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Measuring Speaker Impedance Magnitude vs Frequency with APx500

Created 23 Oct 2009

Any Audio Precision analyzer may be used to measure speaker impedance. However, since the analyzers measure and display in voltage, a utility is needed to convert the readings into ohms. In the following article, we cover the basic theory and test setup, and then present the APx Impedance Measurement Utility for APx500 Series instruments. For users of the 2700 Series/System Two family instruments, a tutorial and macro is available for measuring complex impedance.A macro is also available for the ATS-2.

Figure 1 shows a simple schematic of the speaker measurement circuit, which is essentially a voltage divider. In the Figure, V_gen denotes the generator voltage, R_s is the source resistance, and Z_x is the impedance of the device under test.

Figure 1. Impedance measurement circuit.

In general, Z_x will have both resistive and reactive components, and is therefore a complex quantity (i.e., it has both magnitude and phase). However, if R_s is much larger than the magnitude of Z_x (denoted by |Z_x|), then the magnitude of Z_x can be determined using equation (1).

    .................................................. (1)

As |Z_x| becomes large relative to R_s, the denominator term in equation 1 (V_gen - V_m) becomes smaller, and eventually the results become inaccurate. As a rule of thumb, R_s should exceed |Z_x| by at least a factor of 10 for equation (1) to be valid. For measuring a typical 4 or 8 Ω (nominal) loudspeaker, a 600 Ω source impedance is ideal.

Connections

The APx525 family has a selectable 600 Ω source impedance built in, and a loopback mode that internally connects the generator outputs to the analog inputs. Cable resistance between the generator and the speaker should be as low as possible so that it does not significantly affect the results. For the APx525 family, you can best accomplish this by connecting heavy gage speaker wire to the analog balanced output banana jacks.

Figure 2. Signal Path for the APx525 family using Loopback mode.

Connections to the APx585 family instruments are a little more complicated. First, there’s no internal loopback mode, so a cable is needed to connect the measurement point to the analog input. Second, the impedance is fixed at 50 Ω unbalanced and 100 Ω balanced. This is too low to meet the requirement that R_s should exceed |Z_x| by at least a factor of 10. To overcome this limitation, an external resistance needs to be added in series with the internal source resistance. We suggest using a Pomona Electronics #1469 adapter (BNC male to binding posts with isolated solder turrets) on the analog unbalanced output. Mount a 549 Ω 1.0% resistor across the turrets on the ungrounded side of the adapter, and short the turrets on the other side. The total of the internal and external resistances will now equal 599 Ω ± 1 %. From the binding post end of the adapter, heavy gage speaker wire is run to the speaker, and a standard BNC cable, stripped at one end, is run back to the analog unbalanced input.

Figure 3. Signal Path for the APx585 family using an external resistor in series with R_s.

Using the APx Impedance Measurement Utility

The APx Impedance Measurement Utility and associated project file allow you to measure impedance vs. frequency of a speaker and see the results directly in ohms. The utility was developed in LabVIEW 2009 and uses the APx LabVIEW driver to communicate with the APx500 measurement software. The download includes the source code for those who have the LabVIEW development environment, as well as the compiled application for those who don’t.

Figure 4. The APx500 Impedance Measurement Utility.

When the utility is started, it opens the APx500 measurement software if it isn't already running. It first checks to see what model of APx analyzer is attached, and then configures the analyzer and itself accordingly as shown below.

Table 1. Automatic configuration of the utility.

APx525 family	APx585 family
APx input connector is set to loopback mode. R-ext (Ohms) control on the utility panel is disabled and grayed out. R-source control and the generator output impedance are set to a default value of 600 Ω.	APx input connector is set to match output connector. R-ext (Ohms) control on the utility panel is enabled. R-source control is set to 50 or 100 Ω (the APx generator fixed unbalanced/balanced output impedance).

The utility is designed to be used with the accompanying project file, impedance.approj. To load the file, click Load Project File. The file browser will default to the location where impedance.approj has been installed, so just click OK. Settings in the impedance.approj project file are configured to produce accurate results. If you wish to modify the project or create your own, please observe the following guidelines:

Table 2. Project file settings.

Setting	Default	Comments
Signal Generator Level	6 Vrms	The voltage drop across the speaker is about 50-100x smaller than the generator output. Reducing the output level much below 6 Vrms will produce less accurate results. Increasing it does not seem to produce better results, and depending on the load may exceed the current limitations of the generator.
Coupling	DC	Coupling must be DC. Otherwise, there will be errors in the low frequencies.
Sweep Time	2.5 sec	If the sweep is too fast, the low frequency range will be inaccurate or truncated.
Output Impedance	600 Ω	600 Ω produces the best results when measuring common 4 or 8 Ω speakers. Output impedance generally may range between 600 Ω and 3 kΩ
Output Connector	Analog Balanced	The speaker is an unbalanced floating load, so you can use either the balanced or unbalanced generator output with no change in the results. Choose the connector that works best for your cabling method. For the methods suggested above, use Balanced for the APx525 family and Unbalanced for the APx585 family.
Start Frequency	20 Hz	Set this as desired. With the APx525 family, you may set the Start Frequency as low as 100 mHz. In this case the lowest data point will be zero, or the DC resistance of the speaker.
Stop Frequency	20 kHz	Set this as desired. If you wish to set the Stop Frequency above 20 kHz, you’ll need to change the Bandwidth in the Input Configuration to 40 kHz or > 90 kHz. Making the bandwidth wider than necessary will not improve results and will make the FFT longer.
Input Range	10 Vrms	The default setting will produce accurate results over a wide range of levels. We have found that setting it to Auto or 1 Vrms does not improve the accuracy.

 

The utility checks all the signal paths and measurements in the currently loaded APx project and sets the Selected Signal Path control to the first signal path in the project. It also sets the Selected Measurement to the first frequency response type of measurement in the signal path (if one exists).

Frequency Response is an ideal measurement to use for an impedance versus frequency measurement. It runs very quickly and has very high frequency resolution. To increase the frequency resolution of the measurement, you can increase the length of the sweep using the control in the Advanced Settings panel of the Frequency Response measurement in the APx500 software.

Continuous Sweep and Acoustic Response provide the same results as Frequency Response, except that Acoustic Response offers the option of averaging several measurements, if needed. Note that if the Acoustic Response measurement is used, the time window control should be set to the same value as the sweep time (effectively removing the time window). Otherwise, there will be large errors in the results.

To run an impedance measurement, select the signal path and measurement you want to use from the utility’s front panel and click the Start button. After the measurement is completed in APx, the utility calculates the impedance and displays it on the graph (Figure 5). One cursor is automatically placed at the point of maximum impedance, and another is one at the point of minimum impedance.

Figure 5. Measured impedance curve for a loudspeaker with multiple drivers.

The Print Results button opens a new window that allows you to print the graph image to the PC’s default printer, or to save an image of the graph to a file. This window also has a notes field where you can add details about the test or DUT.

Finally, the Export Data button will export the data as a tab-delimited text file, for import into a spreadsheet program like Microsoft Excel.

Using the Utility's Graphing Tools

The graph provides many controls to customize it as desired. Because this utility is written in LabVIEW, the controls differ from those in the APx500 software.

All graphs and charts, except for the 3D graphs, automatically enable autoscaling, which means they adjust their horizontal and vertical scales to fit the data. By default, autoscaling is enabled for graphs and charts. Right-click the graph or chart and select AutoScale X or AutoScale Y from the context menu to turn autoscaling on or off. To change the upper and lower limits of a graph’s X and Y scale, double-click on the text at the limit of the axis, type the new limit in its place, and then press the Enter key. You should turn autoscaling off when setting the graph axis ranges manually. Otherwise, they will revert to autoscaled values the next time data is displayed on the graph.

You may control cursor appearance and add additional cursors by right clicking inside of the cursor box. If you add cursors, select Visible Items | Vertical Scrollbar so that you can access the entire list.

Figure 6. Graphing palette.

The graphing palette allows you to move the cursors, zoom, and pan. Each button lights in green when you enable it.

 

©2009 Audio Precision

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jb

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