Near field length, rectangular probe

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.

N = k · L² / (4λ)
L = longer element dimension, W = shorter
k = f(W/L), from the shape-factor table