chaparral physics research jay helmericks, duncan marriott, john olson wilson infrasound...

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.


Introduction

▫ Linearity of Chaparral Physics sensors

▫ Frequency response of soaker hose▫Large area vs small area wind noise

reduction systems

▫ New Chaparral Physics sensors


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


Test Chamber


Test chamber opened


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


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


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


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


8680 8700 8720 8740 8760 8780 8800

-40

-30

-20

-10

0

10

20

30

40

90Pa p-p Waveform

Pa

Samples


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


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


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


0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000-40

-30

-20

-10

0

10

20

30

40Two Tone Waveform

Samples

Pa


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


-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


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


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


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


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


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


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


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?

chaparral physics research jay helmericks, duncan marriott, john olson wilson infrasound...

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