The active aperture is the total width of the elements that are pulsed together to form one beam. For a linear array it is simply the number of elements in the group multiplied by the pitch, A = n x p. Pitch is measured centre to centre, so n x p slightly overstates the physical piezo width: the true edge-to-edge dimension is n x p - g, one inter-element gap short.

Aperture controls almost everything else in the setup. Near field length grows with the square of the aperture, beam width at the focus falls in proportion to 1/A, and focusing is only possible inside the near field. Doubling the number of elements in the group therefore quadruples the depth at which the beam can still be focused.

The useful figure of merit is the aperture measured in wavelengths, A/lambda. Below about 5 wavelengths the group behaves almost like a point source: it radiates a wide, weakly directional beam that cannot be focused usefully. Above about 10 wavelengths the beam is directional and focal laws behave as the textbook formulae predict.

The element width e = p - g is what limits steering. A wide aperture built from wide elements gives a strong, narrow beam that will not steer; the same aperture built from finer elements steers well but needs more channels. Pitch and element width are fixed by the probe, so aperture is the only variable left on the day.

Compression or shear

Worked example

Elements in the active group	16
Element pitch	0.6 mm
Inter-element gap (kerf)	0.1 mm
Wave mode	shear
Probe frequency	5 MHz
Material velocity	3240 m/s
Active aperture	9.6 mm
Physical aperture (edge to edge)	9.5 mm
Wavelength	0.648 mm
Aperture in wavelengths (A/λ)	14.81
Element width	0.5 mm

16 elements at 0.6 mm pitch: A = 16 × 0.6 = 9.60 mm, edge to edge 9.60 − 0.10 = 9.50 mm, element width 0.6 − 0.1 = 0.500 mm. At 5 MHz shear in steel λ = 3240/(5 × 1000) = 0.648 mm, so A/λ = 9.6/0.648 = 14.81.