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g/av • Posted by u/dante_dave •

Microphone feedback loop in a church — can't get the system to ring out properly

Small church, 250 seats, problematic acoustic space (hard walls, low ceiling, parallel reflective surfaces). Two Shure SM58s on stage, 4 JBL SRX815P tops. The system rings out around 2.5kHz no matter what I do with the GEQ.

I've pulled 2.5k down 6dB, it rings at 3kHz. Pull 3kHz down, it moves to 2.2kHz. Seems like there's a fundamental feedback problem I'm treating with EQ but not solving.

The system was installed by someone else. I'm being called in to "fix the feedback problem."

💬 3 replies

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u/rack_randy •

Also verify the speaker coverage pattern. If the SRX815s are pointed at the stage (coverage extends past the audience and hits the pastor), that's a coverage problem. Aim the tops to cover audience seats only, not stage. It sounds obvious but I've fixed more feedback problems by repositioning speakers than by anything in the signal chain.

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u/av_wizard •

EQ won't fix a physical feedback problem — it masks it temporarily. The real issues are almost always: (1) mic-to-speaker distance (mics too close to speaker coverage, or speakers spilling onto stage), (2) mic pickup pattern not aimed correctly (SM58 is cardioid — the rear null should face the speakers), (3) gain structure — if the system is being run too hot at the mic preamp, you have less headroom before feedback.

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u/crestron_carl •

Rent or buy a RTA and look at the room frequency response with pink noise. You probably have 2-5dB room peaks in that 2-4kHz range driven by room modes, not the PA. Address the room acoustics (even temporary absorption panels on the worst reflective surfaces) and the feedback threshold will rise significantly.

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