The influence of microphone height in the measurement of low frequency and infrasonic wind turbine noise at typical receivers

For environmental noise measurements, the most common height chosen for the outdoor measurement microphone is 1.5 m, as this corresponds approximately to the ear height of an average receiver. The main disadvantage of measuring at this height is that wind-induced noise can affect the results, particularly at low frequencies. Therefore, it is advantageous to measure at ground level or below ground, where the wind speed approaches zero and the wind-induced noise is minimised. On the other hand, results obtained from any height that does not represent the human receiver should be interpreted with caution, particularly at low frequencies when large propagation distances are involved. The reason for this is that the difference in noise level at various heights above the ground is a function of the frequency, ground surface properties, distance from the source and the height of the source. Therefore, it may not be valid to assume that the noise level at ground level and at a height of 1.5 metres is equivalent for low frequencies with corresponding large wavelengths. Also, the noise level difference between these heights at mid- and high-frequencies may not be exactly 3 dB, as it will depend on the ground impedance. Therefore, further research in this area is necessary before measurement at ground level or underground becomes standardised. This paper presents results of low-frequency measurements taken at two distances from an operating wind farm, where microphones were mounted at a height of 1.5 m and at ground level and protected using secondary wind screens of the same diameter and material. The low-frequency results are compared to a theoretical model that incorporates the parameters described above and similarities and differences are explained.