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We demonstrate a highly sensitive and stable fiber-optic Fabry-Perot cantilever microphone based on fast demodulated white-light interferometry. The cavity length of the low-finesse Fabry-Perot interferometry is absolutely measured by realizing a high-speed demodulation method utilizing a full spectrum, with the advantages of both high resolution and large dynamic range. An acoustic test demonstrates high sensitivities and linear responsivities at frequencies below 2 kHz. The pressure responsivity and the noise-limited minimum detectable acoustic pressure level are measured to be 211.2 nm/Pa and 5  μPa/Hz, respectively, at the frequency of 1 kHz. Comparative experimental results show that the signal-to-noise ratio is over 10 times higher than a reference condenser microphone.

Microstructure and grain boundary segregation of the direct-chill (DC) cast Al-Zn-Mg-Cu (7A60) alloy, with and without low-frequency electromagnetic field, were investigated. The surface quality, hot-tearing tendency, and tensile properties of the ingots manufactured by DC and low-frequency electromagnetic field casting (LFEC) were compared. The results show that LFEC significantly improves the surface quality and reduces the hot-tearing tendency of DC ingots. It is possible to generate a fine, uniform, and equiaxed microstructure with LFEC. Under optimum conditions, the average grain size varies from 30 m near the surface to 45 m at the center of the LFEC ingots. Decreasing electromagnetic frequency or increasing intensity significantly refines microstructure, suppresses grain boundary segregation, and increases as-cast fracture strength and elongation. In the range of frequency and ampere turns employed in the experiments, the optimum frequency is found to be 15 to 25 Hz and the number of ampere turns to be 18,000 to 20,000 A t .