Experimental analysis of the Very Near Field produced by different acoustic sources

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 the most common choice ever. 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, it has become possible to use the microflown in the areas of experimental and operational modal analysis thanks to the so-called Very Near Field (VNF) assumption. It states that in the region really close to 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 two different acoustic sources are analysed: (i) a single excited speaker and (ii) a vibrating PMMA plate, suspended in free-free conditions. In both the cases, FRFs have been estimated by considering a burst random input signal. The output signals have been measured by using (i) a scanning laser Doppler vibrometer and (ii) a microflown probe. It is shown that even when the probe is placed in the closest possible position, the measured particle velocity is not equal to the corresponding surface point velocity, but smaller. The two quantities seem to be in a certain ratio that appears to be quite not dependent on the frequency.