The following explanations refer to an acoustic photo, in which five sources with white noise and sound pressure levels of 90 dB, 80 dB, 70 dB, 60 dB and 50 dB have been simulated. Considering the following hints, the accurate usage of HDR yields the map depicted in Figure 1.
As High Dynamic Range (HDR) is an algorithm based on TDBF, the application of HDR requires the compliance with all guidelines that are essential for the creation of a precise beamforming map.
When generating an acoustic photo, the basic parameters are the distance between the Acoustic Camera and the measured object (= focus), the number of pixels per unit length (= resolution) and the chosen image section. This is a specialty caused by the HDR’s functioning: It is necessary, that all sources are within the image section. Otherwise the algorithm cannot reconstruct the sources and therefore subtracts them incorrectly from the sum signal. This becomes noticeable when sources seem to lay at the image boundary (figure 4). When the other parameters are adjusted incorrectly, the source signal’s time function is calculated erroneously as well and thus the HDR computing a deficient clean map. Figures 2, 3 and 4 are showing clean maps in which focus, resolution and image section have been adjusted wrongly.