https://www.physikinstrumente.com/en/about/news-press/news-detailpage/piezoelectric-spheres-and-hemispheres-in-miniature-format-for-ultrasonic-sensor-applications
Piezoelectric Spheres and Hemispheres in Miniature Format
PI Ceramic also manufactures piezoelectric spheres and hemispheres in miniature format.
spheres and hemispheresin miniaturepiezoelectricformat