|Acoustic Holography Priciples|
Acoustic holography makes it possible to determine the noise radiated by each of the mechanical components of a complex system, it is the near field acoustic imagery. It delivers a fine representation of the distribution of the sound sources on the surface of the equipment or in any parallel plan near this surface. By measuring the pressure in the immediate environment of the system, acoustic holography allows to calculate the field of pressure in any point close to the sound sources or in the far field.
The complex field of sound measured by the antenna is broken up into an infinity of propagatives elementary plane and evanescentes waves. The evanescentes acoustic waves describe the complex field of the sound existing close to the envelope and partly mirroing the vibrations. The level and the direction of each acoustic wave are described by their number of acoustic wave. The principal treatment of acoustic holography is to apply to each acoustic element components (planes, cylinders, etc) an opposite operator of propagation, in order to obtain it sound field on a surface parallel with the plan of measurement in near field. Starting from the same data of measurement, it is possible to calculate the radiated acoustic pressure in the far-field.
The basic equipment used in acoustic holography includes:
When one uses a reasonable number of microphones it is not possible to acquire at the same time all acoustic data necessary to the treatment of holography accoustics. This treatment is based on the use of complex acoustic pressures regularly distributed within measurement. To know the relations of phase between microphones acquired at different moments it is imperative to use fixed references representative of the behavior vibroacoustic of the testing equipment. These references can be acoustic, vibratory or kinematics.
Interferometry or angular imagery has the aim of locating and identify sources of noise on a given far-field site (factory, route transportation...). The acoustic image resulting from the surface observed represent the distribution of the noise level according to two principal parameters which are:
A collecting system that can be implemented is an acoustic linear antenna with multicapteurs. This allows to carry out an interferometric treatment of the acoustic signals received on all the sensors. Need to be known: number of microphonic sensors, steps of space sampling and scale and of dynamics.