There is substantial interest in sub-megahertz ultrasound fields for therapeutic applications. Hydrophones with sensor apertures that are small compared to the wavelength are used to minimise spatial averaging in field characterisation, but data on effective aperture size are limited at these frequencies. This study presents measured directivity and effective element size of capsule and needle hydrophones at 200–500 kHz. Directivity depends on hydrophone structure, and effective element sizes can be many times larger than the nominal size. Internal reflections cause interference in some cases, creating directional sensitivity fluctuations at non-normal incident angles. These findings will help increase accuracy of sub-megahertz field characterisation.
Xu et al. (Fri,) studied this question.