An experimental analysis of the correction factors adopted on environmental noise measurements performed with window-mounted microphones

The ISO 1996-2:2008 standard, currently under review and dealing with the description and assessment of environmental noise, suggests applying corrections to the readings in situations where the microphone is mounted directly on a facade or on some of its elements. To obtain the value equivalent to the sound pressure level in free field, the correction corresponds to subtracting 3 dB or 6 dB from the measured value, for the situations where the microphone is located at a distance of 2 m from the facade or directly placed on it, respectively.In the present study, the influence of the front microphone mounting condition is examined. Also the effect of the velocity and direction of the wind on the correction factor to be applied to measurements is analyzed, for a case where the dominant noise source is road traffic.The results obtained show that, when the microphone is mounted directly on the window, the difference relative to measurements obtained in free field conditions varies between 4.0 dB and 4.4 dB. This difference is associated with the effect that some wind parameters have on sound propagation, namely the wind velocity and its direction relative to the direction of sound propagation. These parameters show remarkable influence for certain frequency bands, particularly at low and high frequencies.In the situation where the microphone is directly mounted on a plate of reflective material, with similar characteristics to those required in ISO 1996-2:2008, a difference of 4.9 dB is found. This value is contained in the interval 5.7 ± 0.8 dB, for which the confidence level of 95% is known.The time evolution of the Ld descriptor is presented, which was obtained based on the sound level values acquired continuously over a period of 47 months. The application of a correction factor to the measured values results in a difference of 2 dB relatively to the value that would be obtained by applying the reference correction factor of 6 dB.Thus these results advise the need for studies “in situ,” whenever this measurement solution is used, since the adoption of the agreed value in the test’s standard may, in some cases, introduce significant errors in the final results of the long duration descriptors.