Investigation of Thermal and Nonlinear Distortions in Small Loudspeakers
Master Thesis of Mecking, Jens
This thesis deals with the characterization and modeling of small dynamic loudspeakers which are relevant due to their use in mobile devices (smart phones, tablet computers, etc.). These so-called microspeakers show considerable deviations from the idealized model of a linear time-invariant system which allows its electrical, mechanical and acoustical characterization by means of a time- independent transfer function. Although the reasons for these deviations are well-known for the case of large dynamic loudspeakers the transferability of the results to the case of microspeakers is questionable due to the altered mechanical composition concerning structure, material and size. An exact description of the occuring distortions would eventually allow their compensation and thus lead to an improved playback quality. During operation the temperature of the system rises which leads to changes in the electrical and mechanical properties and therefore also in the linear transfer properties of the transducer. An uncontrolled temperature increase can cause mechanical damage and eventually even the destruction of the device. An accurate description of the speaker’s thermal behavior is therefore necessary to compensate these undesired effects. In order to carry out thermal investigations on the system, a precise knowledge of the device temperature is essential. Therefore, a method to measure the temperature via the DC resistance of the voice coil was validated regarding accuracy and reproducibility. This method was subsequently used to characterize a loudspeaker in the framework of thermal models which predict the temperature as a function of the electical input power. Furthermore, the temperature dependence of the linear Thiele-Small parameters was investigated. In addition to thermal effects, microspeakers show nonlinear distortions in their transfer characteristic if operated in the large-signal domain. The Harmonic-Balance method was used in order to characterize the loudspeaker in the framework of an extended nonlinear model. Therefore, generalized transfer functions for higher orders of the input signal were derived and fitted to measurement data obtained in the nonlinear regime. A special focus was laid on the integration of the linear loudspeaker parameters into the nonlinear model.