A spherical-cap transducer with Legendre shading holds constant beamwidth above a cutoff frequency.

Evidence

Keele (2000, 2002, 2003a & b) extended the work of Van Buren (Rogers and Van Buren, 1978; Van Buren et al., 1983) to develop the constant beam­ width theory (CBT) arrays. In his papers, the transducer is a circular spher­ ical cap of arbitrary half-angle with Legendre function shading. It provides a constant beam pattern and directivity with extremely low side lobes for all frequencies above a certain cut-off frequency. — David Howard, Jamie Angus, coustics and Psychoacoustics, 2900

Categorization

• DEFINE: Names and defines the CBT array, the chapter’s core technical construct: spherical-cap transducer plus Legendre function shading.
• SHARE: The specific pairing of ‘Legendre function shading’ with a spherical cap and a hard cutoff frequency is narrow enough that only a speaker-array hobbyist would care.