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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.