Besides the geometric configuration, the spatial resolution of the measuring system depends on the chosen frequency range and the distance to the sound source to be localized. The higher the frequency is, the smaller the wave length is, the higher the spatial resolution gets. Furthermore, the spatial details of the mapped sound field increase by diminishing the distance to the object.
Like in an optical system, increasing the angle between object direction and the normal to the camera causes distortions of the object`s mapping. The same effect occurs in the case of mapping a sound field using a microphone array. In the acoustic far field the distortions can be neglected up to a maximum aperture angle of 60°. The larger the dimensions of the test object get, the greater the distance between microphone array and object has to be.
The measuring system AcoustiCam® is based on the near field beamforming algorithm. This algorithm allows acoustic investigations of test objects at great distance as well as in close proximity to the microphone array because the suggested maximum aperture angle increases with decreasing distance to the test object. The minimum distance that must be kept is approximately 25 cm.
Thus, smaller objects can be placed very close to the microphone array, larger objects require a greater distance. For the application of AcoustiCam® there are virtually no constraints regarding the dimensions of the object to be investigated.
The novel algorithm applied by AcoustiCam® is based on splitting the localization result into uncorrelated, that means independent, sound sources. These sound sources are caused by different source mechanisms, leading to different acoustic source strengths.
The application of the orthogonal beamforming technique automatically yields separate images of the different sound sources and their source mechanisms which determine the sound field. Thus, not only the main sound sources but also acoustically masked sound sources, which cause only a low sound pressure level, can be separately localized.
This requires neither the enclosure of single areas of the test object nor the execution of multiple measurements. Additionally, with the help of this technique the signal-to-noise ratio can be improved from 10 – 15 dB up to more than 25 dB.
Virtual deactivation of partial sound sources
The energetic addition of all sound field components corresponds to the mapping of the overall sound field (result of the near field beamforming algorithm). By adding up only selected components, it is possible to virtually deactivate specific sound sources. This allows the prediction of the effect of the optimum elimination of a specific sound source on the overall sound field. Therefore, no modifications on the investigated object are necessary.
This is an important advantage if the goal is to decide between different procedures, with the aim of not exceeding a permitted maximum sound pressure level.