The AKG C391B is a compact, small-diameter condenser microphone designed for professional studio recording and live sound applications where accurate, detailed sound capture is required. Its small-diaphragm design offers several advantages, including an extended high-frequency response, precise transient detail, and a wide, linear frequency pickup, making it particularly well-suited for acoustic instruments, drum overheads, hi-hats, piano, and orchestral recording. The microphone features a cardioid polar pattern, which provides focused front-side pickup while effectively rejecting sound from the rear, helping to isolate the source and minimize room ambience or stage noise. This combination of clarity, precision, and directionality has established the C391B as a versatile workhorse in both fixed installations and on touring rigs.

The microphone incorporates a permanently polarized condenser capsule, eliminating the need for external polarizing voltage and contributing to its reliability. It is powered by standard 48V phantom power and features a robust, die-cast metal body with a satin nickel finish that is both durable and resistant to RF interference. A significant feature of the C391B is its three-position bass-cut switch, located on the body of the microphone. This switch provides preset low-frequency roll-off filters (-6 dB at 75 Hz and -12 dB at 150 Hz), allowing engineers to quickly reduce proximity effect or low-frequency rumble without resorting to external equalization, which is especially useful when close-miking instruments or in live sound situations.

Designed for versatility, the AKG C391B delivers a clean, transparent sound characteristic that faithfully captures the source without adding significant coloration. Its high SPL handling and low self-noise make it suitable for everything from delicate classical recordings to capturing the explosive transients of a snare drum or crash cymbal. The microphone is typically supplied with a protective pouch and a shock mount to isolate it from mechanical vibrations. By offering professional-grade performance in a sturdy, feature-rich package, the AKG C391B provides engineers and musicians with a reliable and sonically accurate tool for capturing a wide array of sound sources with detail and fidelity.