This was a question that for a while completely stumped me whilst mixing my soundtrack for BBB. I had never worked on a film set in the outdoors and with only one previous Post-Production work to my name which was set completely indoors I had no idea of the acoustic properties of fields or forests having shockingly never really paid attention whilst out on walks. Forests I could kind of guess as being fairly reverberant with objects such as tree trunks, branches, shrubs and leaves, to reflect and absorb sound, but how about a large open field surrounded by a forest and hills? With hardly anything in the way of reflective surfaces would there be any tail?
I searched through my plug-in collection for one which might provide me with some inkling as to what settings would be required to recreate the acoustics of these two environments only to find one Ableton preset entitled Forest which confirmed that a forest reverb is potentially darn big. After trying it though I wasn’t all that happy with it. I found myself yearning for something more natural sounding and so after scouring the internet for some much needed advice I began to notice a couple of words surfacing time and again. Those words were Convolution Reverb.
Not having come into contact with with Convolution Reverb too many times before (apart from the excellent Altiverb from time to time) I felt the need to research further to gain a better understanding of this particular method of simulating reverberant spaces. In rather basic terms, to create a convolution reverb setting an Impulse Response (“IR”) of a particular room or area is captured. This is done by recording how a space responds to a sound possessing adequate amplitude and the full audible frequency range in order to fully excite its acoustics. The two most common types of sound source used are Sine wave sweeps and sounds with sharp transients, e.g. starter pistols, popping balloons or snare drum crack. As explained by designingsound.org, Sine sweeps covering the entire audible frequency spectrum tend to be the preferred method of creating IRs because they provide increased accuracy. “Depending on the length of the sweep, they usually provides the best signal-to-noise ratio” (longer length = greater signal-to-noise + greater chance of recording resonances). Transient sounds have one advantage in that they require no post processing once recorded.
Put simply Convolution Reverbs essentially recreate the reverberant behaviour of a real-life acoustic space. Curtis Roads (The Computer Music Tutorial) describes convolution in audio terms as:
“Convolution of two audio signals is equivalent to filtering the spectrum of one sound by the spectrum of another sound. Convolution of spectra means that each point in the discrete frequency spectrum of input a is convolved with every point in the spectrum b.”
In the case of convolution reverb, within a designated piece of software such as Altiverb or Avid’s TL Space, each sample of the incoming audio which is to be processed (reverberated) is multiplied (convolved) by the samples in the impulse response file of the acoustic space you wish to reproduce.
With this in mind I thought I would try out Convolution Reverb for BBB. In my plug-in collection I already possessed good Convolution Reverb software in the from of Waves IR-1 / IR-L. The pre-loaded IRs however didnit really suit forest/field imagery so I decided to purchase the “Outdoor Impulse Response” bundle by Boom Library. From the 68 locations I managed to choose two which most complimented the animation. For the field scenes I used the “Field, Wide, Small Hills and Forest” IR, and for the forested scenes I chose “Forest, Plane”. A nice addition to the IRs was the inclusion of pictures of the locations they were recorded:
How reverb was used
On the whole reverb was used quite conservatively. For close up shots reverb was sparingly added in an attempt to imitate how things would sound naturally. In Forest/Field locations quite often reverberation is not consciously noticed when walking and talking alone or in close vicinity to other people. In fields the acoustics close up actually sound almost dead. More reverb was added to loud sounds such as bangs and crashes and also when a sound source appeared further away from camera (further = more reverb), For the field reverb I ran two versions: the original with a decay of close to 6 seconds, and a tweaked version with a decay of 2.7 seconds. The tweaked version was used mostly for action close to camera where as the original was saved for louder and distant noises. As the film is an animation this created scope to use reverb creatively in ways to achieve effects that would not occur naturally. Large doses of reverb were applied for dramatic effect at various points, such as when Frank brutally murders a second butterfly. Here the largest field reverb was employed, with tail exaggerated so that the sound echoed for a long time, bouncing off distant hills and trees. When this was used in conjunction with muting all ambience the resultant effect emphasised the wickedness of Frank’s action and the shock of Big Buck Bunny and surrounding birds not camera, but who all stop their chattering at that moment.
As well as the main reverbs already described, there were others which were used for specific moments during the film. For example:
- Bunny snoring in burrow – A large, cavernous reverb created in Waves RVerb. This reverb was kept in mono because the camera is set outside the burrow, therefore an immersive stereo reverb would not have been suitable
- Gimera walking through hollow tree trunk – A short, small reverb created in Pro Tools DVerb to capture the crampedness of tree trunk. Again this sounded best and more realistic in mono.
- Slow motion flying butterfly – Another large reverb, this time in stereo to portray the sheer downward force created by the butterfly’s wings. A large tail seemed to match the unrealistic image of a butterfly appearing so large and flying so slowly.