Introduction to Sound Synthesis Effects
Reverb
Reverb is the dense pattern of reflections a space adds to a sound. A sound arrives first directly, then as a few distinct early reflections, then as a wash of reflections that fades away. A reverb effect recreates that pattern for spaces from a small room to a large hall, and can also produce decays far longer than real spaces have.
In any enclosed space, a sound reaches a listener by many paths at once. The direct path is shortest and arrives first. Reflections from the nearest walls, the floor and the ceiling arrive a few milliseconds later as separate early reflections, and then reflections of reflections pile up so densely that they can no longer be heard apart. That dense tail is reverberation, and it fades because each reflection loses some energy to the surfaces it meets and to the air it travels through. The physicist Wallace Clement Sabine, working at Harvard in the 1890s, measured how long reverberation lasted and how that time depended on a room's size and materials, and his measure, the time for the sound to fall by 60 decibels, is still used as RT60.
The tool below generates the response of an imaginary space and plays a source through it. The plot shows that response, the level of the reflections that follow a single click, in decibels against time.
A reverb
Plucks or drums pass through a generated reverb. Decay sets the reverberation time, pre-delay sets the gap before the reverb starts, damping reduces the high frequencies in the tail more as it fades, and mix sets how much reverb is heard.
Listen for A decay under half a second sounds like a small room and gives the drums body without an audible tail. Several seconds sounds like a hall or a cathedral and blurs the plucks together. Pre-delay separates the reverb from each note, which keeps the attacks clear. With damping high, the tail darkens as it fades, as in a room with soft furnishings.
Decay, pre-delay and damping
A longer decay makes the space sound larger. Under about half a second, the reverb sounds like a room and mostly adds body; one to two seconds sounds like a hall; several seconds sounds like a cathedral or a cave. Pre-delay is the gap before the reverb begins. In a large space the first reflections arrive tens of milliseconds after the direct sound, and adding that gap keeps the start of each note clear of its reverb. Damping models the way soft surfaces and air absorb high frequencies more than low ones, so in a damped space the tail grows darker as it fades. In a space of bare stone and glass the tail stays bright for longer, although the air still absorbs high frequencies.
How reverb is made
Recording studios first made reverb with real spaces, sending sound to a speaker in a hard-walled echo chamber and recording it back with a microphone. Plate reverbs vibrated a large steel sheet and picked up its vibration with contact microphones, and spring reverbs, common in guitar amplifiers, sent the signal down a coiled spring. The physicist Manfred Schroeder showed in the early 1960s how to make a digital reverb from a few comb filters and allpass filters, and most algorithmic reverbs since build on networks of delays in the same way. Convolution reverb combines the sound with an impulse response, a recording of how a real space responds to a click, and so reproduces that space closely. The tool above convolves with a response it generates itself: a few early reflections followed by noise that fades at the chosen rate.
Common questions
What is reverb?
Reverb is the pattern of reflections a space adds to a sound. A sound reaches a listener directly, then as a few distinct early reflections from nearby surfaces, then as a dense, decaying wash of reflections arriving from all directions. A reverb effect recreates that pattern.
What is RT60?
RT60 is the reverberation time, the time it takes the reflections in a space to fade by 60 decibels, to one thousandth of their starting amplitude, after the sound stops. A small room might have an RT60 of under half a second and a large concert hall around two seconds.
What is pre-delay on a reverb?
Pre-delay is a gap between the direct sound and the start of the reverb. In a large space the first reflections arrive noticeably later than the direct sound, and a short pre-delay also keeps the start of each note clear before the reverb joins it.
What is convolution reverb?
Convolution reverb combines a sound with an impulse response, a recording of how a space responds to a single click or sweep. Every sample of the sound starts a copy of the impulse response, scaled by that sample's level, and the copies are added together, which reproduces the reflections of that space closely. The tool on this page uses a generated impulse response.
Related modules
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