Get access to a comprehensive guide on noise reduction by Emmy®-winning dialogue editor John Purcell. Learn how to control ambient sounds like wind, traffic, and electrical hum that run the length of a scene, whether they’re broadband or tonal.
This is written from the perspective of film dialogue postproduction, but noise is noise, so what you learn can be applied to other audio recordings as well.
What is ambient noise?
Ambient noises are the unwanted sounds that run the length of a scene. They may be steady, they may change over time, but they are generated by the space and its surroundings rather than by short events within the space. This article will help you control these noises.
There are two basic categories of ambient noise: harmonic (tonal) and broadband. Today you can probably use one plugin to process a track that contains both harmonic and broadband problems and get decent results, but usually you should try to do your ambient processing in two steps. This lets the processors do what they do best, and keeps you from having to overcook the tracks.
Reducing harmonic noise
Harmonic noise is, by definition, harmonic, which means that in most cases you clean it up with a multiband EQ.
Use a multiband EQ to create a custom harmonic filter. Locate and attenuate the fundamental sound and as many of its harmonics that it takes to control the noise. Notice that in the image below, the Q and gain cut are tailored for each band. This enables you to find the greatest harmonic reduction per band with the least damage to the dialogue.
The process is easy:
Remember, listen. Solo and/or mute each band each time you change a setting. Bypass the plugin and compare with the original. Listen past the noise. Stick your head through it and hear the voices of the original. Compare. Listen, listen, listen.
Traps to listen for when using an EQ for tonal noise:
The “Full Reset” menu item in the WaveSystem Toolbar resets the plugin to its factory settings. Sometimes it’s better to give up and start from scratch than to keep fixing a fix of a fix. You’ll likely come up with better results when you’re not burdened with previous misjudgments.
Background noise that’s purely electrical in nature can be reduced or eliminated with a hum filter. These procesors create a notch pattern of based on a harmonic series based on a selected fundamental. Typically, these devices default to 50 Hz or 60 Hz, for obvious reasons.
The cleaner the contaminating noise, the better chance you stand of reducing it. Artifacts in the signal, reflections, and other imperfections in the noise will make the processor less effective.
Reducing broadband noise
Reducing broadband noise is more complicated and more prone to abuse than using an EQ to remove harmonic noises. It’s so powerful and so fun that you can easily be seduced into overdoing it. Take your time. Temper your enthusiasm. Know your limits. Listen.
Broadband noise reduction works by creating a description of pure noise: noise that’s not contaminated by words or other sounds that relate to the dialogue. Some denoisers require a user-created sample of pure sound. These plugins are used primarily for offline work. Other processors extract sample noise from the signal in real time. They also have little or no latency, so they’re well suited for mixing.
All broadband processors work in more or less the same way. Here’s a simplified description of what they do.
Offline broadband processing
Each plugin has its own types and names of controls, and the details of operation differ a bit, so this example is only a basic tour of the essentials. For proper instructions, read the plugin’s user guide (always a good idea!). In this section we’ll look at three different broadband reduction plugins. They represent three different generations of broadband processing. If you understand the basics of these three plugins, you’ll grasp the workings of all of them, regardless the manufacturer.
X-Noise is a classic broadband processor, where you manually locate a chunk of room tone that’s as free from desirable signal as you can find. The plugin then makes a noise profile. You know the rest.
Here’s what you need to know to run this kind of processor:
The other X-Noise controls have to do with attack and release for the gate-like attributes, and a high shelf. Read the manual.
This generation of broadband processors provides miraculous tools for reducing noise without undue damage to high-frequency sounds. They are still widely used. But soon an additional need arose.
X-Noise and its brethren reduce broadband backgrounds using a noise profile and two basic controls. This is effective much of the time, but when Threshold and Reduction are the only controls at your disposal, you at times have to choose between adequate cleaning with some artifacts or no artifacts but less noise reduction than you’d like. The solution is to manipulate the curve of the noise sample itself.
Z-Noise adds a noise profile editing tool (the diamond-shaped makers on the graph). It lets you, in essence, re-write parts of the noise profile. You can change what the processor chooses to attack and what to let pass.
By reducing the amount of model-based attenuation at a specific frequency (move the marker down), those frequencies are processed less (probably resulting in fewer artifacts and perhaps more noise at that frequency only).
When you lower a marker, the associated freqenency will likely get louder.
Something else that developed in the generation between X-Noise and Z-Noise is the ability to extract noise profile from a signal directly. Sometimes you just can’t get a decent bit of noise, or it keeps changing. In such cases, Extract creates an ever-changing noise sample.
Online broadband processing
Since online processors are meant for mixing conditions, a few things are expected of them:
Most online noise-reduction processors use the Z-Noise “extract the noise sample” model. You’ll typically see a bank of faders, each of which controls a processing band. Next, some sort of threshold control, and finally a Resolution control. Its range could be “Low to High,” or “Smoothing 0–100,” or “Smooth to Punch,” or any other such thing. It’s the sharpness of recombination back to the time domain.
If you’re comfortable with the Z-Noise controls, the WNS Noise Suppresor will come naturally:
Of course, if you’re working with inserts, you can always change the settings later. Buy why not try to get it right now, since the sound of the dialogue will affect other elements?
There’s really nothing more to know. Well, that’s not true. There are the matters of skill, experience, curiosity, and diligence that make for a good editor or mixer.
Remember: A little bit of broadband noise reduction goes a long way, so take it easy.
Working with inserts vs offline processing
Whether to work online or offline is not always obvious. Clearly, if you’re mixing, you’re likely going to process on inserts, and if you’re trying to save a scene in which each take of each shot is unique, you’re best served with offline processing. But there are some gray areas to explore.
The upside of using inserts for noise reduction is that you work in real time: you can hear all of the processed dialogue in context, and you can easily tell if you’re doing too much/little work at a specific moment. In other words, it’s live. Inserts provide you quick, flexible solutions. Inserts are more forgiving than offline processing such as AudioSuite.
The biggest downside of insert processing in non-mixing settings is about you, not so much about processors: insert noise reduction processing can make you lazy. By the time you’re ready to work on noise reduction, you’ve already arranged the film by scenes, organized the scene’s clips by shot and other variables, found separate room tone for each shot, created sensible shot transitions that will minimize processing, removed all the transient noises that you have patience for, and basically worked yourself silly to make tracks that really work well and are very mixable. (If you don’t understand what I’m talking about, read my book on dialogue editing. I know this sounds like a promo—it is, of course—but it’s the quickest way for you to learn how this works).
Despite your well-planned tracks, you may break down and shove all the dialogue channels into one buss, slap on some generic noise reduction, and call it a day. This is a common, self-destructive mistake. Even if you do your EQ processing at the track level, and then the broadband processing on a dialogue buss, you undermine your well-orgainzed and caring dialogue edit. All the dialogue is going down one noise reduction highway, and you’ll inevitably have to over-process in order to iron out the mismatching room tones. You’ll probably say, “I’d never do something so obviously silly.” You will.
However, real-time insert processing needn’t result in a compromised, overcooked, boring scene. If you keep the bulk of your processing on the channels (not on the dialogue buss) and customize the settings for each channel in the scene, you can do the correct—and minimum—noise reduction processing for each shot. On the buss you can do whatever processing is common to all the tracks in the scene. This lets you use the dialogue buss do the make the scene relatively quiet and smooth, without having to over-process. Remember to save all of your plugin presets. If you rely on the DAW to save your plugin settings, you’ll lose a lot of flexibility down the line.
One more thing about inserts: latency. Many modern processors work with zero latency, so you can pile lots of them onto a channel with no concern of slowing down the whole channel, buss, or even mix. However, some perfectly good processors have notable latency. This makes them problematic as inserts. Yes, you can offset the channel itself to match the latency, but there’s a limit to how much of this you want to do.
Bottom line on inserts: This is a perfectly sensible way to work, by far the most common way—in fact, the only way—to mix dialogue tracks. Save your high-latency plugins for offline processing.
Certain situations aren’t well suited for insert processing:
Offline noise reduction is not unusual. In some ways it’s more focused and precise, since you are hand-making each clip and you’re probably spending more time doing it. Some processors that offer insert processing provide a “higher quality” offline algorithm that delivers a quality that can’t be delivered in real time. Offline processing clearly doesn’t care about delay, so there’s no worry about latency.
If you want to do some offline processing on a scene or part of a scene, save yourself horrible headaches by paying attention to these steps.
Save your steps and presets
It’s very likely that you’ll be doing more than one step of processesing when you work offline. After all, if your noise reduction worklow were so simple you’d be working online. Since each step is built on the previous, save each iteration. This lets you (1) back up and hear where you went wrong, and (2) blaze a new path from there. If you simply blast over each step you can’t back out of the mess you built, and you’ll have to start from scratch if you run into a brick wall.
This image shows a multi-iteration de-noise process. The original clip is on top. Each step in the process is built on the one above it. Note that as number of processes increases, the “pedigree” names become longer.
Also, at each stage, save a plugin preset of what you’re doing and name it sensibly. You will need to reproduce settings. Really. It’s totally stupid to have to recreate the parameter settings each time you need the same or similar settings.
It’s easy to forget to keep and label each layer of processing, but sooner or later you will wish that you had.
Remember Why You’re Doing This
We’ve pretty well established that every noise reduction job is different. If you can’t think of a better way or working on your project, use this sequence. At every step, compare your work with the previous one and with the original.
« Read the previous article in this series: Controlling transient noises: Clicks, crackles, pops