SOUND PROPAGATION OUTDOORS - COMPARISON BETWEEN NUMERICAL PREVISIONS AND EXPERIMENTAL RESULTS

The aim of this work is to test the accuracy of numerical previsions made following two different approaches: the first is an Image Source code, built up around the computing formulas contained in the new ISO DIS standard 9613 parts 1 and 2. The second one is a general purpose Pyramid Tracing code, called RAMSETE, that is suitable both for indoor and outdoor simulations. The test case was chosen in an area containing all the most interesting acoustic phenomena: large distance propagation over absorbing soil, shielding by embankments and buildings, multiple reflections on reflecting facades. Only adverse atmospheric conditions were not taken into account (strong wind, inverted temperature gradient). A new technique was employed to collect experimental data: the sound source was a directive loudspeaker, which Sound Power Levels and Directivity Balloons in Octave Bands were previously measured in free field conditions. It was fed with MLS (Maximum Length Sequence) pseudo-random continuos signal. The measurement in each point was obtained through asynchronous cross-correlation of the signal coming from a standard Sound Level Meter (recorded for convenience on a DAT tape recorder) and the original MLS sequence, through a fast-Hadamard algorithm, yielding the Impulse Response between the Source and the Receiver positions. With proper synchronous averaging of the incoming signal, a great improvement in the Signal-To-Noise Ratio was achieved, making it possible to make measurements unaffected from background noise even in highly shielded positions. Both the experimental and previsional data were used to build graphical plots, enabling a direct comparison of the results: they show that the capability of accurately model the source directivity produce generally a better estimate using the pyramid tracing algorithm, but the shielding effects are more accurately modeled by the ISO9613 code.