Assessment of environmental noise from long-term window microphone measurements

Requirements for façade sound insulation against outdoor noise based on the assessment of environmental noise on building façades. For noise estimations now usually exploit an annual composite 24-h equivalent sound pressure level LDEN, which is mandatory for noise mapping and may be measured. A one-year duration noise monitoring experiment with two microphones used simultaneously was done in the town of Vilnius near an arterial road with intensive road traffic, occurring mostly in the daytime. The first microphone with common all-weather protector was placed on a special support at a distance of 2 m apart the plate building façade, and the other microphone, in accordance with the ISO 1996-2 description, was simply directly glued to the façade window. Using represented in standard ISO 1996-2:2007 −3 dB correction between the window microphone and the microphone placed 2 m from the façade results, the traffic noise level expressed in LDEN were practically identical for both microphone positions. At the same time, results acquired on these both microphones at night, when the traffic abated and the dominant became the overall town background noise, demonstrated that the difference became smaller and varied from 1 dB to 2 dB. Additional statistical assessment of obtained during full year results shows that calculated the standard deviation of monthly LDEN values reaches 1.3 dB, while seasonal weather conditions give a 5 dB scattering in results. The experiment confirmed that the typical for Lithuanian climate seasonal traffic nose level variations (at winter time due to snow covering and impaired traffic conditions, at autumn due to wet road surface) must be taken into account applying short-term relatively to the annual determining LDEN. When only representative whole-days (day selected to conform normal conditions under ISO 1996-2 standard requirement) measurements data are taken into account to assess annual value, the weekly (from 7 successive days) estimated LDEN values gave a standard deviation of 0.9 dB and the one whole-day estimated LDEN values gave the standard deviation of 1.3 dB.