Beamforming — Steered Microphone Arrays

Beamforming Microphone Arrays

For full beamforming microphone coverage, see audio/beamforming-mics. For AEC interaction with beamforming, see audio/echo-cancellation.

Beamforming is a signal processing technique that uses an array of microphone capsules and DSP spatial filtering to create a directional pickup "beam" that can be steered electronically toward a sound source. Unlike a traditional directional microphone (cardioid, supercardioid) whose pickup pattern is fixed by physical design, a beamforming array can aim at any direction within its coverage zone — automatically tracking active speakers, excluding specific areas (near HVAC registers, near loudspeakers), and combining multiple beams to cover an entire conference table with fewer physical microphones. Beamforming is the foundation of modern conferencing microphone systems (Shure MXA, Biamp Parlé, Sennheiser TeamConnect, Stem Audio, ClearOne Beamforming).

How Beamforming Works

A beamforming array uses multiple omnidirectional capsules arranged in a known geometric pattern (line, grid, circle, or hemisphere). By applying different time delays and gain factors to each capsule's output and summing them, the DSP reinforces sound arriving from the target direction and cancels sound arriving from other directions.

The math is based on delay-and-sum beamforming: sound arrives at each capsule at slightly different times depending on direction. By applying a delay that compensates for this difference, all capsules are aligned in time for the target direction — their outputs add coherently. For other directions, the delays cause destructive interference — signals cancel.

More advanced adaptive beamforming (used in MXA910 IntelliMix and Biamp Parlé) continuously adapts the beam weights based on actual room acoustics and current noise sources, providing better off-axis rejection in real rooms than fixed delay-and-sum.

Key Beamforming Products

Beam Configuration and AEC

Beamforming arrays require careful configuration:

Exclusion zones: define areas the beams should avoid — near loudspeakers, near HVAC registers, at the head of the table where a monitor faces away from participants. Exclusion zones prevent the array from picking up loudspeaker bleed, which would defeat AEC.

AEC reference: all beamforming microphones require a loudspeaker reference signal fed back to the AEC processor. The MXA910 IntelliMix includes onboard AEC; when integrated with an external DSP, the DSP's AEC block needs the loudspeaker output as its reference. Incorrect or missing AEC reference is the most common cause of echo on beamforming systems.

Beam count vs. room coverage: the number of simultaneous active beams determines how many people can speak simultaneously without audio degradation. A 4-beam array covering a 12-person table may struggle when 5+ people speak simultaneously. Some arrays (MXA920, TCC2) use adaptive multi-beam algorithms that automatically allocate beams to active speakers.

Beamforming vs. Traditional Microphone Arrays

Common Pitfalls

• Missing AEC reference causing echo to far end. The loudspeaker output is not routed back to the AEC processor as a reference, so AEC cannot model the echo path. Fix: verify in the DSP design that the loudspeaker output (after all processing and amplification) is routed to the AEC reference input of the MXA, DSP AEC block, or conferencing codec.

• Beamforming array positioned too close to a ceiling loudspeaker. Even with exclusion zones, a loudspeaker at 1 m from the array overwhelms the acoustic isolation. Fix: maintain at least 1.5–2 m between the array and any loudspeaker; use directional ceiling speakers aimed away from the array.

• Exclusion zones not configured for HVAC registers. A ceiling register near the array is picked up as broadband noise; the beams steer toward it. Fix: define an exclusion zone covering the angle from the array to the HVAC register, preventing beam steering in that direction.

• Insufficient capsule density for room coverage. The array's coverage zone is designed for a specific room size; positioning the array to cover a space larger than specified causes weak pickup at the far end of the room. Fix: verify the array's coverage radius against the room dimensions; use multiple arrays for larger rooms.