Loudspeakers are electroacoustic transducers that convert electrical signals into acoustic ones. They are commonly found in our daily lives. One of the most frequently used types, known as an electrodynamic loudspeaker, functions by the movement of a cone membrane (diaphragm). When AC electrical signals are sent to the voice coil, the change of the electromagnetic field causes the diaphragm and the surrounding air to vibrate, thus creating sound. In this blog post, we will focus on dynamic loudspeakers and further restrict our scope to passive ones, where the power is supplied by an external audio amplifier.
Impedance measurements have many advantages over conventional electro-acoustic measurements: it is faster and easier to set up, and requires no anechoic room for the testing. These great advantages make a comphrehensive inspection possible in the production line, ensuring the quality of every single product. If you are interested in seeing how impedance measurements can help, please read on.
Impedance Measurement of a Commercial Loudspeaker
We begin our measurement with commercial loudspeaker Bowers & Wilkins 606. Each satellite of the speaker is two-way, containing a woofer and a tweeter. Connecting the speaker satellite to the MFIA Impedance Analyzer is therefore easy. We measure and then compare the 2-terminal impedance of the woofer, tweeter, or both of them together (in parallel). Here, we use a very weak test signal at 1 mV, in order to avoid any potential damages.
Figure 1 shows the impedance measured between 100 Hz to 20 kHz in the LabOne® Sweeper module. The impedance appears a bit noisy at low frequencies, which comes from the low current (with only 1 mV excitation). At a quick glimpse, none of the three traces look similar to the impedance of a loudspeaker. For instance, the woofer (in red) shows a strong inductive behavior at high frequencies, and the tweeter (in blue) highly capacitive at low frequencies. This indicates that the (audio) crossover is already built-in. To know more about it, we can use the equivalent circuit model in the LabOne software, and find the capacitor used in the crossover is likely in first-order and with ~5 uF in capacitance. For more detailed circuit analysis, interested users can export the data from LabOne and process it in third-party software.
Regarding the 8 Ohm nominal impedance, we find this happens at around 700 Hz. At the most common definition of 1 kHz, the impedance goes up to 11.5 Ohm. This change may arise from the additional impedance of the crossover. While the manufacturer has its freedom to define the frequency of the measurement, it is also possible that other non-electrical parameters play a role in the impedance.
Figure 1: Impedance measurement of a commercial two-way speaker. The red trace shows the woofer, the blue one shows the tweeter, and the green one shows the woofer and tweeter in parallel. (Click to zoom)
It is also interesting to compare the current passing through at different frequencies (note that this frequency response analysis may be different from the SPL test), as shown in Figure 2. The inductor and the capacitor in the crossover serve exactly as a low-pass and a high-pass filter, respectively. By combining the two filters we can have a maximal attenuation of ~2 dB at the trough, agreeing well with the specs.
Figure 2: Current measurement of a commercial two-way loudspeaker. The red trace shows the woofer, the blue one shows the tweeter, and the green one shows the woofer and tweeter in parallel. (Click to zoom)
A Closer Look
In the previous example, the impedance measurement is heavily limited by the crossover. In order to measure the intrinsic impedance from the loudspeaker, we turn to a homemade one built with a full-range speaker, Tang Band W4-655SA. Here the test signal is set to 300 mV, so that we will not see a strong noise in the measurement.
We compare the results with (see Figure 3a) and without a 3.5 L closed enclosure (in Figure 3b). The nominal impedances in the two cases are similar and close to 8 Ohm. However, we can see the two traces look quite different. In another word, the enclosure plays a significant role in the impedance, even if it is not electrically connected. The impedance decreases from 65 Ohm to 40 Ohm, by ~40%. And the resonance frequency also 'blue' shifts slightly, with an additional minor peak also popping up. The main reason for this is the change in air volume and the total quality factor (Q).
These results suggest that the testing conditions of impedance measurements of loudspeakers need to be carefully defined. To distinguish the impedance in these situations, using an accurate and flexible impedance analyzer such as the MFIA can certainly be beneficial.
Fig. 3 Impedance measurement of a homemade loudspeaker with (a) and without (b) enclosure. The blue trace shows the amplitude, and the orange one shows the phase. (Click to zoom)
In this blog post, we present the impedance measurements of a commercial two-way loudspeaker and a homemade full-range speaker. By comparison, we see that impedance measurement is a useful technique to characterize loudspeakers. Given the complexity of the system, a nominal impedance value alone may not be sufficient. A comprehensive study with specified testing conditions would be more helpful.
If you are interested in this type of application, please do not hesitate to get in touch with us.
Acknowledgments: The author would like to sincerely thank his colleagues Wei Yu and Benjamin Schmid for taking impedance measurements and for the technical discussion.