The Very Near Field II An introduction to Very Near Field Holography

In [1] is shown that very close (distance smaller than L/2Π) to a vibrating and flat structure with typical size L, the normal particle velocity and the (normal) structural velocity coincide. It is shown that with an acoustic particle velocity sensor the normal acoustic particle velocity close to the surface can be measured and that this velocity coincides with the surface velocity that is measured with a laser vibrometer or accelerometer.

In [1] it is also shown that in the very near field (contrary to the behavior in the near field) the particle velocity level doesn't depend upon frequency. Furthermore, in the very near field, there is a linear relationship between the particle velocity as measured and the structural velocity to be measured.

In this paper it is shown that the normal acoustic particle velocity appears to be a good representation of the structural velocity up to distances of L/2 (with L the size of the vibrating structure) and lateral movements of the structure are not measured by the lateral particle velocity. It is also shown that the lateral acoustic particle velocity is zero if the normal velocity is maximal.

A simple holography method is presented based on the very near field concept.