Development of a measurement method for the analysis of surface vibrations
Bachelor Thesis of Peckert, Fabian
Mikrospeakers, which are normally designed as dynamic loudspeakers, are very compact and can be found in portable devices. Therefore, many approximations of conventional loudspeakers are only partially valid for microspeakers. In particular the assumption of a loudspeaker membrane as an in-phase vibrating piston radiator does not apply for the whole frequency domain, since for certain frequencies vibrating-modes propagate in form of bending waves on the membrane. In order to investigate the position-dependent surface velocity of a loudspeaker membrane, this bachelor thesis presents a method to measure the membrane. Furthermore, with the help of the developed method the vibration pattern of the membrane can be visualized and analyzed. The scanning of the membrane is done by a laser-vibrometer that measures the velocity of the membrane. The measurement position is shifted by with two step motors which are attached to a measurement table. The measurements take place all over the membrane with an adjustable resolution of the measurement grid. A broad-band excitation signal which covers the whole human perceptible frequency range is used. This study is limited to the measurements of micro-loudspeakers, but can also be extended to analyze other vibrating surfaces. The analysis can be done for arbitrary frequencies by visualizing the different measurements with 2-D or 3-D graphic tools. Moreover transfer functions or impulse responses for individual measurelnent points on the Illmnbrane can be illustrated in order to study the formation of modes in the surface. Additionally a method for the creation of a video was made that shows the vibration characteristics of the membrane for different frequencies. Hereby the propagation of vibrating modes and the phase differences between input and output signal can be studied and analyzed which can help to modify micro-loudspeakers and its radiation characteristics. With regard to the future, the aim is to use the developed measurement method and the visualization tools for teaching purposes. Hereby the methods shall be integrated into the 'Akustische Praktikum'-lab course and further research on microspeakers at the Institute of Technical Acoustics.