Characteristics and Applications of a High Performance, Miniaturized, Infrasound Sensor


Infrasound Sensors have been used for many years to monitor a large number of geophysical phenomena and manmade sources. Due to their large size and power consumption these sensors have typically been deployed in fixed arrays, portable arrays have required trucks to transport the sensors and support equipment. A high performance, miniaturized, infrasound microphone has been developed to enable mobile infrasound measurements that would otherwise be impractical. The new device is slightly larger than a hockey puck, weighs 200g, and consumes less than 150mW. The sensitivity is 0.4V/Pa and self-noise at 1Hz is less than0.63µPa²/Hz. The characteristics were verified using a calibrator traceable to the Sandia National Laboratories calibration chamber. Field tests have demonstrated the performance is comparable to a Chaparral model 25. Applications include man portable arrays, mobile installations, and UAS based measurements.

Figure #1

Acoustic Performance

The miniaturized microphone, shown in figure #1, matches the performance of much larger diaphragm based sensors, such as the Chaparral Physics model 25, in a rugged, field ready, compact form factor. Current consumption is less than 12mA when operated on a 12V power supply. Amplitude and phase characteristics, shown below, are highly stable and feature a 13 octave bandwidth. The low and high frequency cutoffs are 0.03Hz and 240Hz respectively. Two models have been developed.

The model 60 is optimized for low noise performance (figure #4). It has a typical narrow band noise floor of 0.444mPaRMS, an 88dB dynamic range, and a 55Pap-p maximum input level. The model 60VX has been optimized for high dynamic range(figure #5). It has a typical narrow band noise floor of 1.04mPaRMS,a 108dB dynamic range, and a 720Pap-p maximum input level. Below 1Hz the self-noise of both microphones is lower than the minimum global wind noise.The calibrator, shown in figure #2, is used to characterize the microphones. It’s performance has been validated in the Sandia National Laboratories infrasound calibration chamber.

Figure #2

Figure #3

Figure #4

Figure #5

Environmental Testing

The microphones are designed to be inherently insensitive to vibration; minimizing spurious signals resulting from seismic activity and from the use of the microphones on mobile platforms. We measured the z axis vibration sensitivity of the microphone using an L315M shaker table from ETS solutions.

The vibration level was set to 4.9m/s2 andmeasurements were made from 10Hz to 50Hz. Digital filtering was applied to the microphone data to eliminate interference generated by the power amplifier that drives the table. The measured sensitivity is .0408 Pa/m/s2 with very little variation over the range of measured frequencies.

Temperature stability was measured using a Test Equity 1000 series temperature chamber. Two microphones were used. One was the device under test and the other was used asa reference microphone outside the chamber (figure #8). A speaker, mounted inside a sealed enclosure, was driven by a WavetekModel 29 DDS function generator at 10 Hz to serve as a common infrasound source for both microphones. Temperatures were allowed to stabilize for 1 hour between steps. The temperature chamber was turned off for a short time while measurements were being made to eliminate noise caused by air circulating inthe chamber. The measured gain stability is outstanding between 20ºC and 60ºC. The gain increases by 5.5% as the temperature drops from 20 ºC to -40 ºC.

Figure #6

Figure #7

Figure #8

Figure #9


Figure #10 shows a complete man portable, four channel, 100 meter aperture, infrasound array composed of four low noise microphones and a Chaparral infrasound datalogger. The system fits easily into a small backpack and weighs 4.2Kg. It operates for over 13 hours on the data logger’s internal lithium ion batterypack. The array is ideal for expeditions in remote locations. The low power and weight reduces the cost and dramatically eases the task of deploying the system. For short term scientific campaigns, the microphones are mounted inside closed cell foam “koozies”(not shown). The koozies protect the microphones from the elements, reduce wind noise, and provide insulation to prevent thermal transients from inducing spurious signals.

Figure #10

The microphone’s low noise, compact form factor, and low vibration sensitivity makes it uniquely suitable for any portable or vehicle mounted application. Figure #11 shows another possible application. The microphone and a data logger are mounted on a “Ptarmigan” hexacopter UAS,operated by the Alaska Center for Unmanned Aircraft Systems Integration. The low total payload weight of 0.34Kg allows the microphone to fly on the small UAS and maximizes the flight time. 

Figure #11

Proposed missions would have the UAS fly into dangerous areas near volcanoes, or other hazardous locations, land remotely, take data, and then return. Initial flight tests at Poker Flat Research Range confirmed our expectation that the UAS would generate broadband infrasound and audio band noise during flight (figure #13) .The test signal, shown in figure #12, was not recoverable from the noise due tos pectral overlap and the similarity of the signals. A quieter aircraft, such asa low speed fixed wing UAS or a blimp type UAS, could clearly be used for airborne measurements. The blimp, in particular, would allow very low noise measurements to be made due to the low airspeed.

Figure #12

Figure #13


  • The model 60, miniaturized infrasound microphone matches the noise performance of a Chaparral model 25 large diaphragm microphone.
  • •The model 60VX has a 108dB dynamic range, a maximum input signal of 720Pap-p, and retains excellent noise performance below 1Hz.
  • Both sensors have low vibration sensitivity, excellent temperature stability, and low power consumption.
  • The new microphones enable portable applications that are impractical with the previous generation of infrasound sensors.