Plane acoustic holography allows, on complex equipment, to identify them 'sources' responsible for the acoustic radiation perceived in the vicinity. Method reserve here consists, starting from acoustic measurements, to find the vibratory components fields sources while using, on the one hand a process of return towards the sources (or propagation reverses), and in addition the existing relation between vibratory speed and speed particulate in the fluid environment. These hot sources, or 'points', will be visualized under form directly interpretable images.
The guiding principle of acoustic holography consists of measurements of pressures phased acoustics on a regular level of collecting close to the sound-effects man. Since one cannot to acquire all the microphones simultaneously, it is necessary to use fixed references of phase on the body or close to the sound-effects man. It can be a question of:
The acoustic plan of collecting is located in the field close to the sound-effects man closest to radiant zones, thus all the components of the sound field are measured in one limited frequential field depend on the physical characteristics of the sensors used.
The sampling step of the microphones of the plan of collecting defines the high limit frequency of use of the sensors:
For example, for a spacing enters the microphones of 2.5 cm, the maximum frequency analysis will be 6800 Hz. Theoretically there is no limit low frequency of use sensors, however, for certain frequencies, levels acoustic on the images resultants will be constant.
The object of acoustic holography is to solve the opposite problem. From measurements acoustics in close field it is a question of locating and of identifying the sources of noise acoustic "hot points" in a parallele plan (sensor plan). See Maynard and Williams' article for more info.