Thermophotovoltaic power conversion utilizes thermal radiation from a local heat source to generate electricity in a photovoltaic cell. It was shown in recent years that the addition of a highly reflective rear mirror to a solar cell maximizes the extraction of luminescence. This, in turn, boosts the voltage, enabling the creation of record-breaking solar efficiency. Now we report that the rear mirror can be used to create thermophotovoltaic systems with unprecedented high thermophotovoltaic efficiency. This mirror reflects low-energy infrared photons back into the heat source, recovering their energy. Therefore, the rear mirror serves a dual function; boosting the voltage and reusing infrared thermal photons. This allows the possibility of a practical >50% efficient thermophotovoltaic system. Based on this reflective rear mirror concept, we report a thermophotovoltaic efficiency of 29.1 ± 0.4% at an emitter temperature of 1,207 °C.

Very low power electromagnetic (EM) wave sensors are being used to measure speech articulator motions as speech is produced. Glottal tissue oscillations, jaw, tongue, soft palate, and other organs have been measured. Previously, microwave imaging (e.g., using radar sensors) appears not to have been considered for such monitoring. Glottal tissue movements detected by radar sensors correlate well with those obtained by established laboratory techniques, and have used to estimate a voiced excitation function for speech processing applications. The noninvasive access, coupled with the small size, low power, and high resolution of these new sensors, permit promising research and development applications in speech production, communication disorders, speech recognition and related topics.

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Recent experiments using a portable, extremely low-power electromagnetic motion sensor to detect the motion of the posterior tracheal wall during speech production will be presented. The motion of the wall may be related to the driving subglottal pressure through a lumped element circuit model, leading to an approximation to the voiced excitation function of the human vocal tract. Using the excitation and the recorded spoken audio, a stable and accurate transfer function of the vocal tract may be calculated every few glottal cycles in near real-time. The excitation function may be used to calculate very accurate pitch information at low cost, and the transfer functions may be employed as an additional feature vector to enhance the performance of a new class of speech recognizers and synthesizers. [Work supported by NSF and DOE.]

Low Power EM radar-like sensors have made it possible to measure properties of the human speech production system in real-time, without acoustic interference. This greatly enhances the quality and quantify of information for many speech related applications. See Holzrichter, Bumett, Ng, and Lea, J. Acoustic. Soc. Am. 103 ( I ) 622 (1998). By using combined Glottal-EMSensor-and Acoustic-signals, segments of voiced, unvoiced, and no-speech can be reliably defined. Real-time de-noising filters can be constructed to remove noise from the users corresponding speech signal.