Chaparral Physics Research
Jay Helmericks, Duncan Marriott, John OlsonWilson Infrasound Observatories
Geophysical Institute, University of Alaska Fairbanks
Presented at the Infrasound Technology WorkshopBermuda 3 Nov 2008
This presentation does not necessarily reflect the policies or views of the United States Government.
CP Research • ITW 2008 2
AbstractThis talk will cover two areas of research that Chaparral
Physics has been pursuing, both of which are of interest to the general Infrasound community. The first is an investigation of the linearity of Chaparral Physics sensors. The testing shows that there are three regions: with small-amplitude signals the sensor is fully linear; then, as the signal amplitude increases, there is a point where the linearity of the sensor depends on the shape of the incoming wave; and finally, as the signal exceeds 150 Pa peak-to-peak the sensor response completely departs from linearity. The second area of research looks at both the effectiveness and frequency response of wind noise reduction systems, from ~5 Hz to 100Hz. The effectiveness of wind noise reduction systems have been studied extensively, but little work has been done on the frequency response of such the systems. Preliminary results from this research will be presented.
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Introduction
▫ Linearity of Chaparral Physics sensors
▫ Frequency response of soaker hose▫Large area vs small area wind noise
reduction systems
▫ New Chaparral Physics sensors
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Linearity Setup▫ Constructed a test chamber with
heavy plywood with a subwoofer as the signal source
▫ Standard audio power amplifier and sound card to drive the speaker
▫ Pressure reference was a G.R.A.S 40BF ¼” microphone
▫ Able to test from 1 to ~ 200Hz, and 0.5 Pa p-p to 700 Pa p-p
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Test Chamber
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Test chamber opened
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Single Tone Test
▫Drove the chamber with a sine wave of varying frequency and amplitudes
▫Compared the output of the G.R.A.S. microphone to the Chaparral microphones
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0 25 50 75 100 125 1500
25
50
75
100
125
150
Pa p-p Ref
Pa
p-p
Sen
sor
Composite Linearity Plot
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100 Pa Fit
0 10 20 30 40 50 60 70 80 90 1000
10
20
30
40
50
60
70
80
90
100
GRAS Value
M25
Val
ue
Low Gain summary of Linear Fit
M25-1
M25-2M25-3
M25-4
FitL1
FitL2FitL3
FitL4
Norm-Residuals:0.4270.4170.4070.508
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0 2.5 5 7.5 10 12.5 15 17.5 200
2.5
5
7.5
10
12.5
15
17.5
20High Gain summary of Linear Fit
GRAS Value
M25
Val
ue
M25-1 M25-2
M25-3
Fit-1
Fit-2Fit-3
Norm-residualsare:0.1120.1100.110
20 Pa Fit
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8680 8700 8720 8740 8760 8780 8800
-40
-30
-20
-10
0
10
20
30
40
90Pa p-p Waveform
Pa
Samples
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Linearity to 650 Pa p-p
0 50 100 150 200 250 300 350 400 450 500 550 600 650 7000
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
Pa p-p Ref
Pa
p-p
Sen
sor
Composite Linearity Plot
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2110 2120 2130 2140 2150 2160 2170 2180 2190 2200 2210
-150
-100
-50
0
50
100
150
200
310 Pa p-p Waveform
Samples
Pa
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Two Tone Test▫ Used the same test setup▫ Ran two sine tones at the same time
▫A low frequency tone is varied in amplitude
▫A high frequency tone is constant through the tests
▫ Looked at the amplitude of the high frequency tone as the low frequency tone moves the diaphragm through its operating range
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0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000-40
-30
-20
-10
0
10
20
30
40Two Tone Waveform
Samples
Pa
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0 1000 2000 3000 4000 5000 6000-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Samples
Pa
Small signal distortion with 90Pa bass tone
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-60 -40 -20 0 20 40 60-25
-20
-15
-10
-5
0
5
10
15
20
25Sensitivity Change with Pressure Bias
Pressure Bias in Pa
Per
cent
cha
nge
in s
ensi
tvity
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Linearity Summary
▫Below 20Pa p-p non-linearity is not significant
▫From 20-150 Pa p-p small scale non-linearity needs to be considered
▫Above 150 Pa the wave shape will have significant distortion
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Response of Wind Noise Filters
▫Measured the frequency response and the noise reduction▫4 50ft soaker hoses▫2 3ft soaker hoses▫Foam doughnut
▫Used the same signal source as the linearity tests
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Freq Response
100
101
102
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
2Comparison of Wind Noise Reduction Systems Relative to an Open Sensor, Active Signal Source
Frequency [Hz]
Res
pons
e R
elat
ive
to a
n op
en s
enso
r [d
B]
Foam Doughnut
2-3' Soaker Hoses4-50' Soaker Hoses
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10-2
10-1
100
101
102
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
Power Spectral Density of Wind Noise with Various Filters, Wind speeds between 1 and 2 m/s
Frequency [Hz]Pow
er S
pect
ral D
ensi
ty o
f W
ind
Noi
se [
dB r
elat
ive
to 1
Pa2 /H
z]
Open Sensor
Foam Doughnut4-50' Soaker Hoses
2-3' Soaker Hoses
Wind Noise Reduction
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Wind Filter Summary
▫ Short soaker hoses provide no noise reduction and significantly attenuate high frequency signals
▫ Long soaker hoses provided good noise reduction with a slight attenuation of high frequency signals
▫ There appears to be no wind noise reduction relative to an open sensor with a filter that does not have significant averaging area
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New Sensors▫ Added a single port configuration
back to the lineup▫Lighter weight▫Provides an option when a manifold is
not needed▫ New feature reduced low cost
sensor for 30% less then a M25▫ Removed gain selection and sensor self-check
functions
▫ See website for details (soon)▫ www.chaparral.gi.alaska.edu
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Conclusion▫ Linearity of Chaparral Physic sensors
▫20 Pa p-p transition point to needing to evaluate whether the linearity will effect your results
▫ Wind Noise Filters▫Showed the frequency response of
soaker hose and how it changes with length
▫The small filters tested had no noise reduction advantage over an open microphone
▫ New Models from Chaparral Physics
Questions?