Abstract
For the reduction of the acoustic pollution levels of road vehicles, the evaluation of the external noise in operational conditions, together with the identification and quantification of the different relevant sources, represents an aspect of fundamental importance for the automotive industry. One of the most commonly used techniques consists of performing road tests after physically masking the different sources with specifically-designed acoustic shields in order to determine the contribution of each. However this procedure is relatively time-consuming and expensive; considerable work is required to prepare the vehicle, set-up and calibrate the instrumentation, execute the test, and process the measurements, particularly since the tests need to be repeated many times opening one 'window' at a time to quantify the contribution of each individual source with respect to the fully shielded vehicle.
This article describes an activity conducted in collaboration by Centro Ricerche Fiat and Fiat Auto in which an experimental methodology, called the Indicator Microphones (IM) method, was studied and developed in an attempt to overcome some of these difficulties. The underlying assumption of the IM method is that the vehicle can be effectively divided into its main areas of emission (engine, intake and exhaust systems, tyre-road interfaces, etc.), and that each individual contribution to a given point in the external noise field can be represented by the product of a source term, defined as an equivalent volume-velocity for each area of emission, multiplied by an acoustic transfer function measured between the surface of the source and the response point. The source term, i.e. the equivalent volume-velocity of each area of emission, is identified by measuring the acoustic pressure spectra during tests in operational conditions using a set of microphones, called
indicators. The case studies illustrated include the application of the methodology to a current-production vehicle, providing an overview of the general aims and discussing the issues which still need to be addressed in order to permit routine application in industry.