Shure SM91 User Manual

222 HARTREY AVE., EVANSTON, IL 60202-3696 U.S.A.
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THE SOUND OF THE PROFESSIONALS@

GENERAL

The SM91 Microphone is designed specifically for
surface-mounted applications. It is a professional­quality permanently-biased condenser microphone with
a half-cardioid directional pattern (cardioid in the hemi­sphere above the mounting surface.)

The SM91 takes advantage of the well-known prin­ciple that, at a barrier or boundary, sound pressure
doubles compared to its value if the boundary is re­moved. When placed sufficiently near the boundary sur­face, a microphone has effectively 6 dB higher sensiti­vity and approximately 3 dB greater rejection of random
background noise.

Because of its half-cardioid polar pattern, the SM91
surface-mounted microphone discriminates against
sounds originating from the rear, suiting the SM91 for
conditions where an omnidirectional pattern makes
other surface-mounted microphones impractical. The
intrinsic unidirectionality of the SM91 can be a great
benefit when it is desirable to isolate a particular
vocalist, instrument, or group from the rest of an ensem­ble being recorded. Because of a cardioid pickup pat­tern, no physically isolating barriers are required, and
directionality is maintained to low frequencies.

The SM91 can be used for individual instrument

pickup, e.g., mounted inside the lid of a grand piano or
on the floor next to a bass drum. Experimental place­ment and critical listening will demonstrate the best lo­cation for any particular purpose or effect desired.

The meticulously optimized design of the SM91 in-

cludes a totally new cartridge, developed at Shure. The

result is high output, notably accurate sound reproduc­tion over the entire audio frequency range, and off-axis

performance comparable to the finest unidirectional

microphones. The supplied low-distortion, high-

PROFESSIONAL PRODUCTS

clipping-level preamplifier provides switch-selectable

Flat or Low-Cut response for further assistance in ob­taining the best possible microphone signal even under
difficult conditions.

The SM91 is powered either by two readily available

9-volt alkaline batteries (300 hours continuous battery

life) or by an 11-to-52 Vdc simplex (phantom) supply
from broadcast, sound-reinforcement, or recording
equipment. The system is designed so that the battery
supply will automatically switch in should the simplex
power fail.

The SM91 consists of a small, rugged, surface­mounted microphone finished in professional durable
matte-black enamel; a 7.6m (25 ft) small diameter two-

conductor shielded interconnecting cable with two
3-socket miniature Switchcraft connectors; and a stur­dy preamplifier assembly with battery compartment,
battery On/Off switch, green LED battery-condition in­dicator, low-frequency Flat/Lo Cut switch, and standard
3-pin professional audio connector output.

Features:

Wide flat frequency response for accurate sound

reproduction across the audio spectrum

Switch-selectable 12 dB/octave low-frequency
cutoff permits tailoring response to suit conditions

Half-cardioid polar pattern minimizes pickup from

rear of microphone, permits aiming microphone,
e.g., toward performers and away from audience, or
toward singers and away from instruments

High sensitivity and high signal-to-noise-ratio
Very low distortion and high output clipping level

Battery or simplex powering: uses standard 9-volt

alkaline batteries; accepts wide range of simplex
voltages - 11 to 52 Vdc

Low susceptibility to RFI, electrostatic and electro-

magnetic hum

Extremely rugged construction of both microphone

and preamplifier for outstanding reliability

Low profile and matte black finish for unobtrusive

appearance on-camera or onstage; on floor, table,

ceiling, wall, or lectern

Usable over very wide range of temperature and

humidity

Copyright 1988, Shure Brothers Inc
27A2161 (HD)

Printed in U.S.A.

SPECIFICATIONS

Type

Cardioid condenser (electret bias) for surface
mounting

Frequency Response

20 to 20,000 Hz at 30° incidence to infinite surface
(see Figure 1)

TYPICAL FREQUENCY RESPONSE

FIGURE 1

Polar Pattern

Half-cardioid (cardioid in hemisphere above mounting
surface), uniform with frequency, symmetrical about
axis (see Figure 2)

TYPICAL POLAR PATTERN

FIGURE 2

Output Impedance

Rated at 150 ohms (90 ohms actual)
Recommended minimum load impedance: 800 ohms
(May be used with loads as low as 150 ohms with
reduced clipping level)

Output Level

at 30° incidence to infinite surface)

Open Circuit Voltage . . . . . . . . . . . -71.5 dB (0.27 mV)

0 dB = 1 V/µbar

Preamplifier Output Clipping Level

than 0.1% THD)

800 ohm load

150 ohm load

Maximum SPL (at 1,000 Hz, sound source at 30° in­cidence to infinite surface)

800 ohm load . . . . . . . . . . . . . . . . . . . . . . . . . .

150 ohm load . . . . . . . . . . . . . . . . . . . . . . . . . .

Hum Pickup

-13.5 dB equivalent SPL in 1 m0e field (60 Hz)

Output Noise

25.5 dB SPL, A-weighted

28.5 dB SPL, weighted per DIN 45 405

Signal-to-Noise Ratio

68.5 dB re 94 dB SPL

Dynamic Range

121 dB

(at 1,000 Hz, measured with sound source

(at 1,000 Hz, less

. . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

0 dBV (1.0V)

-13 dBV (0.22V)

146.5 dB

136.5 dB

Phasing

Positive pressure on diaphragm produces positive
voltage on pin 2 relative to pin 3 of preamplifier output
connector.

Power

Battery:

mately 300 hours continuous with fresh alkaline bat­teries

Simplex Voltage:
9 Vdc; 1.8 mA current drain; permissible to use
simplex power with batteries in place or removed

Switches and Indicator

Two recessed slide switches and green LED indicator

on top surface of preamplifier assembly

Battery On/Off Switch:

batteries or to use batteries as backup power in case

of interruption to external power supply (no battery

drain occurs as long as simplex voltage source is

greater than battery voltage). Move to Off to prevent

battery drain when unit is not in use.

LED Indicator: Green LED flashes momentarily

when switch is moved to On to indicate at least 8

hours of battery life remaining.
Flat/Lo Cut Switch:

rolloff below 30 Hz; In Lo Cut position, 12 dB/octave
rolloff below 80 Hz (see Figure 1)

Cable

7.6m (25 ft) two-conductor shielded, small diameter,
interconnecting cable with 3-socket miniature Switch­craft connector on each end to mate with microphone
output connector and preamplifier input connector

Case

Microphone:
perforated steel grille with replaceable or cleanable

fine mesh screen and foam pad wind/dirt barrier

Preamplifier: Matte black enamel die-casting

Dimensions

See Figure 3

Environmental Conditions

Operating Temperature:
Storage Temperature:

Relative Humidity (Operating or Storage):

Net Weight

Microphone:
Preamplifier: 435 grams (15.4 oz) less batteries

Two 9-Vdc alkaline (NEDA 1604A), approxi-

11 to 52 Vdc, operational down to

Move to On to power from

In Flat position, 6 dB/octave

Matte black enamel die-cast base and

OVERALL DIMENSIONS

FIGURE 3

-18° to 57°C (0° to 135°F)

-29° to 74°C (-20° to 168°F)
0 to 95%

263 grams (9.3 oz) less cable

-2-