Measurement of infrasound sensor self-noise

Self-noise is a critical performance characteristic of sensors intended for weak-signal detection and localization; however, the lower the self-noise, the more challenging the measurement. A recent international sensor-characterization exercise coordinated by the Provisional Technical Secretariat of the Comprehensive Nuclear-Test-Ban Treaty Organization included self-noise measurement of several infrasound sensors from 0.01 Hz to 10 Hz. Of three common methods for self-noise assessment—isolation of the sensor from external excitation, subtraction of common (coherent) components among several sensors, or de-activation of the sense mechanism—the first two were used in this exercise and the results highlight important measurement issues. In the infrasonic frequency range, isolation from external excitation is challenging. A sealed, thick-walled chamber attenuates ambient noise but heating from dissipation of power in the chamber interior can induce convection with large, low-frequency pressure fluctuations in the chamber. Capping the inlet(s) of a sensor creates a small, closed volume with strong coupling between temperature and pressure fluctuations. Subtraction of coherent components shared by co-located sensors can be effective in reducing the influence of ambient excitation; however, the process may be frustrated by errors in subtraction of large, nearly equal components or by unexpected electrical coupling.