Shure SM91A User Guide

GENERAL

The SM91A Microphone is designed specifically for surface-mounted applications. It is a professional-qual- ity permanently-biased condenser microphone with a half-cardioid pickup pattern (cardioid in the hemisphere above the mounting surface.)

The SM91A 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 SM91A surface-mounted microphone discriminates against sounds originating from the rear, suiting the SM91A for conditions where an omnidirectional pattern makes other surface-mounted microphones impractical. The intrinsic unidirectionality of the SM91A 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 SM91A 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 help to determine the best location for any particular purpose or effect desired.

The meticulously optimized design of the SM91A includes a unique cartridge, developed at Shure. The result is high sensitivity, exceptionally accurate sound reproduction over the entire audio frequency range, and freedom from off-axis coloration comparable to the finest unidirectional microphones.

The furnished ILP-1 preamplifier is a high-clipping level low-noise unit, phantom powered by 11 to 52 Vdc. It has selectable gain of 0 or +10 dB, and switchable lowfrequency response, flat or low cut (rolled off at 12 dB/ octave). The low cut is particularly useful to minimize pickup of ambient low-frequency noise from sources like heating or air-conditioner fans, for instance.

The SM91A consists of a small, rugged, surfacemounted microphone finished in professional durable matte-black enamel, a 7.6 m (25 ft) small diameter twoconductor shielded interconnecting cable with two 3-socket miniature Switchcraft connectors; and the ILP-1 preamplifier assembly with standard 3-pin professional audio connector output.

Features:

•Wide flat frequency response for faithful sound reproduction across the audio spectrum

•High sensitivity, low self-noise

•Very low distortion and high output clipping level

•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

•Low susceptibility to RFI, electrostatic and electromagnetic hum

•Extremely rugged construction of both microphone and preamplifier for outstanding reliability

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

•Switch selectable preamp gain of 0 or +10 dB

•Accepts phantom power of 11 to 52 Vdc, permits use of wide variety of amplifiers or phantom supplies

•Standard 3-pin male XLR-type output connector on preamp can be directly connected to any 3-pin female XLR-type phantom-powered input

•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 1993, Shure Brothers Inc.	Printed in U.S.A.
	27A2669(MC)

SPECIFICATIONS Type

Cardioid condenser (electret bias) for surface mounting

Frequency Response

20 to 20,000 Hz (see Figure 1)

TYPICAL FREQUENCY RESPONSE (measured at 30o incidence to infinite surface)

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 Ω (90 Ω actual)

Recommended minimum load impedance: 800 Ω (May be used with loads as low as 150 Ω with reduced clipping level)

Output Level (at 1,000 Hz, measured with sound source at 30o incidence to flat surface)

Open Circuit Voltage . . . . . . . . . ±71.5 dB (0.27 mV) 0 dB = 1 V/μbar

Preamplifier Output Clipping Level (at 1,000 Hz, less than 0.1% THD)

			GAIN
		0		+10
. . . . . . . . . . . . . .800 Ω load	. 0 dBV			±7.0 dBV
	(1.0 V)			(0.45 V)
150 Ω load . . . . . . . . . . . . .	±13.0 dBV . ±21.0 dBV
	(0.22 V)			(0.09 V)

Maximum SPL (at 1,000 Hz, less than 1% THD) 800 Ω load

ILP-1 gain set at 0 dB . . . . . . . . . . . . . . 146.5 dB ILP-1 gain set at +10 dB . . . . . . . . . . . 136.5 dB

150 Ω load

ILP-1 gain set at 0 dB . . . . . . . . . . . . . . 129.5 dB ILP-1 gain set at +10 dB . . . . . . . . . . . 118.5 dB

Electromagnetic Hum Pickup (maximum)

±0.5 dB equivalent SPL in 1 mOe field (60 Hz)

Preamplifier Controls

Selectable Low-Frequency Response: Flat/Low Cut (12 dB/octave rolloff below 80 Hz)

Selectable Gain: 0/+10 dB

Output Noise

25.5 dB SPL, A-weighted

32 dB SPL, C-weighted

28.5 dB SPL per DIN 45 405

Signal-to-Noise Ratio

68.5 dB re 94 dB SPL

Dynamic Range

121 dB (800 Ω load, gain set at 0 dB)

Phasing

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

Power

11 to 52 Vdc phantom (simplex) voltage (operational down to 9 Vdc with reduced clipping level); current drain 2.2 mA at 52 Vdc, 1.8 mA at 11 Vdc

Case

Microphone: Matte black enamel die-cast base and perforated steel grille with replaceable or cleanable fine mesh screen and foam pad wind/dirt barrier Preamplifier: Matte black enamel finished steel

Cable

7.6 m (25 ft) two-conductor shielded, small diameter, with Switchcraft Tini (Q.G.) 3-pin (female) connector on each end

Environmental Conditions

Operating Temperature: ±18 to 57o C (0 to 135o F) Storage Temperature: ±29 to 74o C (±20 to 168o F) Relative Humidity: 0 to 95%

Dimensions

See Figure 3

OVERALL DIMENSIONS

FIGURE 3

2.