Traceable calibration of microphones at low frequencies

The accurate measurement of infrasound from wind turbines, mining blasts, earthquakes and other natural phenomena is essential to the correct assessment of the health and safety implications of low-frequency sound. Currently many low-frequency measurements are often made using microphones calibrated at higher frequencies (250 Hz or 1000 Hz) attached to sound level meters (SLMs), and assumptions are made based of manufacturers' specifications of the response of the microphones at lower frequencies and of the software in the SLM. Presented here is a simple technique for the calibration of low-frequency microphones by comparison with differential pressure gauges, combining two different traceability paths to determine a microphone's frequency response and sensitivity in the range 0.1 Hz to 20 Hz. (paper)