AN-843, Measuring a Loudspeaker Impedance Profile Using the AD5933 Application Note (Rev. A)

Rev. A | Page 1 of 12 INTRODUCTION This application note describes the circuit architecture and details required to measure the impedance profile of a commercial loudspeaker using the AD5933 impedance-to-digital converter. By evaluating the acoustic properties of loudspeakers through an impedance measurement from 1960 to 1970, two Australian pioneers, N. Thiele and R. Small, defined the Thiele-Small parameters. Thiele and Small analyzed the electro-mechanical behavior of a speaker voice coil, magnet, and cone interacting with the cone suspension and the air in and outside sealed enclosures. To this day, these findings continue to be regarded by manufacturers and hobbyists as a standard for designing high fidelity speaker cabinets and crossover networks1, as well as for testing the final driver networks. Measuring the impedance of a commercial loudspeaker typically involves using various tools ranging from simple lab equipment (for example, signal generators, oscilloscopes, and digital voltmeters) to PC sound cards and expensive audio network analyzers. A fundamental problem exists: the impedance test equipment remains separate from the audio system driving the loudspeaker.