Characterizing Rayleigh damping parameters of post-cured, hybrid, methacrylic materials used in stereolithography systems.
Thomas Howard Burns, Andrew Beeson · The Journal of the Acoustical Society of America · 2009
Stereolithographic systems of methacrylic are commonly used in the large-scale production of custom earmolds for hearing aids. In hearing aids, feedback occurs when energy from the receiver (earphone) is sensed by the microphone; the feedback manifests itself as an annoying whistle. One source of feedback is the mechanical vibration transmitted by the receiver to the microphone through the structural housing or earmold. Mechanical feedback can be reduced by appropriately designing the earmold housing to have proper isolation features and damping so as to minimize mechanical transmissibility. Toward this end, a procedure is described that utilizes both experimental measurements and finite element analyses to characterize an earmold material’s stiffness and damping in the frequency range of 500 Hz–6 kHz. The experimental set-up consists of an electromagnetic shaker, laser vibrometer, dual-channel analyzer, and multiple cantilever (fixed-free) sample beams fabricated from post-cured, hybrid, methacrylic polymers. The dimensions of the samples are chosen so that the resonance frequencies are sufficiently distributed over the aforementioned frequency range. Bending resonances are measured and compared to a finite element model via a half-power bandwidth technique; parameters for Young’s modulus, the loss factor, and Rayleigh damping are amended in the numerical model to match the empirical results.