TW #118 — Reverb and Compression (Airon)

An "info-for-trackers" tutorial on the two studio effects a tracker can only apply offline, in a sample editor (Cool Edit and Sound Forge are the examples): what the parameters mean, numbers for a small club, compressor settings per instrument, and two recipes — gated reverb and "preverb".

Key claims#

Reverb#

• The tracker "echo" of low-volume copies on extra channels (he points to necros's tutorials "in around TW #15") is "a primitive version" of reverb.
• Anatomy, with sound at 343 m/s (15 °C): the direct sound (the dry part; everything after it is wet), then the early reflections off walls and floor — "an important component for our brain for the task of identifying the size of the room" — then the dense reverb that builds to a peak; pre-delay is the time from direct sound to reverb peak; reverb time is the decay to −60 dB; density follows the materials (polished marble reflects, wood disperses, carpet swallows); high-frequency absorption (Cool Edit: High Frequency Absorption Time; Sound Forge: Attenuate high frequencies above; Lexicon PCM 70/80/90 hardware controls both frequency and time for low and high bands) — a crowded room eats highs fast; a high cutoff and long absorption make a room sound "emptier and colder", small rooms "very metallic. Sewer pipes and oil pipelines".
• Worked example, a club 8 × 8 × 3 m (~200 m³), band 4 m away: direct sound 11 ms; first side-wall reflections travel ~9 m = 26 ms, minus the 11 ms the editor's dry sound already covers → early reflections ≈15 ms (Sound Forge offers a list of ER times; Cool Edit has none); reverb time under 1 s (experiment between 0.7 and 1.2 s); pre-delay 30 ms or more; density high (people, wood, obstacles); HF absorption time very low / attenuate above 2000–2500 Hz; wet level modest — "only very smoothed hard surfaces in big cavernous space create very loud reverbs. A church might … but a club doesn't".
• Placement by reverb amount: "Put your snare to the front, the piano a little to the back, and the sax just a little in front of the drums." "Snare always gets reverb." Decide whether you simulate a live band or a multitrack production ("doubling", extreme panning, "in general NOT reality").

Compression#

• Not an effect but a process: the whole signal goes through it ("DRY goes in, WET comes out"). Parameters via the barrel-and-hammer picture: threshold (the barrel's rim), attack (the hammer man's reaction time), decay (how long he keeps hitting after the signal drops back — the release), ratio (how hard); an infinite ratio is limiting. Radio and TV sound is heavy compression and limiting; a vocal peaks above 100 dB SPL while a $50,000 tape machine handles about 86 dB of dynamic range, hence 2:1–6:1 on vocals, 8:1 for radio hosts; don't set the threshold too low.
• For samples: to push a synth sample to the front, threshold −6 dB, ratio 4:1, attack just longer than the sound's own attack (a 14 ms attack phase → attack 15 ms) so it is never crushed, decay 500 ms or more for a single hit but 20–30 ms for a sequence of sounds; normalise afterwards. Bass drum: compress first, then boost the bass; attack ≥30 ms, decay ≥80 ms, never damage the attack. Acoustic guitar: as little as possible — threshold −3 dB, ratio 2:1 or 3:1. Snare: a deliberately short attack that crunches part of the transient "will change a snare considerably" — compression as an effect.
• "Most important rule in digital sound editing is NEVER DRIVE THE SAMPLE INTO CLIPPING" — digital clipping is not analog overdrive and cannot make a distorted guitar.

Recipes#

• Gated reverb (snare, but not only): normalise, copy the sample; on the copy, reverb with dry = 0 and the HF cutoff/absorption set high so the tail keeps its highs; normalise; compress with threshold −12 dB or lower (−13, −14 …), attack 1 ms, decay 500 ms, ratio 12:1 or more; normalise again; fade up the first ~30 ms (Sound Forge's fade function; Cool Edit's envelope); mix the original at 70–80 % (at least −3 dB, the attack must dominate) with as much of the wet copy as you like. The 1 ms attack and the fade-up stand in for the noise gate that gives the effect its name.
• Preverb (reverse reverb): normalise, copy; insert silence of about 90 % of the sample's length at the start of the original; reverb the copy with dry ≈ 0 and a reverb time of ≥1 s (or a large-hall preset, with the dry level turned back down); reverse the reverbed copy and mix it into the silent start of the original — "nothing but a reverse reverb put in front of the original sound"; "great ghost stuff"; impossible with outboard hardware.

Practical takeaways#

Reverb: set early reflections from room geometry (distance ÷ 343 m/s, minus the direct path), reverb time from size and materials, HF absorption from how full the room is; give the snare reverb and place instruments front-to-back with it. Compression: threshold/ratio/attack/decay per sound (table on reverb-and-compression), attack longer than the transient, normalise after, never clip. The two recipes are sample-editor procedures any tracker can run on a drum sample.

Notable quotes#

• "Snare always gets reverb."
• "NEVER DRIVE THE SAMPLE INTO CLIPPING … Analog overdrive gives us stuff like distorted guitars and you CANNOT do that with digital clipping."

Relevance#

The wiki's first source on offline mixing effects: the basis of the reverb-and-compression technique; complements fake-echo (the in-tracker version), instrument-envelopes (looped reverb tails) and stereo-panning (placing echoes).

Pages touched#

reverb-and-compression, fake-echo, instrument-envelopes, instrument-design, recording-samples, sidechain-pump.