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Audio Repair 101: Noise Reduction Basics

May 03, 2018

Get exclusive access to a comprehensive guide on noise reduction by Emmy®-winning dialogue editor John Purcell. Learn how to spot noises in recorded dialogue and other audio, and learn why you should fix some and let others go.

Audio Repair 101: Noise Reduction Basics
This is part 1 in a 3-part series:
  1. Noise Reduction Basics: Understand kinds of noises and how to control them
  2. Controlling transient noises: Clicks, crackles, pops
  3. Ambient Noise: Identifying and removing tonal and broadband noise
In Part 1, you’ll learn how to:
  1. Plan your workflow for repairing various noise issues in an organized way
  2. Recognize the different types of noises you will likely encounter
  3. Decide which noises should be removed and which should stay
  4. Maintain perspective to what should be done in the noise reduction stage, and what should be left for mixing

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 to do about noise

The world’s a pretty noisy place. In the wild, in the city, and even in the library, it’s far noisier than you think: an onslaught of thuds, clicks, snaps; a never-ending stream of hums and buzzes; an irrepressible wash of whooshey, hissy noise. Oddly, these sounds usually go unnoticed in real life.

Shoot a scene on the beach, in a convenience store, near a generator, or even in a quiet office. On the set you may notice a few noises. They may be annoying, but usually not catastrophic. While you film the scene you understand every word of the dialogue, the interaction between the characters is believable and intimate, and the ambient sound goes largely unnoticed. Return to the cutting room and everything changes. The inoffensive background sounds overpower the words, there’s a strange hum you never noticed, and there are clicks of unknown origin. The characters are there, somewhere in the soup, but the track has lost its punch, drive, and accessibility.

When you speak with someone in a noisy environment, your brain filters out information that is irrelevant to the conversation. You can rely on body language and other visual clues, and perhaps a bit of voodoo and guesswork, to magically get you through the conversation.

A mic doesn’t benefit from any of this. It hears everything. Period. The noises you didn’t catch while you were filming have come back to haunt you. The tracks are a noise disaster and it’s up to you to resuscitate them. This is noise reduction.

Set proper expectations

There are very good processors out there that help you clean up messes in dialogue tracks. You can remove all sorts of noise, but you can, in the process, rob the characters of any hope of humanity and reduce room tone to a cellphone-like slosh. To avoid this almost inevitable sin of overprocessing, there are things to consider:

Should all of this noise be removed?

Just because you can remove noises doesn’t necessarily mean that you should remove them. That little chair squeak, the gentle traffic in the background, or the hum of the fluorescent light—there are as many reasons to keep them as there are to lose them. Story, character, space, mood, budget, and common sense lead you to the thumbs up/thumbs down verdict.

A scene recorded next to a busy street can justifiably be noisy. The fact that you have to labor to hear a word or two might help the viewer sympathize with the character, who also must strain to understand and be understood. Remember, though, that a noisy dialogue track saddles you with a noisy scene. The scene will never be quieter than the dialogue premix.

However, even the slightest inappropriate background noise can kill a quiet, intimate scene. A scene of a couple sitting in their living room in the middle of the night, discussing their troubled relationship, will lose its fragility and edge if we hear traffic, the generator, the neighbor’s TV, or a crewmember walking about the set. This scene must have dead-quiet dialogue. The dialogue editor must be able to deliver a believable track with no disturbances: to create a world of two people in a very quiet room, alone with their problems.

What are the issues?

Identify the types of noises you’re up against. What are you trying to accomplish?

What tools are available?

Understand what you need to do and what tools you have to do it with. Create a workflow that makes the most of your available kit.

What are you willing to invest?

There’s a decent correlation between how much time you invest and how good it sounds.

Know your limits

Let’s face it: some noise issues can’t be sorted out. The sooner you make peace with the hopelessness of a scene and steel yourself to dig into alternate takes or wild lines, or rerecord the scene, the better your life will be.

Start with the right tool

Picking a way to start is more complicated than you’d expect. Lots of good processors include the word “noise” in their name. You have a noise problem, so naturally you reach for a processor called “noise-something.” Maybe this is the best choice, maybe not. To give a processor a fair chance of sorting out your problems, it helps to understand the kinds of noises you’ll encounter, and which processors are suited for each.

Talk to the mixer

Before you fire up your DAW, keep in mind that sooner or later the film will be mixed—whether by you or someone else. You and the mixer must have some sort of pact about who’s doing what. Even if you’re mixing the film on your own, you have to decide what you’ll do as you edit and what you’ll do in the mix. With noise reduction this is extraordinarily important and if it’s ignored, then horrible things can happen.

Who is handling the noise reduction?

More than likely it’s a shared job, so the real question is “who’s doing this and who’s doing that?” A mixer would be nuts to want to deal with clicks and pops and other edit-oriented problems. But she may not want you to any broadband processing. Figure it out, make deals, and stick with them.

Who is handling the ambient noises?

You’ll almost certainly process your hisses and rumbles and hums as inserts. No problem. But if your scene has lots of short mismatched clips, you may have to clean them locally with offline processing. Talk to the mixer and make a plan. If you use an offline process to deal with some of the background noise you’ll probably end up edting the scene as best you can, making a copy of all the shots that you will process, and putting them in a safe place in your session. (If you didn’t really get the last sentence, read it again.) Do whatever noise reduction is needed to fix things. If you go to the mix and all of this plays well, you’re a hero. If you get to the mix and you hear that your tracks aren’t as good as you thought, you can mute your nasty processed tracks, bring the edited-but-unprocessed clips online and move along. Still a hero.

Mixing on your own? Ask yourself two questions:

  • Is this process better done in the editing process or in the mix process?
  • Is the step I’m about to take going to paint me into a corner? Am I making choices that I can’t undo later on?

Pay attention to context

If a scene has one shot and no edits, then your noise attenuation job is all about making that one shot sound as nice as possible. You don’t have to match different shots or different takes. But when a scene is made up of several shots, they must match, and when you prepare your dialogue tracks for the mix, keep in mind that it’s probably more important to make shots match than to make any given shot sound its best.

Know the different kinds of noise

There are two (well, three) kinds of unwanted noise. Each requires a different kind of processing. Once you understand this, you’re past the hard part.

Transient (impulsive) noises

  • These range from the tiniest clicks and crackles, to beeps, to longer interruptions like dolly squeaks and footsteps. Such noises may vary in duration and annoyance, but are all, to some degree, time-limited. They start and they stop, perhaps repeatedly. They are events in a scene rather than space-definers.

Ambient noise

  • If transient noises describe the short-term details of what was happening on and around the set when the movie was shot, then ambient noise tells us about the space itself. These noises tend to run the entire length of a scene. They may be constant in nature or they may change over time.

    It’s useful to break ambient noise into two very different types of sound. Generally, they are treated differently:

Harmonic (tonal) sound

  • Dodgy electrical grounding, generators, motors, fluorescent lights, and so forth create tonal noise—things that generate a fundamental frequency with lots and lots of harmonics.

Broadband noise

  • Broadband is non-harmonic, unorganized sound. White noise and tape hiss are good examples. So are wind and crickets.

    The trick is to use the right tool at the right time. The boundary between harmonic and broadband noise can be hard to nail down, so we’ll begin with transient noises, which are clearly different from the other two.

Keep your head

Make a scene as quiet as you reasonably can without taking away the things that touch your heart. As you start to work, step back and remind yourself why you’re doing all this work.

No one cares about your noise reduction. People go to the movies to experience something outside of themselves: To fall in love, seek revenge, be frightened, to feel like a hero, to marvel at the wonders and horrors of humanity. You’d best be invisible. Deliver a dialogue premix that’s as quiet as is needed to make people feel all of those things—without you showing off. Short of a noise disaster, people forgive a bit of noise more than they do the artifacts of overprocessing.

Get a grip. When you’re chasing noises, you become drunk on your power, and it’s easy to lose perspective. “Just a little bit more,” “I’ll stop…soon.” Do what you must, but I highly recommend that you first do what you think is needed to make the track lovely. Leave the room, drink some water, jump up and down—rest your ears and your brain. Go back to the studio and listen. There’s a good chance that your hard-earned tracks are, in fact, overcooked. Reduce the amount of processing. If you can’t fix problem quickly, mark the spot in your session and deal with it tomorrow.

You’re not mixing, you’re reducing noise. Unless you’re the dialogue editor,sound designer, and rerecording mixer all rolled into one, your job at this point is to “make it clean,” not “make it nice.” Noise reduction changes the sound, sometimes for the better. The warm artifact of de-crackling and the focus added from multiband EQ processing may indeed improve the color of a voice. That “better” sound may, however, be a product of artifacts warming up the track. If you don’t know what will happen in the mix, you’re better off aiming for the most neutral voice sound possible, and leave some of the noise. It will make for an easier mix.

Noise reduction boils down to organization, patience, and good listening skills. It gets easier with time.

And finally, give yourself time

Noise reduction is a very repetitive job, often involving many iterations and even more trial and error. At every successful step, save your presets. Name them sensibly. You will thank me for this. Be patient and sit comfortably. Close your eyes from time to time and listen.

Take breaks. Noise reduction is more taxing than you think—you can easily lose perspective.

In the next section, I’ll talk about dealing with transient noises, theshort-term noises that are removed with editing and with click-removing processors.

Read the next article in this series: Controlling Transient Noises: Clicks, Crackles, Pops »

Emmy® winning sound editor John Purcell is the author of Dialogue Editing for Motion Pictures: A Guide to the Invisible Art. He writes for Waves Audio.