Mono bass is one of the few pieces of mixing advice that gets repeated as a rule rather than a decision. Below some frequency, sum everything to the centre. The number people quote is usually 100 Hz, sometimes 120, occasionally 200 with no explanation of the difference.
The rule is sound. The reasoning behind it is worth having, because the right frequency depends on the mix and the wrong one is audible.
Why there’s bass in the sides at all
You didn’t pan the bass. It’s still there in the side channel, for four reasons.
Stereo widening. Any width control works by increasing the side channel relative to the mid. Applied full-band, it widens the bass along with everything else, and the bass had the most energy to move.
Stereo sources. Sampled stereo one-shots, a synth patch with two detuned oscillators panned apart, a room mic pair on a kit. All of these carry low content that differs between channels.
Reverb. Send a kick to a stereo reverb and the tail’s low end lands in the sides.
Recording. Two microphones on one source, at different distances, produce phase differences that read as side content at low frequencies.
None of these is a mistake. They just cost you level on a fold, and low frequencies are where that costs most.
What summing costs
Summing the low band to mono removes the side content there. You lose whatever width the bass had.
You almost certainly can’t hear that width. Human localisation below about 150 Hz works on level differences that barely exist at those wavelengths, and a sub-bass wave is long enough that a domestic room’s reflections dominate any directional cue the recording had. The practical effect of mono bass in stereo is close to nothing.
What you gain is that a fold to mono no longer loses that energy. A mix losing 5 dB in the low band before the crossover typically loses under 1 dB after.
The one thing that does change audibly is the character of a wide stereo synth whose low harmonics were part of its movement. Summing those flattens the motion. This is the case where the crossover point matters.
Where to put it
80–100 Hz is the conservative setting. It fixes sub-bass cancellation and leaves everything above it alone. Use it when the bass is already centred and you’re guarding against surprises.
120–150 Hz is the working default for most modern productions. It catches the kick fundamental and the body of the bass, which is where the fold loss usually concentrates.
Above 200 Hz starts to be audible in stereo. Snare body, guitar low end and the bottom of a pad live up there, and pulling them to the centre narrows the mix in a way people notice. Go this high only when a measurement tells you the problem is that high.
The way to choose is to measure rather than pick. Find the band that loses the most on a fold, set the crossover above it, and check how much you got back. If 120 Hz recovers 4 dB and 200 Hz recovers 4.3, use 120.
A crossover set from a number someone gave you is a guess. A crossover set from the band that’s actually cancelling is a fix.
When not to do it
When the bass isn’t the problem. If the fold loss is at 400 Hz, mono bass does nothing and you’ve narrowed the low end for no reason.
On a stereo bass that’s the point. Some records are built on a wide, moving low synth. If summing it kills the part, the part matters more than the fold. Make the decision knowingly rather than applying the rule.
On material that’s already mono down there. A lot of programmed music has a centred kick and a centred bass and nothing else below 150 Hz. A crossover changes nothing, which is fine, but don’t assume it fixed something.
Before you’ve found a polarity error. If a whole element vanishes on a fold rather than thinning, one channel is flipped. Mono bass will mask the symptom in the low band and leave the rest of it in place.
A starting point
- Check the fold first and find which band loses level. Fixing the wrong band is the most common version of this mistake.
- Set the crossover just above the worst band, not at a number you remember.
- Listen in stereo with it in and out. If you can hear the mix narrow, the crossover is too high.
- Check the fold again and confirm you got the decibels back.
- Leave it there. This is a set-once control, not something to automate.
The related read is how to check mono compatibility, which covers how to find the offending band in the first place.