The Shure SM91 is a boundary‑style electret condenser microphone designed to sit directly on a surface, using the boundary effect to pick up sound cleanly and with fewer phase issues. Its half‑cardioid pattern focuses on sound coming from above the mounting surface while reducing noise from behind, which makes it useful for applications like placing it inside a kick drum, on a piano lid, or on the floor near low‑frequency instruments. It offers a wide frequency response of roughly 20 Hz to 20 kHz and can handle high sound levels while maintaining accurate reproduction and good off‑axis performance thanks to its optimized cartridge design. The included external preamp provides a selectable flat or low‑cut response to help manage rumble or excessive low end, and the system can run on either two 9‑volt batteries or phantom power, automatically switching to battery backup if external power is lost.