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140 Chapter 10 Convolution Reverb: Space Designer
Typical reverb algorithms have parameters for:
• room size (church, club, closet, bathroom, and so on),
• brightness (hard walls, soft walls or curtains and so on) and
• feedback or absorption coefficient (are there people, carpets, and so on in the
room?—how quickly does the sound die?).
• Upmarket reverbs may also contain several filter parameters.
When trying to simulate rooms, take the following into consideration:
• How large is the room? How long will it take for the first reflection to get back to
your ears?
• What are the surfaces like in the room? What material are they made of? Not just the
walls, ceiling, and floor, but are there objects in the room that can reflect sound
waves? As examples: plastic chairs, tables, and so on.
• Are there surfaces in the room that can absorb sound? As examples: curtains, soft
furnishings such as lounges. Even people in the room can also remove energy from
reflected sounds!
Convolving Reverb
The Space Designer is, as you know, a convolving reverb. So, just how does convolution
work? You’ve already learnt how to use the effect parameters, and how to record (or
create) an impulse, and use the impulse response, but now we’re going to look at the
maths behind the process.
Convolution is a fairly complicated software process. It takes each sample in the
impulse response file (of a given room) and multiplies that sample by each sample in
the sound file that we want to place in the room. So each sample input sound file, such
as a vocal sound file that you want to add reverb to, is multiplied by each sample in the
impulse response file.
As an example, imagine a three second impulse file, and a one minute sound file that
you want to add the reverberant characteristics of some space to. At 44.1 kHz, that’s:
3 (sec. − IR) × 44,100 (Samples) × 60 (sec.—audio) × 44,100 (Samples)
= 180 × 680,683,500,000
= 122,523,403,030,000 (Wow!)
This multiplication of each point in each function by every other point in the other
function (in the time domain)—called a cross multiply—produces what we refer to as
the convolution in the frequency domain.
As we’re sure that few of you have applied mathematics degrees, we’ll just say that
there’s a whole lot of computation going on! This computationally intensive (and
remember that we only looked at a 60 second sound!) has not been widely adopted for
the simple reason that the garden variety computer just simply wasn’t fast enough to
do it—until now, that is.