The near field is the region where interference between contributions from different parts of the aperture makes the pressure amplitude fluctuate wildly. It ends at the last on-axis maximum, at distance N, beyond which the field decays smoothly and the beam diverges at a fixed angle. For a circular source N = D²/(4λ).

A phased array group is rectangular, not circular: an active aperture A = n × p in the steering plane and a fixed passive aperture W in elevation. The rectangular case is handled with a shape factor k applied to the longer of the two dimensions, N = k·A_long² / (4λ). For a square aperture k is about 1.37 and it falls towards 0.99 as the aperture becomes a long strip.

Near field length is the single most important number in a phased array setup, because focusing is only possible inside it. It also fixes where the beam is naturally narrowest and where far field amplitude laws such as DGS and inverse-distance TCG start to apply.

Note that N is a distance along the sound path in the test material. For an angle beam on a wedge, subtract the wedge path expressed as an equivalent steel path before comparing N with a depth or a metal path.

Compression or shear

Worked example

Elements in the active group	16
Element pitch	0.6 mm
Passive aperture (elevation)	10 mm
Probe frequency	5 MHz
Wave mode	shear
Material velocity	3240 m/s
Active aperture	9.6 mm
Wavelength	0.648 mm
Aperture ratio (short / long)	0.96
Rectangular aperture factor k	1.322
Near field length	51 mm

A = 16 × 0.6 = 9.60 mm, W = 10 mm, so A_long = 10 mm and the ratio is 9.6/10 = 0.96. Interpolating the k table between 0.9 (1.25) and 1.0 (1.37) gives k = 1.25 + 0.6 × 0.12 = 1.322. λ = 0.648 mm, so N = 1.322 × 10²/(4 × 0.648) = 132.2/2.592 = 51.0 mm.