A specialized audio mixer that senses the presence or absence of a signal from a microphone and turns it on or off without requiring human intervention. Automatic mixers can have a single channel or multiple channels.
Most automatic mixer designs are based on a threshold setting that defines the minimum SPL the mixer will recognize as a valid signal. One of two basic approaches is then used. In the first, the mixer operates as a gate and shuts off all signal from the mic, and then reactivates it once the signal level rises above the threshold. In the second approach, the mixer attenuates the signal by 15-20dB, and then gradually increases it as the source volume increases. Fans of this approach point out that it does away with the “choppy” action of gates, which instantaneously changes from full attenuation to full gain.
Automatic mixers are often found in audio-for-video recording, broadcast TV and radio, and houses of worship.