Reverberation Explained
Introduction
The acoustic phenomenon recognised as reverberation accompanies all natural sounds we hear, in fact we are so conditioned to hearing the accompanying reverberant field that on hearing sound totally lacking in reverberation (anechoic chambers for example) we are immediately struck by its unnatural character.
Natural reverberation informs our ears from which direction the sound is emanating; approximately how far away the sound is; how loud the initial sound was; what type of acoustic space we are in; how large the space is; what the boundaries are like (hard walls, curtained walls, etc.).
Lets say you are in the centre of a small hall and someone, standing a few feet away, claps their hands. Immediately the resulting sound wave will radiate in all directions at an approximate rate of one foot every millisecond. The initial sound you will hear will be that which comes directly towards you from the clapped hands. Thanks to our well-developed binaural hearing this ‘Direct Sound’ will tell us where the sound source is.
Pre-Delay
The early reflections are not heard until the sound has reached the nearest boundary, or obstacle and reflected back to the listener. This initial delay between the direct sound and the first reflected sound provides what is perhaps the strongest clue as to the room size.
Early Reflections
Next come the early reflections; sound is radiated in all directions and some of this radiated energy, on striking room boundaries (walls, floors, ceilings, etc.), will be directed towards your ears. These early reflections will be slightly different in character from the original sound source, since not only is some of the energy absorbed by the boundary upon which the wave strikes, but this absorption also varies depending on frequency.
The early reflections are indispensable in providing information regarding the characteristics of the acoustic space and the distance between the source and the listener.
Density
The density of the early reflections suggests the size of the room, where denser reflections imply a smaller room, as sound quickly reflects and re-reflects from nearby surfaces; the greater the spacing of the early reflections, the bigger the room.
In mixing applications Higher diffusion values are desirable when the material has a lot of percussion. Higher Diffusion can also contribute to a smoother-sounding reverb. With low Diffusion values the early reverb will be grainy and sparse, but will produce a clear, bright sound that is very useful with strings, horns, and vocals.
It is worth noting that density is sometimes labelled as ‘Diffusion’ in some digital algorithmic reverbs, although these two phenomenons are different.
Diffusion
Diffusion affects how quickly the echo density in the reverb builds up over time - essentially the envelope of the delays.
The rate at which early reflections build up differs depending on the position of the listener within the room. A listener at the back of a concert hall will hear the early reflections building up more slowly than a listener near the front.
A properly diffused sound field will benefit from more uniform frequency response and other acoustic qualities.
Late Reflections
Long late reflections can be indicative of large spaces, but much depends on the early reflections that preceded them. For example, a very reflective, small tiled room may have almost as long late reflections as a large hall, but the character of the early reflections and the brightness of the following reverberation tail are what suggest the room’s actual size.
Direct vs. Early Reflections vs. Late Reflections
The closer the listener is to the sound source the greater the proportion of direct sound is heard compared to the reflected sound. Conversely, the further away the listener is from the sound source, the closer in level will be the direct and reflected sounds.
In mixing applications, increasing the reverb wet signal compared to the dry signal will have the effect of moving the sound source to the back of our programmed space.
The further the listener is from the sound source, and or the larger the space, the quieter the reflected sounds and the greater the proportion of late reflections are heard compared to the early reflections. This is due to the boundaries of the room being further away from the listener and thus, travelling longer distances.
Damping
High-frequency damping allows the high frequency decay of the reflected signal to dissipate more quickly than the overall decay time. This emulates the way the materials in real rooms absorb sound. All materials reflect sound more efficiently at some frequencies than at others and typically, high frequencies tend to be absorbed more readily than low frequencies. Thus producing a longer reverberation time at low frequencies than at high frequencies.
Modulation
Modulating the delay times of the recirculation path helps prevent static patterns building up that may cause ringing or other metallic effects.
For mixing applications, mod rates between 0.25 and 0.5 Hz are useful for smoothing out any artefacts in the decay, while rates above 1 Hz are useful for adding a lush chorusing effect. In general, you will want to keep mod depths low when creating realistic smaller spaces, and turn it up when creating large spaces or emulating older digital reverbs.
Stereo Width
Another important aspect of sound in any acoustic space is its binaural nature. Sound occurs in three-dimensional space and even a single handclap is heard in stereo, even though emanating from a single source. The stereo image is created by each ear receiving a slightly different pattern of reflections, coming from different directions at different times. The ‘stereo image’ in artificial reverberation is an extremely important concept and should not be overlooked. We never hear truly mono sounds in nature.
In mixing applications, it is recommended to use the stereo spread parameter within the reverb unit rather than panning the stereo return, as this will minimise phase interactions between the L and R channels.
I believe reverb is a topic many of us struggle with, so it would be great if you could share your reverb editing techniques below and get the conversation going.
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