Wind, pops, and rustle

Three more everyday nuisances, all caused by physical things hitting the microphone rather than by electronics or rooms. They share a theme, which is why they’re together: each is a burst of unwanted energy concentrated in a particular part of the spectrum, and each is removed by acting on that part — sometimes all the time, sometimes only during the burst.

Wind — the low rumble

Record outdoors without a foam or furry “dead-cat” cover and the breeze turbulating across the mic produces a low, blustery rumble — sometimes a roar. Crucially, almost all of that energy sits very low, below the range where speech lives.

That makes the cure simple: a high-pass filter — a tool that lets the high stuff pass and blocks the low stuff. Set its cutoff at, say, 80 cycles per second and everything below (the wind rumble) is steeply rolled off while the voice above is untouched. Cathar’s dewind --cutoff 80 is exactly this, built from a classic, very steep filter shape (a Butterworth) so the rumble drops away fast without disturbing the voice just above it. It’s the same high-pass you hear engineers reach for the instant an outdoor clip starts rumbling.

Plosives — the “p” thumps

Get close to a mic and say “peter piper” and each “p” and “b” fires a little puff of air straight at the capsule, producing a low thump — a plosive. Like wind, a plosive is mostly low-frequency energy — but unlike wind, it’s not constant: it’s a brief burst, only on the plosive consonants.

So instead of filtering all the time, de-plosive watches the low end and acts only when it suddenly thumps: it spots the short bursts of excess low energy and ducks just those moments, leaving the steady low warmth of the voice in between alone. (The physical prevention, by the way, is the round foam ball or mesh “pop filter” you’ve seen in front of studio mics — but when you’re handed a recording that already has the thumps, software has to clean up after the fact.)

Rustle — the clip-on-mic scratch

The little clip-on (lavalier) mics used in interviews and film sit against clothing, and every time the wearer shifts, the fabric scrapes the mic and makes a scratchy rustle. This one is sneakier: it’s a brief burst like a plosive, but it lands in the mid range, right among the consonants of speech, so you can’t just filter it away without dulling the voice.

De-rustle therefore does the same “act only during the burst” trick as de-plosive, but aimed at the mid-range: it watches a band roughly where rustle lives (around 1,500–6,000 cycles) and, when energy there spikes briefly above its normal level, it pulls just that fleeting spike back down, while sustained speech in the same band passes through. It’s the hardest of the three, because the rustle and the wanted consonants are near neighbours.

How the big tools do it

• The high-pass for wind is utterly universal — every DAW channel strip, every mixer, has a low-cut button. Nothing exotic here, in cathar or anywhere.
• For plosives, engineers often just automate a quick low-cut on the offending word, or use a dynamic filter; iZotope RX’s “De-plosive” automates exactly the spot-the-thump-and-duck-the-lows behaviour cathar uses.
• Rustle is genuinely hard, and it’s a showcase for ML: iZotope RX’s “De-rustle” was one of the first ML-driven restoration modules precisely because fabric noise overlaps speech so much that a learned model separates them far better than a rule about bands. Cathar’s transient-suppression approach gives a useful reduction on obvious rustles; deep, speech-tangled rustle is RX’s territory.

Notice the recurring pattern across this whole section: a steady offender (wind) gets a filter that’s always on; a bursty offender (plosive, rustle) gets a watcher that acts only during the burst. That single distinction — always on versus only-when-it-happens — explains an enormous amount of audio software.