We’ve just bought a Behringer Xenyx 1222 mixer. When setting the gain, we’re not getting much past 40 percent on the mic channel before we start getting feedback. The mixer has feedback detection but none of the equalizers are indicating any feedback. We use the Hall effect on the mic channel but the feedback occurs without speaking into the mic, and this also occurred when the effects are off. I’ve seen a YouTube review that states using the compressor can cause feedback. Does it? Any suggestions on what the cause could be and how to eliminate it would be welcome!
SOS Forum post
SOS Technical Editor Hugh Robjohns replies: Feedback happens when the system loop gain exceeds unity. In other words, the mic picks up the sound from one or more PA speakers, either directly or after being reflected around the room. The signal from the mic is amplified by the preamp, through the mixer, and then by the PA amplifier (or active PA), before being emitted by the speaker and getting back to the mic. That’s the loop.
If the sum of the gains in that loop add up to more than 0dB overall, the system will ‘howl‑round’. The preamp and power amp add the gain, while the speaker’s inefficiencies and directionality, the mic’s sensitivity and polar pattern, and acoustic losses through the air introduce attenuation. Because the mic’s frequency response isn’t flat (and neither is that of the PA speakers), there will be slightly more gain at one specific frequency than others, and that’s why howl‑rounds occur at that pitch.
A graphic EQ is often used to reduce the gain at the pitch of the howl‑round, which will kill it, but if you then increase the overall system gain it will howl‑round at a new, different frequency where the overall gain is more than 0dB again! Skilful setting of a graphic EQ can usually achieve 6dB or more extra gain in the system than without, but it’s very easy to degrade the system sound quality with too much of this ‘feedback correction’.
The best way to prevent/reduce feedback is to reduce the amount of sound reaching the mic from the PA speakers and/or floor monitors.
The best way to prevent/reduce feedback is to reduce the amount of sound reaching the mic from the PA speakers and/or floor monitors. You can achieve that by making sure the PA speakers are placed well forward of the mics, and/or using PA speakers with well controlled sound dispersion. Also, aim the null(s) in the microphone’s polar pattern to reject as much sound as possible from the floor monitors/PA speakers — and avoid using omnidirectional mics!
Compressors don’t cause feedback per se, but poor use of them can. Most have a ‘make‑up gain’ facility, which is intended to maintain peak levels while raising quieter elements in the signal, but also adds to the overall gain in the system, and as I’ve explained, that’s what causes howl‑rounds. Compressors can be very useful tools for reducing peaks, but only if configured so as to make loud stuff quieter, not to make quiet stuff louder!