New DSP to measure acoustic efficiency of road barriers. Part 1: Reflection Index

Barriers employed for road traffic noise reduction can be characterized by two indices: reflection index for sound reflection and insulation index for airborne sound insulation. They can be measured following the method described in CEN/TS 1793-5 standard, based on impulse response measurements employing a pressure microphone. The method mandates for averaging results of measurements taken in different points in front of the device under test and/or for specific angles of incidence, employing the obsolete MLS signal for performing the measurements, which can cause severe artefacts due to nonlinearity and time-variance of the system. Furthermore, the CEN/TS 1793-5 standard presents some geometric problems, which could arise if the barrier does not reach a minimum height or if it has a very rough (scattering) surface. During the reflection index measurement on a barrier of limited height, the reflected sound can be contaminated by the ground reflection, compromising the fairness of the whole result. Also the insulation index can be affected by the height of the noise barrier, since the sound passing above the device under test can become mixed with the sound passing through it. It has been noticed how these practical problems, jointly with the assumption of a surface reflecting specularly in the final formula, can significantly over/under estimate the laboratory values of both the indices. Results of in situ tests based on CEN/TS 1793-5 will be shown in comparison with results obtained through a different approach, based on sound intensity measurements, and with the traditional tests performed in the lab.