Reverb and delay (RT60, pre-delay, early reflections, feedback)

Definition

Reverb is the set of reflections of a sound in an environment. It consists of three parts: the direct sound; the early reflections, distinct, which arrive in the first 50–80 ms and convey the size and shape of the space; and the diffuse tail, dense and exponentially decaying.

The reverberation time RT60 is the time in which the sound energy decays by 60 dB after the source stops. The Sabine formula estimates it as RT60 = 0.161 · V / A, with V the volume in m³ and A = Σ S_i · α_i the equivalent absorption in m² (surfaces S_i times their respective absorption coefficients α_i). The pre-delay is the interval between the direct sound and the onset of the reverb; a reflection that travels d meters farther than the direct sound arrives with a delay t = d / c, with c = 343 m/s (2.9 ms per meter).

The delay is a delayed copy of the signal. With feedback g (amplitude ratio between a repeat and the previous one) the n-th repeat has a level of 20 · log10(gⁿ) = n · 20 · log10(g) dB, and the time for a 60 dB decay with delay D is T60 = 60 · D / (−20 · log10 g). With g ≥ 1 the repeats do not decay. The tempo-synced delay time is D = 60,000 / BPM ms for a quarter note; the dotted eighth is 0.75 of a quarter note, the eighth-note triplet 1/3.

Parameters and units

Reverb

• Decay / RT60: 0.3–1 s (room), 1.2–2.5 s (plate and hall for vocals), 2.5–6 s (effects and large spaces).
• Pre-delay: 0–100 ms.
• Size, diffusion, density: apparent size and density of the reflections.
• Damping: faster decay of the high frequencies (onset frequency 4–8 kHz or decay ratio).
• Early/late: balance between early reflections and tail.
• Filters on the return: high-pass 150–300 Hz, low-pass 6–10 kHz.

Delay

• Time: ms or note value.
• Feedback: 0–90% (amplitude).
• Filters in the feedback: high-pass and low-pass, each repeat darker and thinner.
• Ping-pong, modulation, ducking (the delay lowers while the vocal sings).

Musical function

Reverb and delay place instruments in a shared space, create depth (what is far away has more reverb and fewer high frequencies), and give sustain and width. Pre-delay separates the source from the tail and keeps the vocal in the foreground even with a long reverb. A tempo-synced delay adds space without the density of reverb and works on the rhythm. Both mask the following attacks: the length must be adapted to the tempo and the density of the arrangement.

Practical example

• Sabine: room 10 × 8 × 4 m (V = 320 m³), equivalent absorption A = 50 m² → RT60 = 0.161 · 320 / 50 = 1.03 s.
• Times at 120 BPM: quarter note 500 ms, eighth note 250 ms, dotted eighth 375 ms, eighth-note triplet 166.7 ms, sixteenth note 125 ms; pre-delay of 1/64 = 31.25 ms or of 1/32 = 62.5 ms.
• Feedback: g = 0.5 (50%) → each repeat −6.02 dB; after 10 repeats −60 dB; with D = 500 ms the tail lasts 5 s.
• Pop vocal at 100 BPM: plate with decay 1.8 s, pre-delay 40 ms, high-pass 200 Hz and low-pass 8 kHz on the return; dotted-eighth delay (450 ms) with feedback 25%, filtered 300 Hz–5 kHz, ducked by the vocal.

Common mistakes

• Long reverbs on fast, dense songs: the tails cover the following notes.
• Returns without a high-pass: the reverb accumulates energy below 250 Hz and muddies the mix.
• No pre-delay on the vocal: the vocal recedes and loses intelligibility.
• Feedback close to or beyond 100%: the repeats grow until saturation.
• Unsynced delays that clash with the groove.
• Too many different spaces with no coherence: the space of the mix becomes confusing.
• Very wide stereo reverbs without mono control.

How to check it

• App analysis: export the reverb or delay return alone and analyze it: the energy per band shows any excess in the Upper bass and Low mids bands; the per-band width and the Stereo correlation indicate how wide the return is; the Mono sum drop shows how much is lost in mono. On the full mix, an increase in Low mids energy beyond the reference signals unfiltered reverbs.
• Listening to the tail: in breakdowns and at the end of the song the tail should die out before the next section or in a musical way.
• Spectrogram: the decay of the tail in the different bands shows the damping and the actual RT60.