Approved devices are indispensable: Measurements with a malfunctioning microphone near a wind park as a worst-case scenario

Since the proliferation of wind energy generation is accompanied by serious concerns over noise emission and its impact on people, noise measurements play an important role in the goal to expand the use of wind energy in power generation. Measurements must be legitimate and defendable against critics, for which the use of approved measurement devices is indispensable.

To demonstrate the importance of this issue, a worst-case scenario was created where a noise measurement was carried out near a wind park using a microphone with a subtle defect. Such a microphone was created by making a tiny perforation in the membrane of an otherwise intact microphone. This type of defect is actually not that uncommon, but is usually of little consequence in a conventional noise measurement. Following normal best practice, the measurement system including the defective microphone was first tested with a sound calibrator at 1 kHz. The system passed the functionality check showing the expected response (calibration value 93.9 dB for a sound pressure level input of 94.0 dB). The defective microphone system was then used for live infrasound measurements near a wind park, alongside a second proven well-behaving system, and the results were compared. Sound pressure levels in the infrasound frequency range (2 Hz to 20 Hz) obtained with the malfunctioning device were found to be around 20 dB low, i. e. wrong, as seen in Figure 1, despite the sound calibrator check indicating no issues with the system. This clearly demonstrates that a microphone which works correctly at the typical test frequency of a calibrator can deliver completely wrong values at low or infrasound frequencies. Additional test procedures at low frequencies appear to be necessary to improve this situation and assure the validity of on-site infrasound measurements.