Loudspeaker impedance measurements are made for many reasons. In the R&D lab, these range from the simple task of identifying a speaker’s resonant frequency to more complex functions such as calculating the speaker’s Thiele-Small parameters. On the production line, impedance measurement is a key quality control parameter that verifies that the speaker’s motor properties are correct, that the magnet is charged correctly, the voice coil number of turns is correct and that the moving mass (cone and voice coil) is within specification.
There are two basic methods of making impedance measurements: basic single channel measurements, and more complex, but more accurate, dual channel methods.
Single channel measurement is widely used on production lines as it means that both impedance and audio measurements can be measured simultaneously using widely available and low cost 2-channel interfaces. Dual channel measurement requires 2 channels just for measuring impedance, but offers higher accuracy as voltage is measured across the speaker and reference resistor at the same time. A full comparison of these methods can be found here.
In this article, I provide practical tips for optimizing single-channel measurements for a accuracy and repeatability.
Configure test setup to minimize noise
To reduce the noise in your overall measurement circuit, use a low output impedance amplifier designed for audio measurements such as Listen’s AmpConnect or SCAmp. Minimizing cable length will also reduce noise.
Select Sense Resistor for Optimum Resolution
To obtain an optimal signal to noise ratio, the value of the reference (sense) resistor should be 1/10 to 1/100 the nominal impedance of your DUT. For example, if your DUT’s impedance is in the range 1-10 Ohms then a 0.1 Ohm reference resistor is ideal.
The S/N ratio degrades when a resistor outside the recommended range is used because the measurement circuit itself acts as a voltage divider. If the reference resistor value is too low then the measured signal is so low (microvolt range) that the overall system noise (audio interface self noise, cable EM pickup noise, etc.) disrupts the resolution of the curve, causing the impedance measurement to be obscured.
This is demonstrated by measuring a pair of Sennheiser HD580 headphones with an impedance of approx. 300 Ohms, with a Listen AmpConnect and SoundCheck. Measurements were made using the two default resistors in AmpConnect, 0.1 Ohm (Z-Low) and 1 Ohm (Z-High).
The diagram below clearly demonstrates that the measurement using Z-High yields a much cleaner signal.
Keep Stimulus Level Consistent for Measurement Repeatability
Impedance curves can vary based on stimulus level, so it is important that this is kept consistent across your tests for measurement repeatability and comparison.
The graph below shows how the resonant frequency of a 4” driver varies when tested at 4 different voltage levels, 500mV, 1V, 2V and 3V. You can clearly see how the resonant frequency of the curves drifts by as much as ~20Hz depending on the stimulus level. It is therefore important to ensure consistency in stimulus parameters if you intend to compare data from different locations/products/production runs.