C6.4 - Towards a Magnetostrictive Mirco-Loudspeaker

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
C6 - Miniature Audio Transducers
T. Albach, P. Horn, J. Ilg, S. Friedrich, A. Sutor, R. Lerch - Friedrich-Alexander Universität Erlangen-Nürnberg (Germany)
477 - 482


Until now, there does not exist any commercial integrated MEMS-device for sound generation in the audio range. However, some effort has been made using electromagnetic, electrodynamic or piezoelectric actuation principles.
At the Lehrstuhl für Sensorik at University Erlangen-Nürnberg we have been developing a micro-actuator (MEMS) based on Joule magnetostriction taking advantage of the high energy density of this actuation principle. In this contribution we demonstrate its performance as a micro-loudspeaker. The active area has a diameter of up to 3mm and it generates a sound pressure level (SPL) of up to 101dB at 400Hz in a standard 2ccm ear volume coupler.
The presented micro-actuator consists of a comb structure of long and narrow monomorph bending cantilevers on a silicon substrate. Each cantilever consists of one active, magnetostrictive layer (Vanadium Permendur: Fe49Co49V2) and at least one passive layer (SU8-3000). The layers are fabricated using HF-magnetron sputtering and spin coating process. The cantilevers are freed from the substrate in the last processing step using anisotropic silicon etching.
Bending of the cantilevers is achieved by applying an external magnetic field. An additional DC magnetic field shifts the working point to a nearly linear region of the characteristic curve. We connect the 2ccm ear volume coupler to the micro-actuator via a small rubber tube, 5mm in length and 1mm in diameter. The microphone is attached directly to the coupler volume. SPL measurements for microactuators of different sizes are presented.
We also introduce a calculation model for the coupled mechanical-acoustical system. The mechanical properties of the micro-actuator are first calculated by means of a finite-element-simulation. This result is incorporated into a lumped element network analysis, which realizes the coupling between the mechanical and the acoustical system. The presented sound pressure level measurements validate our calculation results.
Variations of different design parameters of the micro-actuators such as layer thickness ratio are performed. The results lead to an optimized setup in order to maximize the sound pressure level.