An experimental analysis of the correction factors adopted on environmental noise measurements performed with window-mounted microphones Mário Mateus ⇑ , João Dias Carrilho, Manuel Gameiro da Silva ADAI/LAETA, Department of Mechanical Engineering, Faculty of Science and Technology University of Coimbra – Pólo II, 3030-201 Coimbra, Portugal article info Article history: Received 10 January 2014 Received in revised form 25 June 2014 Accepted 30 June 2014 Keywords: Environmental noise Facade correction factors Long-term assessment abstract 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 differ- ence 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 L d 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. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction In the measurement of environmental noise sound levels, when the microphone is placed on the facade or in its vicinity, it is required that the measured levels be corrected to compensate for the influence of reflection and diffraction. These phenomena occur due to the interaction of sound waves with the facade wall. The values of 3 dB or 6 dB are indicated as the corrections to be applied to the measured values in situations where the microphone is positioned respectively at a distance of 2 m or directly on the facades of masonry, concrete, wood, glass or other reflective material of similar characteristics ([1–4]). The values of these corrections were studied at various sites [5], where distinct configurations of streets were considered, in partic- ular with L and U shapes and some of its variants such as the width and distance from the opposite wall when the variant configura- tion is in L. The sound field was the result of the emission and propagation from the passage of vehicles (mobile source) in adjacent traffic lanes. The microphone height placement relative to the ground is always 4 m, which is placed on an aluminum plate with 5 mm thickness and 60 cm  90 cm dimensions. Considering all studied http://dx.doi.org/10.1016/j.apacoust.2014.06.019 0003-682X/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +351 239 708 580; fax: +351 239 708 589. E-mail addresses: mario.mateus@adai.pt (M. Mateus), joao.carrilho@dem.uc.pt (J. Dias Carrilho), manuel.gameiro@dem.uc.pt (M. Gameiro da Silva). Applied Acoustics 87 (2015) 212–218 Contents lists available at ScienceDirect Applied Acoustics journal homepage: www.elsevier.com/locate/apacoust