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Acoustic Holography - Using Stationary and Transient Array Based Sound Source Identification

Does anyone else use the QUALIDB and SUIVIDB modules in LMS to post-process array microphone measurements in the near-field in to sound power and acoustic holograms?

 

The measurement process in my case is to measure 320 microphone positions a uniform distance away from the engine surface in a semi-anechoic chamber. The grid is uniform and the spacing is 0.05 m with 18 positions in Y and 20 positions in X. The array is approximately 0.17 m away from the furthest component on the measured face of the engine.

 

I have been having a plausibility issue with the data for some time now. The noise spectra in 1/3rd Octaves has OALs of between 90 dB(A) and 95 dB(A) in the near-field and 84.3 dB(A) at 1 m from the radiating surface. This is intuitive as one expects the levels at the surface to be higher and can approximate the decay from the engine in a semi-anechoic chamber.

 

However when one computes the sound power level following the process laid out in the help documentation, the sound power level in 1/3rd octaves has an OAL of 85.9 dB(A). This suggest the SWL at the radiating surface is ~ = to the SPL at 1 m. My expectation is approximately a difference of 8 - 10 dB(A) due to the intensity reduction as one propagates away from the engine.

 

Has anyone faced this issue or have you had a different experience when processing out the sound power levels?