Near Field Acoustic Characterization system

In November 2016, the DRDO had handed over four Naval Systems to the Indian Navy including ABHAY – Compact Hull Mounted SONAR for Shallow Water Crafts, HUMSA UG – Upgrade for the HUMSA SONAR System, NACS – Near-field Acoustic Characterisation System, and AIDSS – Advanced Indigenous Distress SONAR System for submarines.  We have already discussed them in other article, here we discuss the Near field acoustics.

NACS determines the in-situ performance of the SONAR systems which are used to find the frequency-dependent 3-D transmission and reception characteristics of the SONAR. It is also used to measure the magnitude and phase characteristics of the SONAR transmission and reception electronics and the transducers. The NACS has been integrated with the HUMSA-NG SONAR array and proved onboard Naval platforms and this will be fitted on all frontline ships.

What is Near-field Acoustics?

In sonar theory the near-field/farfield boundary is an important concept. The reason for this is that a targets scattering characteristics is different in the near-field than in the far-field. In long ranges a underwater target is seen as point expressed by a single highlight. But from short ranges an extended underwater target can have several distribution characteristics.

The radiating near-field is the region between the reactive near-field and far-field. In this region the reactive fields are not dominant and the radiating fields begin to emerge. In this region the shape of the beam pattern may vary greatly with distance.  Within the near-field the wave fronts produced by the transducer is not parallel. The intensity of the wave oscillates with range. This means that the echo levels from targets within the near-field region can vary greatly with small changes in location. If the source is located close to a transducer, the wave front of the propagating wave is curved with respect to the dimensions of the transducer and the wave propagating direction depends on sensor location