Shure SM-91
A boundary microphone that solved the omnidirectional problem - and made carpet strips obsolete.
Overview
Shure's own advertising for the SM-91 opened with a claim that was, for once, literally true: the world's first unidirectional surface-mounted condenser microphone. Before the SM-91, every boundary or "pressure zone" mic on the market was omnidirectional. Getting any directionality out of one meant building physical baffles, laying carpet strips, or otherwise putting a crude barrier around the capsule. Shure's pitch was blunt about it - no carpet strips or plastic baffles needed. The SM-91 did the isolation electrically and acoustically instead of mechanically, using a condenser cartridge with a half-cardioid pattern.
The half-cardioid pattern is the whole story here. Boundary microphones work on the boundary effect: put a capsule close enough to a large flat surface and the reflected sound arrives essentially in phase with the direct sound. That is why a pressure-zone mic picks up so much more cleanly than a free-standing one. Shure's advertising described the effect as as much as 6 dB higher sensitivity and roughly 3 dB greater rejection of random background noise, compared to the same capsule in free air. That's a real gain for a mic meant to sit on a conference table or a stage floor. The catch, historically, was that boundary mics were omni, so they grabbed the whole room along with the source. The SM-91's half-cardioid element changed that. Mix (December 1984, p.109) explained that the unidirectional pattern allowed effective isolation without the physical barriers that pressure-zone models normally required.
The preamplifier is a small in-line unit. It runs on either two standard 9-volt batteries or phantom power from 11 to 52 VDC, a range wide enough to cover essentially any mixer or snake input a working engineer is likely to meet. The preamp has a switch for flat or low-cut response. The low-cut position rolls off below 80 Hz at 12 dB per octave, which is the setting for a large hollow stage or a conference table that transmits every footstep and chair scrape. Shure advertised the preamp as "ahead of its time," with an ultra-low noise design, a strong signal-to-noise ratio, and a high output clipping level. That last spec matters for a boundary mic on a drum kit or a piano lid, since the preamp needs headroom before anything else.
Mix (July 2008, p.57) documented a drum tech's kick-drum setup running an SM-91 alongside a Beta 52 inside the drum. Kick drum and stage floor are where the boundary effect earns its keep. The mic gets the low-frequency reinforcement of the surface without the phase cancellation that plagues a free-standing mic shoved inside a drum. Shure also pitched it for meeting and conference rooms, a less glamorous but equally sensible job. The half-cardioid pattern keeps HVAC rumble and the room's far wall out of the pickup while the boundary effect does the heavy lifting on the voices around the table.
The SM-91 is physically small and built to be stepped on. The die-cast base is finished in matte black enamel with a perforated steel grille. The capsule sits behind a replaceable fine-mesh screen, with a foam pad serving as both wind and dirt barrier. Audio (June 1986, p.124) listed the microphone at 9.3 ounces and the preamplifier at 11.3 ounces including cable, with the mic itself measuring roughly 2.36 inches wide by 4.37 inches deep by just over an inch tall. It lies on the surface rather than standing on it.
Shure also offered the SM-90, which the company described as identical in appearance to the SM-91 and just as rugged. The difference is that the SM-90 is the omnidirectional version, for engineers who wanted the boundary effect without the directional pattern. Worth knowing when hunting for either model, because the two look the same on a used-gear listing and the model number is the only reliable way to tell them apart.
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