1. Chaparral Physics Research
Jay Helmericks, Duncan Marriott, John Olson
Wilson Infrasound Observatories
Geophysical Institute, University of Alaska Fairbanks
Presented at the Infrasound Technology Workshop
Bermuda 3 Nov 2008
This presentation does not necessarily reflect the policies or views of the United States Government.

2. Abstract
This 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.
CP Research • ITW 2008

3. Introduction Linearity of Chaparral Physics sensors
Frequency response of soaker hose
Large area vs small area wind noise reduction systems
New Chaparral Physics sensors
My main topic is one that is important to the infrasound community as people look at infrasound in the near field, and therefore look at larger signals.
We have characterized for the first time the linearity of CP sensors to giving you more info about how the sensors perform.
The equipment that we developed for testing the linearity also let us look at a question that at least the US infrasound team has been grappling with recently, the frequency response of soaker hose. With more interest in the combining of low audio frequencies with infrasound this has become an issue. As a part of this I have a quick comparison of the effectiveness of large area vs small area wind noise reduction systems.
I will wrap up with a brief mention some new CP sensors.
CP Research • ITW 2008

4. 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
To be able to test the linearity, we ….
The limit on the low end of both frequency and amplitude is set by the background atmospheric noise contamination
The high frequency limit is set by non-uniform pressure amplitudes throughout the chamber.
The high pressure limit is set by the gain of the amplification system
CP Research • ITW 2008

5. Test Chamber
CP Research • ITW 2008

6. Test chamber opened
CP Research • ITW 2008

7. 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
No change with frequency so I wont bring that up again.
CP Research • ITW 2008

8. CP Research • ITW 2008 Results of the test.
Can see that the amplitudes agree well to 125 Pa p-p.
CP Research • ITW 2008

9. 100 Pa Fit
With linear fit norm of the residuals are less then 0.5 Pa
CP Research • ITW 2008

10. 20 Pa Fit
With a 20 Pa max the residuals drop to about 0.1 Pa
CP Research • ITW 2008

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12. Linearity to 650 Pa p-p CP Research • ITW 2008
Ok, lets push the amplitude way up and see what happens when taking the sensor to the limit.
Three sections
Linear, non-linear with agreement, non-linear with out sensor agreement
In all of the non-linear area the pressure amplitude are high enough that the diaphram is moving significantly
CP Research • ITW 2008

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14. 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
Want to explore the effects of the distortion that is clear at high amplitudes.
CP Research • ITW 2008

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16. Can see the reduction of sensitivity with -45Pa of bias and the increase of sensitivity with +45Pa of bias
CP Research • ITW 2008

17. Most signals are in the center area here where there the max error is only a few percent.
CP Research • ITW 2008

18. Linearity Summary Below 20Pa p-p non-linearity is not significant
From Pa p-p small scale non-linearity needs to be considered
Above 150 Pa the wave shape will have significant distortion
Most signals are in the <20Pa p-p range, except when the sensor is right next to the source
CP Research • ITW 2008

19. 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
CP Research • ITW 2008

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21. Point out four sensors of note, wind set.
CP Research • ITW 2008

22. One of the challenges with field work is maintaining a consistent setup.
CP Research • ITW 2008

23. Freq Response CP Research • ITW 2008 Long Soaker 3dB = 70Hz
Short Soaker 3dB = 8Hz
CP Research • ITW 2008

24. Wind Noise Reduction CP Research • ITW 2008
Long soakers with a 100 foot dia show significant wind noise reduction, while both small system do not show significant noise reductions. Clearly see the total signal reduction of the short soaker.
CP Research • ITW 2008

25. 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
CP Research • ITW 2008

26. 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)
CP Research • ITW 2008

27. Conclusion Linearity of Chaparral Physic sensors Wind Noise Filters
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
CP Research • ITW 2008

28. Questions?