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.

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

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

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

Test Chamber CP Research • ITW 2008

Test chamber opened CP Research • ITW 2008

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

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

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

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

CP Research • ITW 2008

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

CP Research • ITW 2008

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

CP Research • ITW 2008

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

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

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 Most signals are in the <20Pa p-p range, except when the sensor is right next to the source CP Research • ITW 2008

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

CP Research • ITW 2008

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

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

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

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

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

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 CP Research • ITW 2008

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

Questions?