The effect of the angular error on the determination of the sound power levels of industrial coolers

Drycoolers are relevant noise sources in many industrial plants due to their typically exposed positioning. For this reason, drycoolers are often subject to strict acoustic requirements in the form of maximum permissible sound power levels. Thus, the suppliers have to provide proof of compliance with these requirements before delivery. The acoustic verification measurement is usually carried out by measuring the sound pressure level using the enveloping surface method in accordance with EN 13487. According to EN 13487, the measurement is carried out at discrete points on a cuboid measuring surface. Since drycoolers are sound sources that extend over a large area, an angular error occurs when measuring sound pressure levels on a cuboid measuring surface, which leads to a superelevation of the measured sound pressure level, since the acoustic particle velocity on the cuboid measuring surface is not perpendicular to the measuring surface at almost any point. Therefore, the sound power level determined is methodically too high. By measuring the sound intensity levels in accordance with ISO 9614-2 instead of the sound pressure levels, the influence of this angular error can be excluded and brought into good agreement with theoretical considerations and calculations of the angular error.