Sound Photographs to reveal vehicle pass-by sources with a calibrated source-strength level

In national and European discussions, it appears that the conventional sound measurement techniques are insufficient to answer some relevant questions with respect to source strength of road vehicles. An example of such a question is: What is the importance of tyre-road noise on the one hand and drive-line noise on the other hand in every day traffic? TNO TPD has developed measurement techniques, based on array technology, that enable the assessment of source strengths of complex sound sources, such as passing road vehicles, passing trains or industrial sources. These methods aim to distinguish the different partial sources in one dimension (Syntacan) or two dimensions (Acoustic Camera) and to measure their calibrated source-strength levels. The Dutch ministry of Environment (VROM) supported the application of these techniques by TNO TPD within the European HARMONOISE project. The objective was to show the ability of array measurements to provide both sound exposure levels (SELs) and heights. Tests were conducted at TRL (UK) to determine the accuracy of two array methods developed by TNO TPD by comparison with the subtraction method. The data measured by TNO TPD resulted in one-dimensional as well as two-dimensional 'sound photographs' of the vehicles, in different circumstances. The 'sound photographs' present calibrated source strength levels with similar accuracy as those obtained with single microphone techniques measured by TRL and on top of that, provide spatial information of the source strength distribution. Based on these results it can be concluded that the Acoustic Camera and Syntacan can serve in the assessment of road, vehicle noise. This could be done in various ways and for various objectives such as monitoring of tyre-road noise in real traffic, determining effects of the road surface on tyre-road noise and drive-line noise.