Shure Beta 91A
A half-cardioid condenser that turns whatever surface it sits on into part of the pickup. 470 grams of boundary mic with the preamp built in.
Overview
The Beta 91A is a boundary microphone, so it doesn't work like the mic most people picture. Lay it flat on a floor, a drum shell, or the bottom of a piano and the surface becomes part of the pickup. Shure's own description calls it "very lightweight, unobtrusive and space-saving due to its special design," and at 470 grams it is all three. The half-cardioid pattern - cardioid in the hemisphere above the mounting surface - is what makes that possible. It also means the mic picks up the floor, the shell, and whatever else the mounting surface is doing. Whether that is the whole point or the whole problem depends on how the room sounds.
The big difference from a conventional mic on a stand is the integration. The preamplifier and the XLR connection live inside the body, so there is no inline preamp to lose and no mini-XLR cable to fail. The original Beta 91 needed an inline preamp connected via mini-XLR. The 91A also has an updated capsule tuned for a more natural high-frequency response than the original. The numbers back that up: roughly 6 dB less self-noise (29.5 dBA against 35 dBA) and higher sensitivity (3.8 mV/Pa against 1.1 mV/Pa), according to RecordingHacks' comparison of the two. Shure's own literature describes it as a combination of the Beta 91 and the SM91.
The spec sheet reads like a mic built for abuse. Frequency response runs 20 Hz to 20,000 Hz, output impedance is 146 Ω, sensitivity is rated at - 48.5 dBV/Pa, and maximum SPL is 155 dB at a 2500 Ω load. That ceiling is the number that counts for the intended job. You won't overload this mic with a snare hit or a close-miked kick. Phantom power draw is modest at 5.4 mA, and the mic accepts 11-52 V, so nearly any mixer or interface will run it.
One quirk: maximum SPL, clipping level, and dynamic range all shift with the input load impedance of the preamp. Shure recommends a minimum of 1000 Ω, and most modern preamps clear that easily, but the 2500 Ω column reads better across the board. Higher impedance yields better performance on those specs.
The contour switch on the bottom of the body handles tonal shaping without tools. Flat is the natural setting for most work. Low-Mid Scoop applies 7 dB of attenuation to tailor the response for a strong punch in the low frequencies and plenty of attack up top. Shure frames the switch as a way to enhance the attack and clarity of bass-heavy instruments - bass drum on one setting, piano or voice applications on the other. The manual's spec table prints the attenuation center as "400 kHz," which is almost certainly a typo for 400 Hz. The surrounding text and the 7 dB low-mid scoop description only make sense that way. [VERIFY: contour center frequency]
Key Features
Half-Cardioid Boundary Design
A boundary mic's response is tied to the surface it sits on, and the 91A is designed around that fact. The half-cardioid pattern picks up the hemisphere above the mounting surface, which is why the mic can sit inside a kick drum or on the underside of a piano lid and still deliver a usable signal. It is also why placement and room treatment matter more with this mic than with a conventional stand-mounted design. Shure's own manual notes that location and room acoustics strongly affect sound quality, and that the best result for a given application may take some experimenting with placement and room treatments.
Integrated Preamplifier and XLR Connection
The preamp and XLR output are built into the body. Compared to the original Beta 91, which needed an inline preamp on a mini-XLR cable, that means fewer parts to lose, fewer connections to fail, and less stage clutter. The output is a standard three-pin XLR male, balanced. For long-term installation, the mic can be mounted to a surface using the mounting holes on the bottom of the body.
Contour Switch
The two-position switch sits on the bottom of the mic and needs no tools. Flat is the natural setting for most applications. Low-Mid Scoop reshapes the response for a stronger low-frequency punch and more high-frequency attack - useful when a kick drum needs to cut through a dense mix, or when a piano needs emphasis without mud. The switch is the feature most likely to be damaged on a used example, since it sits exactly where the mic gets set down on a stage floor.
Load-Dependent Performance
Maximum SPL, output clipping level, and dynamic range all vary with the input load impedance of the preamplifier. Shure recommends a minimum input load impedance of 1000 Ω, and most modern preamps meet that. Higher impedance yields better performance on those specs, which is why the 2500 Ω figures, 155 dB max SPL among them, show the mic at its best.
Historical Context
The Beta 91A is an update of the original Beta 91, with a new capsule design for a more natural high-frequency response and a built-in XLR jack replacing the inline preamp arrangement. Studio Sound (1999, p.49) listed the Beta 91 in a microphone roundup, noting it was designed for live performance and recording applications and describing a neck drum mount usable in horizontal or vertical configurations on drum rims, cymbal stands, or typical hardware. Shure's advertising through the 2010s positioned the whole Beta line on construction quality, low handling noise, and high gain-before-feedback. Mix (April 2013, p.7) ran a Beta rebate promotion listing the 91A among the instrument and drum models in the family. That is marketing, not a verdict.
The 91A shows up in working rigs more than in studio closets. Mix (March 2014, p.27) documented a touring drummer's kit running a Beta 91A alongside a Beta 52 on kick, with Beta 57 and SM57 on snare, a KSM 137 on hi-hat, and a Beta 98AMP on toms. In that two-mic kick setup the 91A sits inside the drum for attack and the 52 sits outside for body. That gets around the boundary mic's one real limitation: it needs a surface, and it needs to be inside or against something. It is not a handheld, and it is not a mic you point at a source from across the room.
Collectibility & Value
The 91A is not a collector's piece. It is a tool, and it trades on the used market as one. The main inspection point on any used example is the contour switch, which sits on the bottom of the mic where it takes the abuse of being set down on a stage floor. Buyers should check that it still detents cleanly at both positions and that the XLR connector has not been stressed. A working example should ship with the 26A21 storage bag, which is the original accessory. New retail listings have shown around $370 on a Canadian site, though no date was attached to that figure, and rental houses have listed it from €16.90 per month. There is no sourced original launch price or current market price data, so treat any figure with caution. [VERIFY: current used market pricing]
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