Focal spot size

A focused probe — a curved element, an acoustic lens, or a phased-array focal law — concentrates the beam at a chosen distance. The −6 dB diameter of the spot it makes is BD = 1.02 · λ · F / D, where F is the focal length in the material and D is the aperture. Smaller wavelength or bigger aperture gives a tighter spot; a longer focal length gives a wider one.

The hard limit is the near field. A probe cannot be focused beyond its own near-field length. The normalised focal length S_F = F/N must be less than 1; at S_F = 1 the ‘focus’ is just the natural last maximum of the unfocused beam and no gain is available. Practical focusing is normally limited to about S_F = 0.6 or less, which is why increasing the aperture (or the frequency) is the only way to focus deeper.

Focusing buys sensitivity and lateral resolution in a narrow band of depth and pays for it everywhere else. A tight spot raises the echo from a small reflector at the focus by many dB, so focused probes are used for small-defect detection, near-surface resolution, corrosion mapping and flaw sizing — but the beam diverges rapidly past the focal zone and the technique must be qualified at the depth of interest.

Use the focal zone length calculation alongside this one: the spot diameter tells you how fine the beam is at the focus, the focal zone tells you over what depth range that focusing is actually useful.