Mixing a lesson so people can follow it

You film yourself playing something, or cooking something, or taking something apart, and then you talk over it. Two sounds, one video, and every version you try makes one of them worse.

That is not a failure of your mixing. It is the shape of the problem. Decide which of your two sources is the lesson, protect that one completely, and accept that the other is now in service of it. A lesson that sounds gorgeous and cannot be followed has failed, and so has one that is perfectly clear over an instrument you have flattened into cardboard.

Two goals in one mix

The first is that the thing being demonstrated should sound the way it deserves to sound. A piano should have weight, decay and some room around it. A pan should sizzle like a pan. Whatever you are showing people, you showed it because it is worth showing.

The second is that the teaching on top has to stay legible. Every word, on a phone, at half volume, to somebody who does not yet know the vocabulary and cannot fill in a word they missed.

Both matter. Listen to the words and the demonstration in context, then give the important one room at each point in the lesson. The priority can change without the other source becoming disposable.

Where do the two compete?

Two mechanisms, and they stack.

The first is masking, which is the plain fact that when two sounds occupy the same pitches at the same moment, the louder one hides the quieter. Speech lives in the middle of the range. So does the body of nearly every instrument, and so does the sound of a hot pan. That collision is closer to physics than to taste: the exact rules of what hides what are more tangled than a straight sum, but no amount of care over your headphones talks two sounds out of the same part of the range.

The second is attention, which catches people who have already solved the first. Most people hold a single stream in the foreground at a time, and a viewer who is trying to learn is not going to split their attention to rescue your sentence. Make the instrument beautiful enough and they will listen to the instrument, and your sentence will go past intact, audible and unheard. That is the failure which looks like success: every word technically clear, nothing learnt. You cannot catch it in your own edit, because you already know what you said. You catch it by watching one other person watch it.

Which of your two sources is the lesson?

Ask what a viewer would complain about if it were missing.

If they came to learn the piece, the playing is the lesson and the talking is signposting. If they came to learn a technique, the words are the lesson and the playing is the illustration of it. Most people filming lessons have never made that choice out loud, and a video that has not made it drifts between the two, which is why it is tiring to watch and impossible to mix.

It can also change inside a single video, and usually does. Two minutes of teaching and then thirty seconds of demonstration are two different balances, not one compromise held across both.

What does protecting the lesson cost?

Something real, which is why the decision has to be made.

During teaching, make space where the voice needs it. That might mean a level change, selective EQ, timing or a passage without the instrument underneath. Removing low end or width is not a requirement of every mix.

During the demonstration, nobody talks, and everything comes back: weight, width, the room it was played in. The join between those two states is where home mixes give themselves away, because one setting has been applied to the whole video, so either the demonstration is thin or the teaching is buried.

Try letting the important demonstration speak for itself. A passage without commentary can reveal detail the explanation was covering. Give it the time the action needs, then bring the words back where they help.

One recording decision sits behind all of this, and it is worth knowing before your next shoot. If the voice and the instrument arrive on the same recording, their balance is close to fixed. The newer separation tools can prise the two apart to a degree, but imperfectly and with artefacts, so two separate recordings remain the only way to keep the decision open for as long as the project exists.

Outside music

It applies to anything with a sound worth hearing underneath somebody explaining it.

A cook talking over a pan. A mechanic over a running engine. A potter over a wheel. A coach over a room of people. The question is the same every time and the answer is not always the voice, which is the part people get wrong. In a video about how a particular fault sounds, the engine is the lesson and the words are labels on it, and mixing it the usual way produces a video where a diagnostic sound has been politely turned down until it can no longer be diagnosed.

Where the balance stops being a setting

The way out is two settings rather than one. The demonstration keeps its depth where the voice stops, and gives some of that depth up where the teaching has to carry. One setting can only serve one of the two goals, and a setting that serves both equally serves neither.

Watch a lesson and mark where the listener needs the voice, the demonstration or both. Where they compete, try a different balance or separate the moments. Keep simultaneous explanation when it helps the learner follow the action.

Sources and evidence

What here is established hearing science and instructional research, and what is our own method.

What is established. That two sounds sharing the same part of the range cannot both stay clear, the louder hiding the quieter, is auditory masking, a long-studied property of hearing set out in the psychoacoustics literature (Moore); the exact rules are more tangled than a simple sum. That a learner holds roughly one stream in the foreground, and that segmenting a lesson and giving control of its pace help people follow it, comes from research on educational video (Brame, and the systematic review by Fyfield and colleagues). These describe tendencies, not a fixed recipe.

Our method. Naming which of the two sources is the lesson, mixing in two states rather than one, and giving the demonstration a stretch of its own with nothing over it, is the practice here.

References.

• Brian C. J. Moore. An Introduction to the Psychology of Hearing (6th edition). books.google.com . Auditory masking / frequency selectivity / critical band: two sounds sharing a frequency region, the louder hides the quieter. Canonical psychoacoustics textbook; exact masking behaviour is more complex than a simple sum, as the copy states.
• Cynthia Brame. Effective Educational Videos. pmc.ncbi.nlm.nih.gov. Cognitive load, engagement and active learning should guide educational video design. Review/guideline, not a fixed-length law.
• Fyfield et al. Improving instructional video design: a systematic review. ajet.org.au. Segmenting and learner control are relatively robust; redundancy effects vary. Instructional contexts differ.