this report gives the results of measurements made of the acoustic performance of a noise barrier erected alongside the m40 at a point where the motorway passes to within 150 metres of a residential estate. the average reduction in l10 (18-hour) obtained at previously directly exposed locations along the first row of houses was 5 db(a) but the barrier did not reduce the noise levels along the first row where there was already a considerable degree of shielding and at the other measurement locations situated beyond the first row. the relationship between sound level and the velocity of the wind vector normal to the road has been explored. it is shown that wind can reduce the effectiveness of the barrier and increase the range of variation in the sound level following the construction of the barrier.(a)

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