A rectangular element does not behave like a circular one of the same area. The near-field length is governed by the long dimension, corrected by a shape factor that depends on how square the element is: N = k · L² / (4λ), where L is the long side and k is read from the ratio of short side to long side.

For a square element (ratio 1.0) k is 1.37 — a square element has a near field 37 % longer than the simple L²/4λ estimate. As the element becomes more elongated k falls towards about 0.99, so a long, narrow aperture behaves close to the plain formula. The table below is the standard shape-factor set used for rectangular transducers.

This is the calculation that matters for phased-array apertures and for the rectangular elements in twin-crystal and immersion probes. For a linear array the long dimension is the active aperture — the number of elements fired multiplied by the element pitch — and the short dimension is the element elevation. Growing the aperture by firing more elements lengthens the near field as the square of the aperture, which is why a large aperture can push the whole inspection depth into the near field and make amplitude sizing invalid.

Enter the two element dimensions in either order; the calculator sorts them. Remember that in shear mode the wavelength is roughly half the compression value, so the near field is roughly double.

Compression or shear

Worked example

Wave mode	compression
Material velocity (override)	0 m/s
Probe frequency	5 MHz
Element dimension A	20 mm
Element dimension B	10 mm
Velocity used	5900 m/s
Wavelength λ	1.18 mm
Long dimension L	20 mm
Short dimension W	10 mm
Aspect ratio W/L	0.5
Shape factor k	1.01
Near field length N	85.59 mm
Start of reliable far field (3N)	256.78 mm

20 mm × 10 mm aperture, 5 MHz compression in steel. λ = 1.18 mm; W/L = 0.5 so k = 1.01; N = 1.01 × 20² / (4 × 1.18) = 404 / 4.72 = 85.59 mm. A 20 × 20 mm square aperture at the same frequency would give k = 1.37 and N = 116.1 mm.