Measurement system for harmonic source noise directivity definition, based on phase correlation in multimicrophone configuration

The paper is aimed at describing the measurement system for noise propagation of strictly harmonic noise source like electrical transformer. The system consists of fourteen microphones with phase calibration, equally spaced around the tested object in three distances. Electrical transformer is source of stationary and harmonic noise creating acoustic filed with many phase interactions. This acoustic wave correlations create positive and negative phase interference in the acoustic near field called also reactive field. The described measurement system can define the wave front for every frequency emitted by the source, thus allows to analyze the phase interference in the acoustic field. The phase interference comes from noise propagation of the transformer tank having complex vibration deflection shapes. The study and results allow for better understanding and definition of the critical distance for the reactive acoustic field created by multi-harmonic noise sources like transformers.