum.

Short the leads together, let the level stabilize, and tap Set Minimum.

You are now ready to measure impedance.

Measuring Impedance with the Direct Method

Make sure you have selected Direct on the setup screen measurement type selector.

Clip the test leads across the load, and run either the Impedance Meter, where you will select a specific frequency, or the Impedance Sweep, which will produce a plot of load impedance by frequency.

direct setup

Note that the level control is disabled automatically by the software.

Shunt R Method

The Shunt R method uses both the generator output (1/4") and the Line Input (1/4"). The generator runs through the load and a known shunt resistor. The voltage across the shunt resistor is measured, and voltage across the shunt + load is also measured. The impedance is computed from these two voltages.

Shunt R Method Theory

IShunt = VShunt / ZShunt

ILoad = IShunt

Also, the voltage across the load + shunt is measured. The voltage across the load is computed by

VLoad = VTotal - VShunt

And then the load impedance is computed by

ZLoad = VLoad / ILoad

Impedance Schematic

Sizing the Shunt Resistor

The Shunt resistor size determines the range of impedance that can accurately be measured. A rule of thumb is:

range of impedance = R ÷ 25 to R × 25

So, with a shunt resistor value of 100 Ohms, you would be good measuring 4Ω to 2500Ω accurately, with accuracy slowing dropping as you move away from those values.

Shunt R Method Setup

First, you need to build the special cable setup. You will need two 1/4" TRS (stereo) plugs,  three alligator clips, and a shunt resistor, normally around 100 Ohms. A small calibration resistor is also useful, around 8-10 Ohms.

Connect a jumper wire from the tip of one of the TRS plugs (this will plug into the Generator Output) to the ring of the other TRS connector (this will plug into the Line Input). This wire is shown in blue in the picture below.

Connect the three alligator clips to the three terminals of the Line Output TRS plug. Here's how we connected the setup below:

cable detail

Here is the connection list for this setup, and the colors we used:

Blue - Output TRS Tip to Input TRS Ring
White - Input TRS Ring
Red - Input TRS Tip
Black - Input TRS Sleeve (ground)

Impedance cable

Advanced Tip:

You could also opt to build the shunt resistor into the cable, if you have determined that a single shunt resistor value will work well for you in your situation. In this case you would just have two alligator clips for the load, and the resistor would go between the ground in the TRS input plug to Tip of the same plug. The black lead shown would not be used.

Shunt R Method Calibration

To calibrate, open the Setup screen on the Impedance Meter or Impedance Sweep.

Step 1 -- Enter Shunt Resistor Value

Select Shunt R on the measurement mode selector.

Enter the value of the shunt (reference) resistor. Typically 100 Ohms will be used, although other values can also work. This is required, and the accuracy of the resistor value will impact your results. Use a 1% resistor, or measure the resistor with a good VOM.

Calibration for the Shunt R method includes two more steps. Both are optional; you will get good results even without doing these steps. By doing them you will get the best possible results.

If you are going to do these steps, do the Voltage Offset calibration first.

Step 2 -- Voltage Offset

Connect the shunt resistor from the Line Input Ground to the Line Input Tip and Ring leads, which are shorted together.

Turn on the Generator output.

Tap the Calibrate button next to the Voltage Offset field.

voltage cal

You should see a value of 0.0050 Volts or less for the Voltage offset.

Step 3 -- Calibration Resistor

Next select a small calibration resistor, 10 Ohms or less if available. Enter this value in the Calibration Resistor Value field, and set up as if you were doing a normal load test, but use your calibration resistor for the load.

Connect the calibration resistor from the Line Input Tip to the Line Input Ring, and connect the shunt resistor from the Line Input Tip to the Line Input Ground.

Turn on the Generator output, and tap the Calibrate button next to Calibration Resistor Field. You should see a value near 8 Ohms.

cal resistor

Shunt R Measurement Method

Make sure that you have selected Shunt R on the setup screen measurement mode selector.

To use this method, you will need the cable described above and a shunt resistor, typically 100 Ohms.

The setup for this measurement is shown below.

Connection List:

Load - Line Input Tip & Ring (white and red)
Shunt R - Line Input Ground and Tip (black and red)

impedance method

Note that the software automatically disables the output level control and uses a fixed output level.

Impedance Meter

Impednace Meter

Use the Impedance Meter to spot check impedance at a few frequencies, and also to compute the power required at a particular voltage for high-voltage audio distribution systems.

Turn on the Generator, selected a frequency with the slider, and directly read the impedance.

Impedance Plot

Impedance Plot

Use the Impedance Plot module to create a curve of impedance versus frequency.

Sweep Time

You can select 10, 20, or 30 second sweep times. Longer sweeps will produce smoother plots.

Smoothing

You can turn smoothing off, or select 1/24, 1/12, 1/6, 1/3, or Octave smoothing. Smoothing can be changed after the plot has been created.

Graph Scale and Frequency Range

Frequency range is 20-20kHz, and the Ohm scale is set automatically to fit the curve.

Settings

Tap the wrench icon to open the Settings page.

Driver Parameters

Turning on the Drivers Parameters option will cause these Thiele/Small driver parameters to be displayed on the graph:

Zmax -- Maximum impedance
Re --Impedance at lowest frequency
Ro -- Zmax / Re
Fs1 -- Impedance = ( sqrt(Ro) * Re ), below Zmaz
Fs -- Resonant frequency of the driver in Hz
Fs2 --
Impedance = ( sqrt(Ro) * Re ), above Zmaz

Difference Mode

When a graph has been recalled, show the difference between the current plot and the stored plot.

Average Plots

All recalled plots are averaged together into a single plot.

Reference Curve

Brings up the Reference Curve screen. From here, you can show Reference Curves on the plot screen, for comparing the current plot with stored reference plots.

See the Reference Curve help file for more information.