Carbon microphone
Roman Doronin
Vitaliy Matiunin
Aleksandr Severinov
Vladislav Tumanov
Maksim Tumakov
Russia
IYPT
The problem
For many years, a design of microphone
has involved the use of carbon granules.
Varying pressure on the granules
produced by incident sound waves
produces an electrical output signal.
Investigate the components of such a
device and determine its characteristics.
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3
The principle
of a carbon
microphone
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Design of a carbon microphone
Front plate =
diaphragm
Carbon
granules
Back plate
Battery
Output voltage
Transformer
Edison’s microphones
5
Concept of a “loose contact”
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The electrical resistance of a carbon powder
changes under strain due to:
z
z
a change in the number of microscopic hills which
are in contact with each other;
a change in the area of their junctions.
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7
Pressure
experiment
Carbon microphone MK-16
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Scheme of experimental setup
Buttery
and
ammeter
Pressure
recorder
Microphone
Pressure
sensor
Current (A)
Pressure & current vs. time
Pressure (kPa)
Time (s)
Time (s)
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Frequency
response
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Ratio of output/input intensities
Frequency response graph
Frequency
Measuring the frequency response
z
13
There are some ways to measure the
frequency response of a microphone.
z
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The first way is to sweep a constant-amplitude
pure tone through the operating range.
The second way is to apply a signal with a
constant power spectrum density and to observe
the spectral response.
White noise
White noise is a random signal with
a flat power spectrum density.
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Experimental procedure
Spectrum analyzer
White noise
generator
Microphone
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MK-16 frequency response
+10 dB
+5 dB
–0 dB
–5 dB
–10 dB
100 Hz
1000 Hz
10000 Hz
Frequency response of audio path
Sound card
Loudspeaker
Microphone
Sound card
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Test procedure
Sound card
Loudspeakers
Microphone 1
Microphone 2
Sound card
Electret microphone MIC-01
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+10 dB
+5 dB
0 dB
–5 dB
–10 dB
100 Hz
1000 Hz
10000 Hz
20
Hand-made
construction
Design of our microphone
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Frequency response
+10 dB
+5 dB
0 dB
–5 dB
–10 dB
100 Hz
1000 Hz
10000 Hz
23
High-frequency
response
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Forced oscillations
Equation of motion
mx = F0e
iω t
Steady oscillations
x(t ) = x0e
F0
x0 (ω) = −
2
mω
Amplitude decreases
as the square of frequency
iω t
Resistance — current — power
The variable component of the resistance
is proportional to the amplitude of oscillations:
a iω t
R = R0 + 2 e
ω
The variable component of the current
is proportional to the amplitude of oscillations:
⎛
U
a iω t ⎞
I = ≈ I 0 ⎜1 −
e ⎟
2
R
⎠
⎝ R0ω
The power of oscillated signal
is proportional to the square of the amplitude:
a2
N∼ 4
ω
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High-frequency response
0 dB
–10 dB
–20 dB
104 times
–30 dB
–40 dB
10 times
100 Hz
1000 Hz
10000 Hz
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Resonant
frequencies
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Frequency response
+10 dB
+5 dB
0 dB
–5 dB
–10 dB
100 Hz
1000 Hz
10000 Hz
Oscillations of the diaphragm
Video 1000 fps
oscillation frequency = 95 Hz
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Natural tones of the box
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10 cm
z
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Sound wave length
4 · 0,1 m = 0,4 m
Oscillation frequency
340 m/s : 0,4 m ≈ 850 Hz
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Frequency response
800 Гц =
air oscillations
in the enclosure
?
+10 dB
+5 dB
0 dB
–5 dB
–10 dB
0 Hz
1000 Hz
2000 Hz
32
Filling the box with foam rubber
Decreasing
of the resonance
?
+10 dB
+5 dB
0 dB
–5 dB
–10 dB
0 Hz
1000 Hz
2000 Hz
33
Quality
of carbon
34
Coking anthracite
+10 dB
+5 dB
0 dB
–5 dB
–10 dB
100 Hz
1000 Hz
10000 Hz
35
Pounded charcoal
+10 dB
+5 dB
0 dB
–5 dB
–10 dB
100 Hz
1000 Hz
10000 Hz
36
Microphone
with charcoal
tablets
Design of the microphone
Charcoal
tablets,
R0 ≈ 200 Om
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The surface of a charcoal tablet
100 μm
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Frequency response
+10 dB
+5 dB
0 dB
–5 dB
–10 dB
100 Hz
1000 Hz
10000 Hz
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Response
linearity
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Scheme of experimental setup
Voltage
Loudspeaker
Sound level
meter
Microphone
Current
Resistance vs. time
Nonlinearity
R (Om)
145
85
Frequency = 1000 Hz, volume of sound = 113 dB
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Excitation of high harmonics
1000 Hz
1000 Hz
2000 Hz
3000 Hz
4000 Hz
…
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Response to 1000 Hz
24 dB
1000 Hz
2000 Hz
26 dB
3000 Hz
33 dB
4000 Hz
31 dB
5000 Hz
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Summary
Conclusions
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The carbon microphone operates on the
principle of varying the resistance of loosely
packed carbon granules as they change their
contact area under the varying pressure of
sound waves.
To study the frequency response of an
acoustic path a white noise can be used.
Response of a carbon microphone decreases
sharply at frequencies > 3000 Hz. This
decreasing is explained by the inertia of the
mechanical parts of the system.
References
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z
47
Goucher F. S. (1934) “The carbon
microphone: An account of some researches
bearing on its action”. J. Franklin Inst. 217,
407–442
Calvert J. B. (2003) Microphones.
48
Thank you for
your attention!