Famed Mastering Engineer Bob Ohlsson once said that all signal processing is a trade-off between sonic degradation and a perceived improvement in sound quality. Nowhere is this truer than with the many noise reduction and audio repair software packages that have developed over the last decade-and-a-half. The first time you remove hiss, air, handling noise, or camera whir from a recording, it’s like the sun comes out and clarity and goodness rules the land. There is, however, that pesky trade-off to consider.
All broadband noise reduction processes, regardless of developer, work on the principle of dividing the audio spectrum up into many individual frequency bands, each with its own level detection and gating process. The system is “trained” by sampling some unwanted noise and the frequency response trace created mirrors its spectral balance, in turn becoming the threshold for each band’s gate. When program level exceeds the threshold, it passes through unhindered but when it drops to, or below, the threshold, the band’s volume drops by a user-set amount. In well-designed software the transition from gated to un-gated audio and back again is all but imperceptible.
A key drawback comes when this process damages the ambience of a recording by applying a sudden cut-off to its natural decay. The human perceptual system is extremely sensitive to such changes, often to the point where any benefit of the reduction is undermined by the distraction. When this happens, a quick fix is to apply some artificial reverb to smooth out and mask the discontinuity; however, unless care is taken to match the character to that from the original recording, the mismatch will force you to use an excessive amount could make things too wet and murky.
A great way to match the reverberation character of the recording space is to capture its impulse response (IR), and then use that sample with an IR reverberation device or plug-in. Capturing IR simply requires setting up some good, flat broadband speakers, playing the test signal – either a swept tone or a short click or pop – and recording the result with a microphone array. Once recorded, the IR can be used with any number of software plug-ins.
With this powerful array of tools, recordings can be made in environments that would have been unacceptable in former times. Free of the interference of unwanted noise, location recordings can also retain the subtle ambience cues that create a sense of compelling realism, providing a more immersive and involving experience for the listener.
Frank Lockwood is a Location Music Recording Engineer specializing in capturing performances in concert halls, churches, and other live performance venues. Examples of noise-reduced and IR reverberated recordings can be found at www.lockwoodars.com/discography/choral.