Control of Voicecoil transducers: Design and implementation of a Motional Feedback Loudspeaker Woofer (2013)

This master thesis describes the dynamical behaviour of a loudspeaker woofer and the modelling of it. By the use of the knowledge gained from the model, a feedback controller is implemented in order to enhance the woofer’s performance in terms of acoustic total harmonic distortion (THD) and low frequency bandwidth. The first objective of the research is to build a woofer set-up that suppresses the THD below 1% during operation. The second objective is to evaluate diaphragm break-up and compensate for this break-up by the use of feedback. The woofer is equipped with an accelerometer sensor. By the use of feedback, the motion of the voice-coil of the loudspeaker is controlled. This enhanced motion results into an acoustic enhancement. Since the motion of only a single point on the woofer diaphragm is measured and used for feedback, only the local distortion is reduced. Even when feedback is applied, the surround of the woofer is radiating acoustic distortion. Initially a woofer is used that has some very specific characteristics in terms of acoustic radiation. The contribution of distortion by the surround is large with the chosen woofer. Therefore, the increase in performance measured on the accelerometer is not identical to the increase in performance measured by the microphone. Where the accelerometer is mounted, the motion of the diaphragm is improved, but the acoustic distortion radiated by the surround is not reduced. After analysing the first woofer, the knowledge gained from the experiments is used to determine a better candidate for the experiments. The experiments are repeated on a second set-up. Two important observations have led to the final result. One observation is that the surround of the woofer is a large contributor of the acoustic radiation. Choosing a woofer that is very uniform in terms of the distortion profile across the diaphragm, is advised. In that situation, when the motion of the location where the sensor is mounted is improved, the same holds for the surround of the woofer. Secondly, it is observed that when using a piezoelectric sensor, the sensor output is not only determined by the acceleration. Stress leading to deformation of the sensor is measured too. When the contribution of measured deformation becomes dominant over the measured acceleration, the magnitude of the signal is that of the deformation instead of the acceleration. For low frequencies, this leads to a limitation in terms of potential loop gain. To some extend, both problems have been solved by the use of a different type of sensor mount and by a careful selection of the woofer. The second woofer used, has an acoustic distortion profile that is nearly identical across the membrane. This observation indicates that for the operating bandwidth of the woofer, the motion of the diaphragm is a close match to that of a rigid piston. Increasing the performance in terms of the motion of the centre, therefore leads to an increase in performance throughout the entire diaphragm. In order to solve the problem of the measured deformation of the sensor, a different type of sensor mount is designed. This sensor mount reduces the deformation of the sensor. This leads to a steeper roll-off slope in the sub-resonant frequency band of the woofer. This steeper slope makes it possible to design a controller, that leads to higher distortion suppression. The achieved reduction of harmonic distortion measured on the accelerometer sensor is up to 22 dB, a factor 12,5. The acoustic reduction of harmonic distortion measured with a microphone is up to 22 dB too. In practice, the frequency band of this high suppression is narrow. The frequency band in which the suppression is over 17 dB, a factor 7, is between 40 Hz and 150 Hz. When the woofer excursion becomes large, the THD without feedback can be over 12,5%. Suppressing the distortion by a factor 12,5 therefore does not lead to the target THD of under 1%. The diaphragm break-up is evaluated, but a controller that suppresses this motion is not implemented. A controller would only compensate for the break-up effect on the diaphragm location where the sensor is mounted. The actual break-up in the diaphragm would remain.