Chaparral Model 5 Microphone

The University of Alaska has acquired the rights to manufacture Chaparral infrasonic microphones at the Geophysical Institute under the name of Chaparral Physics. The Chaparral Physics web site address is: http://www.chaparral.gi.alaska.edu.

 The Chaparral Model 5 microphone is used at I53US in Fairbanks, Alaska and at I55US in Windless Bight, Antarctica. These microphones are equipped with a special input manifold that has a hole on the top that will accommodate a field calibrator so that the microphones can be calibrated in the field after installation in the array. The characteristics of the Chaparral Model 5 microphone are given in the table below. The microphone is pictured in Figure 1.

Chaparral Physics Model 5
	Division of the Geophysical Institute University of Alaska, Fairbanks P.O. Box 757320 Fairbanks, Alaska 99775 (907)474-7107
Standard Sensitivity:
High	0.04 volts/microbar @ 1 Hz (0.4 volts/Pa)
Low	0.01 volts/microbar @ 1 Hz (0.1 volts/Pa)
Output:
Maximum	18 volts peak to peak
Frequency Response:
Electronic noise:	20 microvolt, maximum, 1 Hz @ Low sensitivity
Output Impedance	200 ohms in series with 2200 mfd.
Power Requirements:
DC Source	12 volts, (9 to 18 volts) dc, (24-36 volts dc, special order)
Current Drain	Less than 34 ma. @ 12 v, or (less than typically 7 ma @ 24 volts)
Operating Temperature	-40° F to +150° F (-40° C to +65° C)
Humidity	100% (non-condensing)
Physical Dimensions:	Sensor will function in any attitude
Size	13 in. (33 cm.) high with legs 8.875 in. (22.54 cm.) diameter
Weight	9 lbs.(4.1 kg)
Acoustic Inlet	1 in. screened inlet as standard or choose from a variety of inlet manifolds are available, with typically, four to 12 each, male garden hose threads, inlets, and a calibration port.

Chaparral sensors have a very low seismic sensitivity compared to other infrasound sensors. The extremely low mass of the capacitive element and patented capacitive measurement technique ensure wide bandwidth and precise low noise measurements. Sensors used for infrasound arrays used to locate signals should be phase matched, Chaparral sensors have good phase matching within their stated flat bandpass. If your needs require specific phase tolerances, please contact Chaparral for specifics.

Field Calibrator for Chaparral Model 5 Microphones

A calibrator was designed and built at the Geophysical Institute that would allow pressure calibrations of the microphones to be made in successive years in the field after the microphones had been installed in the array. The calibrator mates with the microphone’s top manifold in a way such that the noise-reducing pipe system does not have to be disconnected from the microphone-calibrator assembly while the calibration is taking place. The calibrator produces a sinusoidal pressure signal at the input to the microphone at three selectable frequencies (1.0 Hz, 0.1 Hz and 0.02 Hz) and at two different selectable pressure amplitudes (peak-to-peak) of one Pa or 10 Pa. The calibrator also produces step function compression and rarefaction and permits phase synchronization. Figure 2 shows a picture of the calibrator.

The calibrator is shown in Figure 3 at I55US in Windless Bight before it is mounted in the equipment vault on top of the microphone as pictured in Figure 4. After the calibrator gears are set for the appropriate frequency and the desired pressure amplitude the vault is closed and the system is allowed to come to temperature equilibrium before calibrations can begin. The calibration is monitored at the vault by viewing the calibration signal waveform with a laptop computer that stores the calibration data. The calibration signal is transmitted to the hub site at McMurdo Station in real time so the operator there can advise the field calibration crew about the quality of the  calibration signal. Successful calibration can only be done during calm wind periods. In a typical calibration at I55US the pressure amplitude of the calibrator is set at 0.1 Pa and the frequency is set successively at one Hz for 5 minutes, 0.10 Hz for 10 minutes, and finally at 0.02 Hz for 30 minutes.

The vault must be opened each time the frequency of the calibrator is changed and the system is allowed to come to temperature equilibrium before proceeding. An example is shown of the microphone output during a typical calibration at all three frequencies in Figure 5. The microphone output pressure is shown in Figure 5 for a 30 minute period of time. The calibration sinusoidal signal can be seen starting at 1 Hz, then 0.1 Hz ending with 0.02 Hz frequency. The peak-to-peak pressure for the calibration is about one Pa. In the 0.1 Hz and 0.02 Hz calibration signal traces the recovery to equilibrium pressure at zero Pa can be seen as equilibrium temperature is reached. Background ambient pressure fluctuations cause the ripple in the envelope of the 1 Hz calibration trace. Narrow bandpass filtering, centered at 1 Hz, of the calibration trace is used to remove the low level ambient noise from the calibration signal as can be seen in Figure 6.