Shure SM81 Instrument Microphone

General Description

The Shure Model SM81 is a high-quality, unidirectional condenser microphone designed for studio recording, broadcasting, and sound reinforcement. Its wide frequency response, low noise characteristics, and low RF susceptibility have made it a standard for applications involving acoustic instruments, especially guitar, piano, and cymbals. The SM81 is ruggedly constructed. It operates on phantom power and performs over a wide range of temperatures and humidity conditions. It is furnished with a swivel adapter, attenuator-switch lock, foam windscreen, and case for carrying and storage. Other accessories are available.

Power Requirements

This microphone requires phantom power and performs best with a 48 Vdc supply (IEC-61938). However, it will operate with slightly decreased headroom and sensitivity with supplies as low as 11 Vdc. Most modern mixers provide phantom power. You must use a balanced microphone cable: XLR-to-XLR.

Selecting Low-Frequency Response

A three-position switch allows you to adjust the low-frequency response of the microphone. Use these settings to reduce wind noise, room noise, or proximity effect.

• Flat response: Provides the most natural sound in most applications.
• Low-frequency cutoff: Attenuates 18 dB-per-octave below 80 Hz. Helps eliminate floor rumble and low-frequency room noise from heating and air conditioning systems. This setting may also be used to compensate for proximity effect or to reduce low frequencies that make an instrument sound dull or muddy.
• Low-frequency rolloff: Attenuates 6 dB-per-octave below 100 Hz. Compensates for proximity effect or to reduce low frequencies that could make an instrument sound dull or muddy.

Setting Attenuation

The attenuation switch reduces the signal level from the cartridge by 10 dB without altering the frequency response. This can prevent extremely high SPLs (e.g., close-miked drums and guitar cabinets) from overloading the microphone. To activate attenuation, rotate the switch to the "-10 dB" position.

Note: Turning the ring part way does not produce incremental levels of attenuation.

The attenuation switch may be locked in place. Use the following steps:

1. Unscrew the grille and cartridge assembly.
2. Rotate the attenuation switch to the desired position (0 or –10).
3. Insert the ring lock (small, clear piece of plastic) in the area behind the ring between the pin and slot.
4. Replace the grille and cartridge assembly.

Proximity Effect

Unidirectional (cardioid) microphones progressively boost bass frequencies by 6 to 10 dB below 100 Hz when the microphone is at a distance of about 6 mm (1/4 in.) from the sound source. This phenomenon, known as proximity effect, can be used to create a warmer, more powerful sound. To prevent explosive low frequency sound during close-up use, the bass response gradually rolls off. This provides greater control and helps the user take advantage of proximity effect.

Specifications

[1] 1 Pa=94 dB SPL

[2] S/N ratio is difference between 94 dB SPL and equivalent SPL of self noise, A-weighted

Diagrams and Visuals

Block Diagram

The block diagram illustrates the signal path within the SM81 microphone. It shows the signal flow from the Capacitor Cartridge through an FET Impedance Converter, an LF Response Filter, a Class A Compound Amplifier, a Balanced Transformer, and an RFI Filter, before passing through a Regulator/Reverse Voltage Protect circuit to the output.

Typical Frequency Response

The frequency response graph displays decibels (dB) on the vertical axis (ranging from -20 dB to +20 dB) against frequency in Hertz (Hz) on the horizontal axis (from 20 Hz to 20,000 Hz). It shows three distinct curves, representing the microphone's response at a distance of 1 meter. The curves indicate a generally flat response across the audible spectrum, with a slight attenuation at very low frequencies and a subtle boost in the upper mid-range frequencies.

Typical Polar Patterns

Two polar pattern charts are presented, illustrating the microphone's directional sensitivity. Both charts use degrees on the outer ring (0° to 180°) and decibels (dB) on the inner concentric circles (-20 dB to -5 dB). The left chart shows patterns for 100 Hz, 500 Hz, and 1000 Hz, all demonstrating a clear cardioid pickup pattern, with maximum sensitivity at 0° (front) and significant rejection at 180° (rear). The right chart shows patterns for 2000 Hz, 5000 Hz, and 10000 Hz, also confirming a cardioid pattern, with similar directivity across these higher frequencies.

Overall Dimensions

The overall dimensions of the SM81 microphone are provided. The total length is 212 mm (8.35 inches). The diameter at the grille end is 23.5 mm (0.93 inches), and the diameter at the connector end is 20 mm (0.79 inches).

Accessories

Furnished Accessories

Optional Accessories

Replacement Parts

Certifications

This product meets the Essential Requirements of all relevant European directives and is eligible for CE marking. The CE Declaration of Conformity can be obtained from: www.shure.com/europe/compliance

Authorized European representative:

Shure Europe GmbH
Global Compliance
Jakob-Dieffenbacher-Str. 12
75031 Eppingen, Germany
Phone: +49-7262-92 49 0
Email: info@shure.de
Website: www.shure.com