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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.

According to ISO 16063-11 (1999), at frequencies below 1600 Hz primary calibration of accelerometers may employ two methods: fringe counting or sine approximation. During a recent intercomparison (APMP.AUV.V-S1) small but systematic differences were found between the results obtained by using these two methods, and by the use of different amplifier modes to drive the shaker at frequencies between 0.5 Hz and 20 Hz. The influences of distortion and noise on the two methods are explored. The results and a discussion of the differences are presented in this paper.

Artificial mastoids are devices which simulate the mechanical characteristics of the human head, and in particular of the bony structure behind the ear. They are an essential tool in the calibration of bone-conduction hearing aids and audiometers. With the emergence of different types of artificial mastoids in the market, and the realisation that the visco-elastic part of these instruments changes over time, the development of a method of traceable calibration of these devices without relying on commercial software has become important for national metrology institutes. This paper describes commercially available calibration methods, and the development of a traceable calibration method including the traceable calibration of the impedance head used to measure the mechanical impedance of the artificial mastoid.