VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

3.10. MEASURING SOUND SOURCES 553.10.3 The comparison methodIn the ’comparison method’ one uses a reference source which is normalised.The method itself is also standardized in many industries. The test is conductedin a so called acoustic hard room, since one does not usually haveaccess to an acoustic dead (anechoic) room. The reference source can generatefor example a power L ′ W . Firstly, one has to install multiple microphonesspaced out in the room and not to close to the machine (in the far field). Onemeasures the sound pressure levels L pi and the spatial averages L pm of thesound radiated by the unknown source (use Equation 3.12). Secondly, onethen replaces the machine with the reference source and one measures L ′ pm inan identical fashion (microphones on identical positions). In each frequencyband the following equations hold true : L W = L pm +C and L ′ W = pm+C L′(where c is a constant that is the same for the two measurements) The unknownpower can be calculated from :L W = L ′ W +(L pm −L ′ pm ) (3.14)3.10.4 Power measurement with an intensity meterDue to the fact that the measuring locations for the described comparisonmethods are located in the far field, that method is not applicable whendifferent objects are placed close to each other. In this case one can use anintensity meter to measure the sound power. The following procedure mustbe used to do this (ISO 9614) :Define a random surface S that includes the source. This does not haveto be placed in the far field but can be situated close to the source.Measure the intensity I i in N discrete points on the surface S.Calculate the power level : L W = 10log ∑ N W ii=1 W 0with W i = I i S i .Instead of discrete measuring locations an alternative method can beused where the intensity meter scans in a continuous fashion over the definedsurface (the so called sweep method, described in ISO 9614-2). Care must betaken that the sweep speed is as constant as possible, moreover the intensitymeter must be held perpendicular to the surface.