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13
9.
Carbon Microphone
Marco Bodnár
13
Task
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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13
Basic scheme
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13
Basic scheme and principle
Resistance
decreases
Current is dynamic
dynamic
Voltage drop across resistor is
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Factory assembled microphone
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5
13
Carbon powder
Chamber
Cover
Elastic
membrane
Covering
membrane
Microphone base
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13
Voltage is needed
MICROPHONE IS PASSIVE
- WITHOUT VOLTAGE APPLIED – NO VOLTAGE CHANGE
CAN BE MEASURED
Dynamic resistance
Dynamic voltage, current
Measureable
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13
Membrane
 Radius
– change in frequency
characteristics
 Weight
– as light as possible
 Elasticity
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13
Why carbon powder?
 Bad conductor
(in comparison with metals)
- sensitivity of microphone
- ideally no added substances
 Stable
- does not change structure with
temperature or other conditions*
Very high sensitivity (of its resistivity)
to pressure change
*to a certain extent
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13
Covers

Protect inner parts of
microphone (not necessary
for correct functioning)
Back cover – protects
from mechanical damage
Front foil – protects
from dirt
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13
Hole in diaphragm

Hole in diaphragm used for
pressure equalisation
With hole – equal pressure inside the
microphone and outside– diaphragm
remains same
Without hole – overpressure inside the
microphone – diaphragm bulks out
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Now that we know how it works…
…we will try to make our own.
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Flexible membrane
Voltage
source
Chamber
Switch
Resistor
Conducting pistons
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Voltage
source
Resistor
Chamber
Membrane
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13
Home made record
Home made
R=5000Ω
Home made
Further measurements with factory assembled microphone only.
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13
Apparatus
Sound
production
Loudness
meter
Sound
analysis
16
Frequency response at 94 dB
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0
0
2000
4000
6000
8000
10000
-10
Relative Loudness [dB]
-20
-30
-40
-50
-60
-70
-80
-90
-100
Does the hole has an
effect on the frequency
response?
Resonant frequencies
of mechanical parts
Frequency [Hz]
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12000
Frequency response at 94 dB - hole effect
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0
0
2000
4000
6000
8000
10000
12000
-10
Relative Loudness[dB]
-20
-30
-40
-50
Without hole
-60
-70
-80
-90
-100
Frequency [Hz]
18
Frequency response at 94 dB - hole effect
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0
0
2000
4000
6000
8000
10000
12000
-10
Relative Loudness[dB]
-20
-30
-40
-50
Radius 1…
-60
-70
-80
-90
-100
Frequency [Hz]
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Frequency response at 94 dB - hole effect
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0
0
2000
4000
6000
8000
10000
12000
-10
Relative Loudness [dB]
-20
-30
-40
-50
Radius 2 mm
-60
-70
-80
-90
-100
Frequency [Hz]
20
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Frequency response at 94 dB - hole effec
0
0
2000
4000
6000
8000
10000
12000
-10
Relative Loudness [dB]
-20
-30
-40
There is no hole effect
Without hole
Original hole
Radius 1 mm
Radius 2 mm
-50
-60
-70
-80
-90
-100
Frequency [Hz]
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Frequency response at 94 dB - hole effec
0
0
2000
4000
6000
8000
10000
12000
-10
Relative Loudness [dB]
-20
-30
-40
Without hole
Original hole
Radius 1 mm
Radius 2 mm
-50
-60
-70
-80
-90
-100
Low frequencies (speech)
- Low sensitivity
- Low quality of recording?
Frequency [Hz]
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Hi, how are you…? Speech record
Factory made microphone
Actually, not that bad !
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Frequency response at 94 dB - hole effec
0
0
2000
4000
6000
8000
10000
12000
-10
Relatively flat area –
high quality of
recording?
Relative Loudness [dB]
-20
-30
-40
Without hole
Original hole
Radius 1 mm
Radius 2 mm
-50
-60
-70
-80
-90
-100
Let’s record these frequencies - song
Frequency [Hz]
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Song record
Factory assembled
R=100Ω
Factory made
Song has significantly worse quality
How is it possible?
Wide range of frequencies
possibly irregular input from microphone – different amplitude
response
Let’s investigate amplitude dependence25 of
Static amplitude dependence
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Turning
screw
increases
pressure
Conductivity vs. pressure measurement
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Static amplitude dependence
0.035
0.03
Conductivity [S]
0.025
Response is non-linear
0.02
Let’s look at the amplitude
characteristics
0.015
0.01
0.005
0
0
1
2
3
4
5
6
Turns
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7
Amplitude
response
characteristics
-60
80
85
90
95
100
105
Relative Loudness [dB]
-65
The slope is different than 1 for log-log
-70
Slope
graph
1
-75
– non-linear
characteristics
y = 1,991x
- 269,47
-80
-85
-90
-95
-100
-105
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100Hz
Loudness [dB]
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Amplitude response characteristics
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-50
70
75
80
85
90
95
100
Relative Loudness [dB]
-60
y = 1,4737x -
Slope 1
-70
-80
-90
-100
220Hz
-110
Loudness [dB]
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105
Amplitude response characteristics
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0
30
40
50
60
70
80
90
100
110
Relative Loiudness [dB]
-20
-40
Slope 1
y = 1,375x 165,16
-60
-80
-100
440Hz
-120
Loudness [dB]
30
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Amplitude response characteristics
-30
50
60
70
80
90
100
110
Relative Loiudness [dB]
-40
-50
Slope 1
-60
-70
y = 1,7228x - 208,05
-80
-90
-100
-110
1000Hz
Loudness [dB]
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Amplitude response characteristics
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0
30
40
50
60
70
80
90
100
-10
Relative Loiudness [dB]
Slope 1
-20
-30
-40
y = 0,6554x 83,392
-50
-60
3200Hz
-70
Loudness [dB]
32
110
Comparison for different
0
frequencies
30
50
70
Relative Loudness [dB]
-20
13
90
94 dB – frequency
110
response
measurement
Each frequency
-40
- Different sensitivity
(relative loudness)
-60
- Different slope
(change in relative loudness)
-80
-100
-120
100Hz
220Hz
440Hz
1000Hz
3200Hz
Not very optimistic
- For ideal microphone
should be one curve
Loudness [dB]
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Why is sound distorted?
• Speech – narrow range of frequencies
- sound card will adapt its
sensitivity
• Song – wide frequency span – great
variation
in relative loudness –
sound card
unable to adapt its
sensitivity
Microphone
can be used only to narrow range
of frequencies (speech) – due to its non34
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Directional
pattern
L=94dB
f=3200Hz
10°
Relative axes
0 = -60 dB
(relative
loudness)
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Thank
you for your attention
Conclusion




Explained the principle
Investigated components of microphone
Our own microphone 
Characteristics of microphone
 Frequency response
 Investigated the ‘hole’ effect
 Amplitude response
- Narrow range
 Direction
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APPENDICES
Thanks to Martin Ferianc for technical support
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Directional – relative magnitude
Relative magnitude [dB]
25
20
15
10
5
0
0
50
100
150
200
250
Angle [°]
300
350
38
400
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Carbon Powder
Graphite – is a very bad conductor !
big changes in resistance
Idealy no add. Substances
Van der Waals (170 pm)
Group, period, block
14, 2, p
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Carbon Powder
Stable
Triple point is at
10.8 ± 0.2 MPa
4,600 ± 300 K
Sublimates at 3,900 K.
Carbon powder of the
carbon microphone
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Chamber with carbon grains
Membrane
Resistor
Voltage
source
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Hi, how are you…?
Home made
Factory made
Factory records speech better – small range of frequencies –
what about song?
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Comparison
Home made
R=5000Ω
Factory assembled
R=100Ω
Home made
Factory made
The record of the song with factory made has similar ‘quality’ as
with home made – as predicted by its characteristics
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