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Operations and Applications of a Laser Microphone
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Ed Fortin and David Cui
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University of the Fraser Valley, Department of Physics
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(Dated: April 28, 2010)
Abstract
A laser microphone was constructed using a red Helium-Neon laser and a photoresistor circuit as
a detector. It was tested using a glass window as the vibrating membrane and a speaker powered
by a function generator as a monotone sound source. The detected signal voltage waveform was
then analyzed and its frequency compared to the frequency of the sound source. We found that
our detected signal frequency was the same as our sound signal frequency within experimental
error, allowing us to conclude that we could detect and reproduce sound waves using a window as
a vibrating membrane.
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I.
INTRODUCTION
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One method of surveillance which was developed in the mid 1950’s enabled people to
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listen in on conversations at a distance through the use of an infrared laser. By bouncing
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the laser off a window and collecting the reflected signal with a detector, they were able
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to bug any room with a view. Sound waves travel through the air and interact with the
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window, causing it to vibrate. The laser’s angle of reflection changes slightly with this
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vibration, resulting in variations of intensity on the detector. By translating the variations
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of intensity into a change in voltage, one can reproduce the sounds that are being generated
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from the vibrating window pane.
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The goal of this experiment was to determine the accuracy of a laser microphone. We
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will compare the frequency of the signal detected by the apparatus to the frequency of a
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given sound signal. In the apparatus section the different parts of the apparatus will be
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explained. The theory section will outline the different equations which will be used to find
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the output frequencies. The procedure section will go through the steps of setting up the
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apparatus and measuring test frequencies. In the analysis section, the recorded data will be
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confirmed using error analysis and a graph comparing the sound signal to the detected signal
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will be produced. The results section will then display the confirmed fidelity by showing the
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output versus input graph. The discussion section will go over where errors or difficulties
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were encountered as well as some possible solutions. Finally, the conclusion will detail the
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ways this lab may be expanded and improved.
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II.
APPARATUS
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The apparatus for this lab was spilt up into three parts; the emitter, the membrane,
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and the collector: as seen in Figure. 1. The emitter part consisted of the laser, including
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mounting pieces that controlled the incidence angle, and the speaker powered by a frequency
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generator which provided the sound signals. The membrane consisted of the surface off of
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which the laser is reflected. The collector gathered the reflected signal, interpreted the
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change in intensity created by the window vibrations, and converted the intensity signal
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into a voltage. Using the collector’s electronics, the voltage change as seen on a oscilloscope
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could be interpreted as sound waves.
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FIG. 1: Laser Microphone Apparatus
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A.
Emitter
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The first part of the emitter apparatus consisted of the Helium-Neon 10mW red laser
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with inventory code 114400733, which was aimed at the window so it reflected back to the
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collector. The other part of the emitter was the REALISTIC 8 Ω speaker, which created
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the input signal to be collected by the laser’s vibrations. We controlled the frequency
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and amplitude of the input signal by connecting the B+K PRECISION 4017A 10MHz
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SWEEP/FUNCTION GENERATOR to the speaker.
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B.
Membrane
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We tested several reflective objects prior to using the window. Initially the bottom of a
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beaker was used; however due to its size it did not oscillate easily and the output signals
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were full of noise. A drum made from a plastic bag was used next. We observed that only
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frequencies around 200 Hz seemed to give any coherent output signal to the drum when
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using the PasPort light detector. This was probably the resonant frequency of the drum,
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which would explain why only one frequency worked. Higher frequencies were extremely
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difficult to detect due to the limit of 1000 Hz in the PasPort light detector sampling rate.
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Finally a window approximately 1.1 metres by 1.45 metres was selected. The window was
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large enough to vibrate easily when a speaker emitted sound waves towards it. The window
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was also selected because it held true to the concept behind this experiment: that this laser
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microphone was used as a surveillance device on exterior windows.
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C.
Collector
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The collector part of the apparatus firstly consisted of the photo resistor voltage divider,
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which converted the varying intensity of the laser into a voltage signal. A photo resistor is
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a device that changes resistance with respect to the intensity of light it is exposed to. To
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minimize amount of background light, a red filter plate was mounted to the end of a small
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box with the photo resistor at the other end of the box. Closing the box ensured that the
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only light reaching the photo resistor was the light from the red laser. The photo resistor
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was followed by a series of high pass filters. These were used to filter out background noise
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from the window. This background noise was likely noise from outside, ambient noises from
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inside the room, and was possibly due to the harmonic frequency of the glass. The voltage
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signal was then sent through an amplifier which boosted the signal and created a readable
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signal for the oscilloscope. The oscilloscope was used as the display for the output signal. A
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741 op amp was selected for the amplifier for its simplicity and ease of use.1 Using the scope,
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the output signal was visible as a waveform and was analyzed using the run/stop function
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to determine the frequency and magnitude of the output waveform. Fig. 2 shows a circuit
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diagram of the detector unit.
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III.
THEORY
A laser microphone is a device used to detect vibrations due to sound on a distant,
reflective object. Sound waves interact with the object, a glass window in our experiment,
causing it to oscillate. When the window vibrates, so too does the laser’s mark on the photo
resistor. The collector detects these vibrations as minute changes in intensity; these changes
are the sound waves from inside the room. The collector amplifies this voltage signal so the
small vibrations become an oscillating voltage signal. This output signal is interpreted as
sound waves which power a speaker so the eavesdroppers can listen to sound from inside
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FIG. 2: Circuit Diagram of the Detector Unit.
the room.3 The detected signals are found by measuring the period of the oscillations of
the window using the oscilloscope. The period of multiple wavelengths are measured and
converted into frequency by the Equation:
f requency =
n
Hz,
T
(1)
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where n is the number of wavelengths being measured, and T is the time to traverse those
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wavelengths.
The glass membrane can oscillate due to signals other than sound, such as ground vibrations and wind. These appear as relatively low frequency (<50 Hz) signals, but are much
stronger than the signals created by sound. In order to remove these background noises,
a high pass filter was included in the apparatus. Passive filters placed in series were used
in this experiment to quickly attenuate unwanted frequencies. A background frequency of
16 Hz was noticed during the developmental stages of the experiment, so high pass filters
were included that attenuated frequencies below approximately 33 Hz. We chose a cut off
frequency for the filter above our background noise since the passive filters attenuate frequencies according to a logarithmic scale.1 A high pass filter was constructed by having the
input run through a voltage divider consisting of a capacitor then a resistor as seen in Fig.
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3. The cut off frequency of the filter was determined using the equation:
f requencycutof f =
1
,
2πRC
(2)
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where R is the resistance of the resistor in ohms and C is the capacitance of the capacitor
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in farads.
FIG. 3: Schematic for a High Pass Filter
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IV.
PROCEDURE
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The apparatus was set up as described in the previous section. The laser was turned
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on, leveled, and trained on the window so the beam reflected into the collector’s housing.
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Once the laser was properly trained onto the photo resistor, the detector’s housing was
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closed to minimize the amount of external light that reached the photo resistor. The sound
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source was then set up approximately 0.05m from the window to simulate someone talking.
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The frequency generator was connected to the speaker and was set to a test frequency.
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The oscilloscope was then used to find the waveform detected from the laser. A range of
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frequencies were tested using the frequency generator and the output waveform’s periods
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were recorded in Table I to find the output frequency, and compare it to the original input
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signal.
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TABLE I: Frequencies (Hz)
Source Measured Quality Source Measured
Quality
120
120.48
clear
570
566.89
clear
150
150.15
clear
600
603.86
clear
180
179.21
clear
630
628.14
clear
210
210.08
clear
660
652.74
unstable and jagged
240
240.38
clear
690
692.52
clear
270
270.27
clear
720
718.39
clear
300
301.20
clear
750
748.50
jagged
330
322.58
clear
780
783.70
unstable
360
362.32
jagged
810
809.06
clear
390
383.14
jagged
840
841.75
unstable
420
420.17
clear
870
862.07
unstable
450
452.49
clear
900
886.07
unstable
480
478.47
clear
930
925.93
unstable
510
507.61
clear
960
961.54
unstable
540
534.76
jagged
990
988.14
unstable
1020
1028.81
unstable
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It should be noted that some of the output signals were classified as either jagged, or
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unstable. A jagged waveform is one that has a measurable period; however the overall
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waveform is not smooth. An unstable waveform is one in which the waveform is smooth,
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but not continuous. In other words an unstable waveform would be present for a time, and
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then it would became unrecognizable noise for a short amount of time. These unstable waves
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were measured with the oscilloscope’s run/stop toggle, capturing a snapshot of the waveform
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which could be analyzed. Another point of interest was that all of the signals became smooth
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and coherent once the output level was maximized on the frequency generator. If the output
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level was left unchanged, some of the signals did not appear.
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V.
ANALYSIS
To determine the fidelity of our output signal, we directly compared the frequency of the
output signal to the frequency of the input signal at a given input frequency. We compared
them directly by finding their ratio:
Ratio =
OutputFrequency(Hz)
,
InputFrequency(Hz)
(3)
where the output frequency is obtained from our detected signal, and the input frequency is
obtained from the sound signal. A frequency ratio of 1.0 would indicate that our detected
signal and the sound signal are identical. Ratios of output frequency to input frequency were
created for each data point. The error present in both input and output frequencies were
combined and shown as the error of the ratio. A linear approximation was then constructed:
Ratio = (a)
Out(Hz)
+ b,
In(Hz)
(4)
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where a = (−0.5 ± 5.0) × 10−6 and b = 0.998 ± 0.003. A linear approximation was chosen
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because the ratio was not expected to change with the signal, and was expected to be
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horizontal. The ratios at each point, the error bars, and the linear approximation can be
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seen in Fig. 4.
FIG. 4: Ratio of the frequency of the Input frequency to the Output frequency
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VI.
RESULTS
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The purpose of this experiment was to reproduce a sound signal by reflecting a laser off
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of a vibrating membrane and measuring the change in intensity of the reflected beam on a
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detector. We found that we could reproduce an output signal of the same frequency as the
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input signal. This was confirmed since the error bars shown in Fig. 4 contain the ratio value
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of 1.0. In the linear approximation, the coefficient a in Equation 4 was extremely small,
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though it had a large error relative to its value, indicating that the ratio was essentially
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constant over the range of our experiment. As the coefficient b of Equation 4 included the
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ratio value of 1.0 within its error, the sound signal frequency and detected signal frequency
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were found to be the same over the range of our experiment.
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VII.
DISCUSSION
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Originally the collector circuit made use of a PasPort high sensitivity light sensor. The
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light sensor was connected by USB to a computer and used software which plots the intensity
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of light versus time. The sampling rate of the PasPort was unfortunately limited to 1000
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frames per second. This sample rate was sufficient for detecting footfalls or other shockwaves,
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but it was far too slow for measuring sound waves. The solution was to use a photo resistor in
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a voltage divider circuit to maximize the sample rate. This also required further electronics
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to purify and amplify the detected signal to make it large enough to power a speaker.
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In order to collect the highest possible output waveform amplitude, the detected signal
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was sent through an amplifier. Other resistor combinations on the amplifier with lower and
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higher gain ratios were attempted; however beyond a gain of 100× there was not a drastic
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change in the output signal size. This might have been due to the pre-loading effect a series
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of high-pass filters has. Pre-loading occurs when there is unwanted feedback in a series of
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devices. This feedback causes a cascade of altered signals throughout the circuit, altering
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the output. Since a high-pass filter works by a relationship between its input voltage and
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its output voltage, only the first filter would have worked as expected. The remainder of
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the series would have received differing voltages; which would have drastically reduced our
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signal strength. Another possibility is the signal collected by the photo resistor was too
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small to amplify to the desired magnitudes.
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Constant background noise which formed a 16Hz wave was minimized by the series of
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high-pass filters; persistent background noise remained and was amplified along with the
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desired signal. One solutions was to raise the selected cut off frequency to approximately
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100Hz. The high-pass filter causes a drop off of signal amplitude at a logarithmic rate
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below the cut off frequency.1 Alternatively, we found that raising the cut off frequency also
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decreases the amplitude of the output signal. Additional amplification could allow us to
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power a speaker using this low amplitude signal.
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VIII.
CONCLUSION
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In this experiment, we constructed and tested a laser microphone. From testing a range
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of input frequencies, we found that it is possible to detect and reproduce, with great fidelity,
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a sound signal on a glass pane. The range of frequencies we used was within the range
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of normal speech frequencies for humans; however it did not include the extreme lower or
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upper limits of human hearing which ranges from 20 Hz to 20 kHz.2
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We began to test polytonal signals with the intent to progress to recognizable speech but
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were hampered by time constraints. What little was observed using two sound input signals
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was promising, as the combining of sound waves in our output coincided with expected
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wave functions. It may be difficult to determine what waves are present in a polytonal
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signal using an oscilloscope, so a new measurement method may be necessary. A qualitative
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test may be useful, simply amplifying the signal and sending it through another speaker to
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be heard and compared with the original sound signal. Perhaps there is a program available
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to dissassemble such waves forms.
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Further amplification of the signal, or increased sensitivity of the detector, would be
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necessary in order to detect normal speech. The input in this experiment was at maximum
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volume so lower volume situations would also have to be tested, possibly revealing additional
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unwanted background noise.
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1
A. S. Sedra, K. C Smith, Microelectronic Circuits (Oxford University Press, 2004), pp. 69,10841099.
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2
Sensitivity
of
the
Human
Ear,
http://www.hyperphysics.phy-
astr.gsu.edu/4base/sound/enrseus.html , date accessed, April 7, 2010.
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157
R.Nave,
3
“Laser Microphone,” http://williamson-labs.com/laser-mid.htm, date accessed, March 24, 2010.
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