Measuring Voice Coil Temperature using Ultrasonic Pilot Tones

Measuring the voice coil temperature of speakers during audio playback is useful for optimizing thermal design and preventing overheating. The established DC method uses a constant current to measure resistance and determine temperature. Therefore, it suffers from noise induced by low frequency audio signals and creates an unwanted constant voice coil displacement. These problems are solved by the HF method, where the voice coil temperature is derived from a high frequency (HF) impedance measurement with an ultrasonic pilot tone. This study extends on the previous research by Gautama and Anazawa on the HF method, for example by showing that in addition to the voice coil temperature, the pole plate temperature can be measured. In this study, impedance measurements at different temperatures of an example microspeaker are used to calibrate the HF method. A comparison to the DC method with different test signals heating the speaker demonstrates that the HF method works well in this case. However, it is susceptible to errors from the skin effect in large diameter voice coil wire, changes in cabling, close metallic objects or drifting of the average voice coil position over time or with large amplitudes. The measured pole plate temperature rises with applied high frequency audio signals, which may be explained by induction heating. Overall, the HF method seems especially suited for applications where impedance measurement and non-linear excursion control are part of the design.