On the Very Near Field assumption in vibration measurements by p-u probes

Non contact techniques are of fundamental importance in vibration measurements in all that cases in which the use of accelerometers is not suggested or actually possible. In this area, Laser Doppler Vibrometry (LDV) has represented ever the most common choice. Since the microflown (p-u) probe has been industrially developed and introduced on the market, it has become an interesting alternative, adding also the flexibility related to the fact of being actually a sound intensity probe. Several industrial applications have been thought and developed in both the field of noise and vibrations. In the latter, in particular, the microflown employment in the field of experimental and operational modal analysis has become possible thanks to the so-called Very Near Field (VNF) assumption. It states that in the region really close to a vibrating source, e.g. a vibrating surface, the air particle velocity measured by the microflown in that point where the probe is positioned equals the velocity of the corresponding point on the surface. The structure-borne noise produced by the vibration is measured by the probe, in terms of sound pressure and particle velocity and this information can be effectively utilised as a direct measurement of the vibration itself in the VNF. In this paper the preliminary results of a still ongoing research activity are presented: A stationary sound source consisting of two different speakers belonging to the same frame has been considered, (i) in the case they vibrate in phase and (ii) out of phase and the vibration of the whole system has been measured by using a 3D p-u probe. Several configurations have been, in particular, investigated, by changing the relative distance between the speakers, their position inside the frame and the composition of the sound emitted, with the aim of showing what does happen in these cases