The hook
The mix is soup. Not broken, just soupy. Everything's audible, nothing's clear, and the whole low half of the room feels thick, like the band is playing under a blanket.
So you go hunting. Solo the acoustic: sounds fine. Solo the keys: fine. Vocals, electric, even the pastor's lav — every single channel, soloed, sounds perfectly respectable. Nothing is wrong anywhere, and yet everything is wrong together.
Here's what the solo button can't show you: each of those channels is carrying a little cargo of low-frequency junk (stage rumble, mic handling, room boom, the AC) down where its actual instrument has nothing to say. One channel's worth is inaudible. Sixteen channels' worth, summed on the main bus, is the blanket. This lesson is the cheapest fix in live audio: one filter, applied almost everywhere, that removes the cargo nobody ordered.
The lesson
You met the high-pass filter in 3.1 (its address on the strip) and used it blind in 3.5. Now the full story.
The reasoning starts with a fact about sources. Almost nothing on your stage produces useful energy in the bottom octaves. A singing voice barely reaches below 100 Hz. An acoustic guitar's lowest string is 82 Hz. An electric guitar amp speaks mostly above 80. Cymbals live above 300. The pastor's voice, above 100. Only the kick drum, the bass, and certain keyboard/track content genuinely own the bottom.
But every microphone on that stage picks up the bottom anyway — not from its instrument, from the environment: footsteps and drum thump through the stage floor, handling noise, plosive pops, HVAC, and the low end of every other instrument washing around. Below its instrument's range, a channel is pure liability: no signal, all junk.
And the junk adds. That's the trap from the hook: sixteen channels each carrying a quiet, individually harmless rumble sum into a low-end haze that eats headroom, muddies the mix, and, because strong low energy masks upward (4.2), smears the clarity of everything above it. The soup was never on any one channel. It's the sum.
The rule: high-pass every channel except the ones whose actual job is the bottom. Kick, bass, subs-bound tracks — those keep their lows (even they often get a gentle cut in the very sub-bass region). Everything else gets a filter, set deliberately.
How to set it by ear, per channel:
- With the source playing, raise the filter frequency slowly from the bottom.
- Keep raising until you hear the sound thin out: the body starts to leave, the instrument gets smaller.
- Back it off a little. That's the spot: everything below what the source needs, gone; everything it owns, intact.
Typical landing zones, as sanity checks rather than gospel: lead vocals 80–120 Hz. Speech mics like the pastor's lav, 100–150 (speech needs less body than singing, and lavs pick up chest thump and clothing noise). Acoustic guitar 80–100. Electric guitar 80–100. Toms 60–100. Overheads anywhere from 150 to 400 depending on whether they carry the whole kit or just cymbals. Keys and tracks: often left low or unfiltered if the content is clean. But verify; a synth pad with accidental sub rumble is still cargo.
Go deeperoptional
Why the sum is so much worse than the parts. The junk on different channels is mostly uncorrelated, so it sums on a power basis (1.2's deep end): sixteen equal uncorrelated rumbles stack to about +12 dB over any one of them (10·log₁₀ 16). But correlated components — the same drum thump arriving through every stage mic, the same HVAC tone — sum coherently, up to +6 dB per doubling, potentially +24 dB over one channel across sixteen. The real bus carries a blend of both regimes, which is why the haze always seems disproportionate to anything you can find soloed. The solo button auditions channels one at a time; summation is precisely the thing it cannot audition.
What the junk costs even when you can't hear it. Low frequencies dominate the amplitude of a waveform: a big subsonic swing rides under the whole signal and eats peak headroom (2.2) on the channel, the bus, and the system processor without contributing any loudness. Downstream it gets worse mechanically: for constant SPL a loudspeaker cone's excursion quadruples per octave down, so amplifier power and driver travel spent reproducing 30 Hz stage rumble is stolen from the program at real multiples. And in the monitor world, low-mid buildup is prime feedback fuel: wedges hoot in exactly the region the filters clear. High-passing is simultaneously an EQ move, a headroom move, an amplifier-budget move, and a feedback-margin move. No other single control buys four things at once.
Slopes, and what the filter doesn't do instantly. An HPF rolls off at a slope set by its order: 12 dB/octave (2nd order) is the gentle default, 18 or 24 dB/octave cut faster below the corner. Steeper isn't automatically better: a 24 dB/oct filter at 120 Hz genuinely erases 60 Hz (−24 dB), but any minimum-phase filter also rotates phase near its corner, and the steeper the slope, the more rotation. On a solo channel, inaudible. On multi-mic sources (the 3.1 polarity discussion's territory) mismatched filter slopes between, say, the two snare mics shift their relative phase in the overlap region and change how they sum. The working habit: consistent, moderate slopes on related mics; steep slopes when you truly need the guillotine.
The corner frequency is −3 dB, not a wall. "HPF at 100 Hz" means the response is already 3 dB down at 100, and the rolloff reaches well above the corner: a 12 dB/oct filter at 100 Hz is still shaving a little at 150. That's why "raise until it thins, back off" works as a procedure — you're finding the corner placement where the audible shaving stops mattering. It's also why a filter set exactly at the instrument's lowest note is slightly too high: the fundamental sits in the filter's knee. When in doubt, the backing-off step has physics on its side.
Remember
- Below its instrument's range, every channel is pure liability: no signal, all rumble. And the rumble sums.
- High-pass everything except the channels whose job is the bottom: kick, bass, sub-bound tracks.
- Set by ear: raise the frequency until the source thins, back off a little. Numbers are neighborhoods, not gospel.
- The payoff is fourfold: clearer mix, recovered headroom, saved amplifier and driver effort, wider feedback margin.
- The soup was never on one channel. Solo can't hear summation. The fix is sixteen small filters, not one big one.
Your turn
- Audit every channel on your board: filter on or off, and set where? Fix each one with the by-ear procedure.
- Do the all-filters bypass test at rehearsal: ten seconds off, then on. Listen for the blanket lift.
- Set the pastor's lav filter deliberately: speech tolerates a higher corner than singing, and lavs carry the most junk per useful Hz on the stage.
- Check your board's filter slope options (12/18/24 dB per octave). If you have multi-mic sources, keep related channels' slopes consistent.