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.
