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Chapter 10 Convolution Reverb: Space Designer 13 9
So, Just What Is Reverberation?
Put simply, reverberation is basically a delay of the source signal x number of times (by
a very small time value), which is then fed back onto itself, simulating the way sound
bounces around a room.
Prior to the invention of digital systems, engineers used a variety of techniques to
create reverb-type effects. As examples:
Reverberant chambers—sending sound via a loudspeaker into a tiled room and re-
recording the reverberant sound—ever recorded a vocal in the toilet?
Reverb units with springs and metal plates hung from the walls—with the latter being
close and distant-miked then re-recorded alongside the original signal.
There are some essential elements for creating a reverberant sound, with the key thing
to remember being that reverb is simply a number of (delayed) copies of the original
signal—which emulates the multiple versions of a sound bouncing around a room.
In a software-based reverb plug-in there may be one copy of the signal dedicated to
making the first reflection: the very first reflected sound we hear when a signal is
introduced into a reverberant space. Other algorithms within the plug-in may be
dedicated to early reflections—the first sounds we hear after the initial reflection. The
(delayed) reflections are fed back, and added, to the original signal at a lower level. This
creates an effect known as comb filtering (a short feedback delay that emphasizes
specific harmonics).
Following on from the delayed signals (first and early reflections) you’ll generally find
that filters are used to make the reverb tails sound as if they are far away. Filtration of
higher frequencies—say 5 kHz and up—makes a signal sound farther away. This is
because high frequencies have very little energy, and don’t travel far. Filtration may also
be used to soften any harmonics introduced by the comb filtering mentioned above.
Needless to say, emphasis of particular harmonics can make a reverb sound very
artificial, which may be just what you’re after … but not if you’re simulating a room!
Note that not all reverb units/plug-ins offer this sort of filtration. Another approach is to
both feed back and feed-forward the same signal simultaneously, phase inverted, to
create an all-pass filter. With this type of filter/delay the comb filtering effect doesn’t
occur (because of the phase inversion of the feed-forward signal—this fills in the
missing frequencies created by the comb filter of the feedback signal). This creates a
blended reverb.