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Characterization of a Laser Microphone Andrew Zeidell, James Vanderburg, Danny Garcia, Brad R. Conrad Department of Physics and Astronomy, Appalachian State University Boone, North Carolina 28608 Purpose Setup and Design This project developed an easy to use circuit, a wireless sound transmission system using a modulated laser beam, for non-physics major students to build. Circuit designs were chosen based on ease of construction as well as functionality. This project focused on building the laser microphone and characterizing its spectral response. The setup used was optimized for introductory laboratory use. The chosen design, as seen in Figure 2, allows for the students to vary input frequency by directly modulating the laser beam. This minimizes circuit complexity, leaving time for frequency response characterization. Modulated Beam 1000μF Capacitor Function Generator 4.5 V Battery Pack Signal Voltage, Arbitrary Scale (Vp-p) Figure 2: Finalized circuit design, with an illustration of the improved setup. A function generator can be replaced with any audio input. Figure 1: Illustration of a laser microphone. An unmodulated laser beam strikes a reflective surface, which modifies the beam intensity. A photovoltaic device observes the signal and amplifies for sound reproduction. • Response range (fig. 4) within human hearing range (fig. 5) •Hi-fidelity output with economical design •Minimal design complexity and simple set-up •Use of photovoltaic cells eliminates need for complex receiver circuit and maximizes fidelity. Photovoltaic Cell Theory A primary concern was simplicity of design. Ideally, a laser is shown onto a reflective surface, such as a sheet of glass, that is modulated by acoustic waves. The beam path is altered by the moving glass and the changes in light intensity are measured by a photodiode or photovoltaic device. Conclusion ƒ = 20 Hz Vp-p = 1.14V Figure 4: Signal intensity as a function of frequency for our circuit. Device provides high quality output up to 2 kHz before diminishing. ƒ = 200 Hz Vp-p = 780mV Figure 5: Human hearing range by level of perceived volume (Rossing et al., The Science of Sound) ƒ = 2 kHz Vp-p = 328mV ƒ = 20 kHz Vp-p = 49.6mV Figure 3: Oscilloscope receiver output voltages from 20 Hz to 20 kHz. Graphs display quality of sound reproduction. Acknowledgements: Appalachian State Department of Physics and Astronomy, Brad Johnson, Dr. Rahman Tashakorri, and the STEP program.