A transient shaper reacts to how fast the level of a signal changes, whatever that level is.
Inside there are envelope followers watching the same signal at different speeds. A fast one follows every hit almost instantly. A slower one takes its time to rise. At the start of a hit the fast one jumps and the slow one lags behind, and the gap between them is the attack. Turn up Attack and that gap becomes gain: the first few milliseconds of every hit get louder.
Sustain works from the other end. A follower that rises quickly but falls slowly hangs above the signal while a hit dies away, and that gap is the tail. Cut it and the snare stops ringing sooner. Boost it and the room in the overheads comes forward between the hits.
Why it isn’t a compressor
A compressor has a threshold. Signal above it gets turned down and signal below it is left alone, so what a compressor does to a hit depends on how loud that hit is.
A transient shaper has no threshold. A ghost note and a backbeat both start with a jump in level, so both get the same Attack boost in decibels, and a drum loop that plays harder in the chorus gets the same shaping it had in the verse. You set the shape of the hit once and it holds for the song.
The same property has a cost. A shaper can’t tell a hit you want from one you don’t, so the tambourine gets what the snare gets. Between hits, a Sustain boost pulls up whatever is there, noise included, which is why a well-behaved shaper fades itself out below a floor. Some also let harder hits take more of the effect than soft ones, so ghost notes stay small under the backbeat.
What changes in mid/side
Split a drum bus into mid and side and you’re holding two different drum kits.
The mid has the close mics: kick, snare, the centre of the overheads, anything panned down the middle. The side has what spreads out: the room mics, hi-hats and toms panned wide, the width of the overheads, and every stereo reverb on the bus.
That gives each lane an obvious job. Attack on the mid makes the kick and snare hit harder without adding anything to the room in the sides. A Sustain cut on the side makes the room shorter, so the space between hits gets cleaner while the hits keep their size. Turn the side’s Sustain up instead and a small room sounds bigger while the centre stays dry.
A stereo shaper can’t separate those two moves. Cutting the room’s tail cuts the snare’s tail with it, because both are in both speakers.
One hit, two detectors
The catch comes from the maths of mid and side.
Left is mid plus side, and right is mid minus side. A sound in the centre lives only in the mid. A sound panned hard left is split evenly between mid and side, and the two halves cancel in the right speaker.
A transient shaper isn’t linear, since the gain it applies depends on the signal. Give the mid and the side separate detectors and, during a hit, the mid can be 4 dB up while the side isn’t. For anything off-centre the two halves stop cancelling where they used to. The hard-left hi-hat leaks into the right speaker for the length of the attack and then moves back, and on a busy pattern that sounds like the image flickering.
A shaper built for this usually offers a linked mode for exactly that reason: one detector drives both lanes, the same gain lands on mid and side at the same moment, and nothing in the image moves. Reach for separate detectors when the centre and the sides need different treatment, keep the side lane’s settings moderate, and listen for anything panned wide jumping on the hits.
Setting the times
Two times decide what counts as the attack and what counts as the tail.
Attack Time is how long the boosted window lasts. Around 5 ms it catches the click of the beater and the stick. By 30 ms it takes the whole front of the snare, body included, and the drum gets bigger as well as louder.
Sustain Time is how far into the decay the sustain gain reaches. Around 150 ms trims the ring of a snare. At half a second or more it reaches into the room, which on the side lane is usually the point.
On a drum bus, a short Attack Time on the mid and a long Sustain Time on the side is a sensible place to begin.
Where it goes wrong
- Peaks. Attack gain goes straight onto the peaks. Nine decibels of attack is up to nine more decibels into whatever limiter comes next, and a limiter that flattens the transients again undoes the move. Bring the output down, or use the shaper’s own limiter if it has one.
- Reverb that isn’t the room. A Sustain cut on the side of a full mix can’t tell the drum room from the stereo reverb on the vocal. Both get shorter.
- Mono. Attack added on the side disappears when the side cancels, so hits that sound punchy on speakers go soft on a phone. Put the attack that has to survive mono on the mid.
- Louder sounds better. Adding attack also adds level. Compare against bypass at matched loudness before deciding it helped.
A starting point
- Start in stereo, with Attack only, until the kick and snare land where you want them.
- Switch to mid/side if the room needs its own treatment. Take 3–6 dB of Sustain off the side and leave the mid’s Sustain alone.
- Solo the side for a moment. Hats and toms panned wide shouldn’t jump on the backbeat.
- Check it in mono, then against bypass at matched level.