Avoid feedback disasters: PA, stage monitor and mix errors
Nothing kills a rehearsal faster than a vocal disappearing in the mix or a monitor screaming right into your face. A solid PA setup is not about buying the most expensive gear—it’s about making every song feel tight, loud and under control.
Monitor feedback, underpowered mains and tangled wiring turn rehearsals into chaos. A band member, DIY sound tech or small-venue promoter with limited budget often has to size speakers, sort monitor mixes and finalise wiring while keeping instruments, venue acoustics and power limits in balance.
How to size PA & calculate SPL
Sizing speakers starts with the speaker sensitivity and continuous RMS power, not peak specs. This gives a realistic estimate of the loudness the system can reach at the audience area.
RMS, sensitivity & SPL
Sensitivity is the SPL a speaker makes at 1 watt measured at 1 metre. Use the formula: SPL = sensitivity + 10·log10(power). This converts watts to expected SPL and shows the effect of amplifier power.
The most common mistake at this point is choosing by peak wattage only. Peak numbers sound impressive but say little about continuous output and headroom.
Headroom matters because drums and vocals have high crest factors. Allow at least 6 dB headroom above calculated required SPL to avoid distortion.
Worked examples & calculator
Example 1: trio in a 150-capacity room.
- Speaker sensitivity: 96 dB @1W/1m. - Desired SPL at FOH position: 105 dB. - Required power: solve 105 = 96 + 10·log10(P) → 10·log10(P) = 9 → P ≈ 8 watts.
Because the next feedback squeal might be warning you about a much bigger mistake...
The analysis in avoid feedback disasters pa stage monitor puts this into broader context.
