Near Field Acoustic Holography Methods
This technique allows the 3D acoustic field reconstruction from the complex numerical part of the pressure measured on an hologram surface which is located in the source near-field.
The acquisition of pressure maps of the complex numerical part of the pressure around a broadband radiation source is itself a major problem which can be carried out according to several techniques.
1. A solution is to use an antenna comprising an array of of microphones to simultaneously measure the pressure at each point of the grid.
The pros are that it is applicable to monochromatic sources and to extended spectrum sources. The cons are that it requires a large and specific instrumentation.
2. The 2nd technique, called method BAHIM, uses a scanning probe comprising a reduced number of sensors. It is based on an grid of independant measurements of the intensity and pressure at each point using an intensimetric probe. Its originality lies in the use of quadratic measures to rebuild the phase of the complex pressure on the hologram. However, this technique is quite sensitive to the sound field and to diffuse scattering.
The use of a measured signal correlation (quadratic pressure) at each point with the same reference signal (as signal of reference related to the source). In this case, the measurment is done with phase as a reference. One carries out finally the ratio interspectre/measurement means. This method is called 'technique of measurementof the transfer function'. For vibrating structures, the output signal (as signal of reference related to the source) is cautiously selected. The problem is that in an industrial environment, it is not possible to use the same reference of phase in all frequency bands.