Audio Effects: Convolution

Convolution reverberation is different from other forms of reverberation in that it is based on actual impulse responses from real world spaces. This effect is looked upon as the holy grail of space representation in audio recordings because it is based on actual rooms, arenas and open spaces. This is also an excellent effect for creating a space that the listener can relate to, such as a bus station, underground tunnel or large open fields where ambiance can be detected but there are few reflections from walls or closed areas.

The impulse response used in this effect may only be a second in length and is stored in a DSP system that can be recalled by selecting presets or by using a plug-in based VST to create the desired space effect. Convolution effects can also be created for guitar cabinets or for drum rooms and vocal booths to use real world effects for an audio program. Once these impulse responses are input, they are mixed with the original audio program to create the space the audio engineer needs for the audio material. This can be useful with several different tracks or as a main mix for the audio material. Once the effect has been tailored to the audio engineer’s needs, the final result can be almost impossible to distinguish from real world spaces.

The way impulse responses are created is simple but time consuming. A “shot” or wave form is played in a space that the audio engineer wants to create a response for, and the resulting reflections from the walls, floors and ceiling of the environment are recorded. These reflections are then stored digitally for later use in professional recording suites that might contain hundreds of these impulse responses. Once they have been stored, they are usually sold in massive convolution kits that contain many different types of environments. These environments can range from small live venues, churches, baseball fields and other sought-after environments that audio engineers want to re-create in the studio.

By taking a recording session of a band and mixing it with different types of impulse responses, an audio engineer can take an otherwise dry recording and place the final mix into a large space, thus creating the effect of a bar or concert arena. This allows the listener to be immersed in the space the engineer has created and makes an audio program “feel” like it was recorded in a place other than a studio.

Many audio engineers use these impulse responses over normal software versions of reverberation because of the realism the convolution impulse responses provide. Most software-based audio workstations can use either software reverberation or convolution reverberation because both are standards used by the industry. But most professional audio engineers will use the convolution systems over any other because of the realistic reflections. Most common reflections are wood, stone, open ambiances and carpet, but most systems that use impulse responses will contain hundreds of different types of materials to create an unlimited palette of responses.