How to rid recordings of hiss, buzz, and traffic noise
Fig. 1. iZotope Denoise provides controls for independently attenuating tonal and broadband components of noise. Output Noise Only (at the top of the GUI) is checked to solo the removed noise. NOTHING IS worse than recording a terrific performance on an acoustic instrument, only to realize during playback that hiss from noisy mics and preamps has polluted the irreplaceable track. Similarly, a once-in-a lifetime electric guitar solo quickly loses its luster when blighted by power-mains buzz. And while sonic filth can often be avoided in a properly grounded, soundproof studio wielding high-end gear, it’s often completely unavoidable in location recordings for film and video shoots, where noise from cameras, traffic, and HVAC systems rule the roost. Let’s face it, every mix and post-production sound engineer can benefit from having noise reduction software in their armory.
This article will explore how to drain the dreck from infected tracks to restore their purity. I’ll focus on techniques using the superb iZotope Denoise plug-in, which is included in the company’s RX2 and RX2 Advanced noise-reduction bundles.
Identify the Enemy Find a section of your track where noise exists alone. While the noise plays back, click Denoise’s Learn button to teach the plug-in the spectral content of the racket you wish to eject. If the snippet of noise is quickly followed by desired signal, you can avoid including the latter in the scrap heap by looping only the noise in your DAW during the plug-in’s learning process. Click the Learn button again to finish the analysis and ditch the din. Boosting the Noise reduction control increases the hush.
Go Solo You’ll get better results if you also adjust Denoise’s other controls. Select the GUI’s Advanced tab. Check the Output noiseonly box to solo the removed noise while adjusting the respective Threshold controls for tonal (for example, buzz) and broadband (hiss) components of the noise (see Figure 1). Find the best settings to avoid including any desired signal in the removed noise. If buzz is the main problem, you can ostensibly treat it more heavily by raising the Tonal Threshold and Reduction sliders while leaving the Broadband Reduction control set closer to 0.0dB. But don’t be attached to the controls’ intended targets; I sometimes find the broadband controls to be more effective for removing buzz. Experiment and see which control set works best for the noise you want to remove. Regardless, the golden rule is to never apply any more processing than you need to make noise unobjectionable—not necessarily eliminated— while preserving the fidelity of desired signal. Cranking the reduction sliders too high will make the track sound muffled, less detailed, and devoid of depth and nuance.
Keep Baby and Bathwater Separate While you work, frequently toggle the Output Noise Only function to assess what is being removed (box checked) and the quality of the remaining, desired signal post-processing (box unchecked). If you hear watery-sounding artifacts in the desired signal, carefully raise the Global Smoothing control. You’ll know you’ve raised it too far if you begin to hear noise modulating in level when transients in the program voice (with Output Noise Only unchecked) or you hear bits of desired audio signal when Output Noise Only is selected. Find the best compromise setting, then raise the Fine Smoothing control just enough to eliminate any remaining artifacts and no higher.
The Denoise plug-in in the more fullfeatured RX2 Advanced also provides a Release control you can raise to reduce artifacts. Be careful not to boost the release time too much, though, or transients and reverb tails will get munched.
Fig. 2. The Denoise plug-in in RX2 Advanced offers an expanded control set, including facilities for dynamically attenuating noise that changes over time. Adapt If Necessary If you’re treating tracks—such as those recorded outdoors during a video shoot—in which the quality of embedded noise changes over time, check Denoise’s Adaptive Noise Learning box (included only in RX2 Advanced; see Figure 2). In this mode, the Learn button at the bottom of the GUI serves no function, so ignore it. Instead, adjust the Learning Time slider (located to the right of the Adaptive Noise Learning checkbox) for the best sound during playback. The shorter the time value, the faster Denoise will react to changing input signal, nipping changes in traffic noise, for example, in the bud. If you hear transients getting munched or watery-sounding artifacts in desired signal such as dialog, increase the learning time.
This adaptive functionality is identical to that provided by the checkbox labeled Adapt to Changing Noise Profiles and its associated Learning Time slider under the GUI’s Simple tab. The two checkboxes are titled differently for no apparent reason.
Put It In Neutral If the noise in some frequency bands sounds more audible than in others, increase RX2 Advanced’s Residual Whitening control until the noise spectrum sounds more neutral. (I find it helpful to loop an isolated selection of noise while making this adjustment.) Doing so will preclude your having to crank the reduction sliders too high in an attempt to improve results in one frequency band while hammering the others too hard. In cases where the noise spectrum sounds really lopsided, you can create a custom noise-suppression response curve— containing up to 26 frequency nodes—in either RX2 or RX2 Advanced.
Get Excited To preserve high-frequency harmonics and prevent dulling, try raising RX2 Advanced’s Harmonic Enhancement slider until the de-noised track sounds sufficiently present. If you hear high-frequency noise pumping, lower the slider to the point where the modulation ceases. If that makes the signal sound too dull again, try raising the HF Synthesis slider, but be careful: Too high of a setting can make the track sound glassy.
Clean Deeper Denoise is just one of the noise-nuking weapons in RX2 and RX2 Advanced’s arsenals. Other plug-ins included in the two bundles effectively discharge AC hum, clicks, crackling noises, clipping distortion, and even chair squeaks and dog barks from tracks. Let the world rumble and roar. There’s a plug-in for that.
Michael Cooper (myspace.com/michaelcooperrecording) has worked with Academy Award-winning actor William Hurt and the Emmy Award-winning ABC News correspondent Barry Serafin. Cooper is a contributing editor for Mix magazine.