The Audio-Technica AT 849 stereo condenser boundary microphone
The AT 849, an X/Y stereo condenser boundary microphone with full mono compatibility, is intended primarily for surface-mount applications in broadcast, professional recording, and sound reinforcement. The AT 849 provides full, natural stereo ambience in a rugged, compact design.
Equipped with a pair of wide-range, optimally positioned miniature condenser cardioid elements, the AT 849 provides the spatial impact and realism of a live sound field, while simultaneously remaining full and uniform when operating in mono (L+R). Consistently natural-sounding response is maintained over an arc of approximately 220°, and the L+R mono polar pattern is virtually free of lobes and comb effects. Frequency response is smooth and peak-free over an extended 30 Hz to 20,000 Hz range, with uniform polar response vs. frequency.
The AT 849 requires 9V to 52V DC phantom power on each channel for operation. A built-in two-position switch allows selection of flat or low-roll-off response. Enclosed in a rugged die-cast case and protected by two layers of sturdy perforated steel, the AT 849 is finished in low-reflectance black. The combination of a heavy case and a rubber non-slip bottom pad minimizes mechanical coupling of surface vibrations to the microphone. The included 25' (7.6 m) shielded cable features a five-pin TA5F input connector and two standard three-pin XLRM-type output connectors. A soft pouch is provided to hold and protect the microphone when not in use.
Installation and Operation
The two stereo outputs are low impedance balanced. The balanced signals appear across Pins 2 and 3 of the TB5M output connector for the left channel and Pins 4 and 5 for the right channel. The ground (shield) connection for both channels is Pin 1. Output is phased so that positive acoustic pressure produces positive voltage at Pins 2 and 4 in accordance with industry convention.
The three-pin XLRM connectors for the left and right channels are marked with gray and red bands, respectively. The balanced output signals appear across Pins 2 and 3, while the ground (shield) connections are Pin 1. Outputs are phased so that positive acoustic pressure produces positive voltage at Pin 2. By locating the AT 849 near the sound source, stereo image width will be enhanced, but the room ambience will be decreased. Conversely, as the mic position recedes from the sound source, a narrower left/right stereo image will be achieved and more of the “room sound” will be noted. Experimenting with placement will provide operators with the “feel” for where to position the AT 849 for the best overall effect.
The symmetry and area of the mounting surface directly affect the sensitivity of the boundary microphone at low frequencies, as well as the overall stereo balance and image. Ideally, the mounting surface should be circular; however, square or rectangular surfaces are most often used. If the mounting surface is rectangular, the smaller dimension tends to determine low-frequency cutoff. The microphone should be centered on the surface and positioned with the front of the microphone facing the sound source along the longer dimension of the mounting surface. The sound source should not be below, or higher than 60° above, the plane of the mounting surface.
The AT 849 can be mounted to a flat surface using two panhead screws in conjunction with the keyhole slots on the microphone’s underside. Simply remove the small cut-out section of the rubber pad to reveal the keyhole slots and the low-frequency roll-off switch. The high sensitivity of the AT 849 assures useful output under most circumstances, and it is designed to provide a distortion-free signal even in very intense sound fields. In some cases, however, an attenuator such as the Audio-Technica AT8202 may be required between each microphone output and its preamplifier to avoid overloading sensitive input stages.
Avoid leaving the microphone in the open sun or in areas where temperatures exceed 110° F (43° C) for extended periods. Extremely high humidity should also be avoided.