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Resident Editor
Mixing Audio for Video

by Craig Tollis

In an ideal world, your edited project would be sent to an audio post production house for sweetening by an engineer, but in the independent world, and increasingly in the budget-strapped world of television post production, you as the editor may have to perform the final sound mix.

The most basic thing is to be aware that a sound mix is not a recreation of reality. It's something artificial that serves your creative purpose. This is both empowered by and limited by capabilities of the technology you and your audience will use to communicate, and by the real world environment in which they will experience it.

Of course, there will be some situations in which you will have problems that need to be fixed with equalization, noise reduction or other techniques, but for now, let's stick with the basics of setting appropriate levels. Firstly, let's look at a few technical elements, then some practical ideas.

Dynamic range: technically, this is the difference between the loudest and the softest sound your equipment can produce. In a real application, it's the difference between the loudest and the softest sound your audience will understand. This will vary with circumstances. In a cinema, you have high fidelity equipment in a controlled environment that gives you more flexibility. For television or internet delivery you are more limited and frequently need to think of the worst case -- say an old TV or small speakers in a laptop. Your audience may also be in a noisy environment, like a living room with other people talking or involved in other activities.

For these reasons, a practical sound mix tends to stay within a smaller range of volumes, a smaller dynamic range, than the sounds we hear naturally. Also keep in mind, in the physical world we naturally adjust our relationships to sound sources to achieve a comfortable "mix." If something is loud, we turn away from it. If something is soft, we lean towards it. We move to block noise or to increase the clarity of something we want to hear. Your audience will probably not want to constantly change their positions or even the volume of their equipment while watching your work. So, you'll want to keep your sound within a comfortable listening range most of the time.

There are also technical requirements that limit the practical range of volumes you can use. This is particularly true for broadcast media, where signals are highly controlled and often severely compressed, but it also applies to other delivery systems where there are standard expected uses of sound levels. You want your DVD or media file to play at more or less the volume the audience is expecting.

To help you with your sound mix, there are usually level meters on your edit system. They are marked in decibels, written as dB. A +10dB change means something sounds twice as loud to us. A -10dB change means it sounds half as loud. Think about this when setting your audio levels. When you're watching television, you wouldn't want something to suddenly get twice as loud, or twice as soft, would you? For that matter, when you are talking with somebody you wouldn't want them to suddenly get twice as loud. You might do this for dramatic effect, but most of the time you are going to want to stay within a smaller, more comfortable range of sound levels.

The final thing to keep in mind is that your edit suite is not a very good representation of the listening environment your audience will be in. You are probably sitting close to your speakers; perhaps they are studio quality monitors. It may also be a controlled, quiet environment so it's easier for you to hear nuances and subtleties that the audience may or may not. Conversely, your audience might have a home theatre system with a subwoofer and much better speakers than you do. Something that seems unimportant when you are editing might be jarring on their system. For this reason it helps to take especial technical and creative care.

Now, some practical considerations.

At the beginning of a mix, it's common to mute all tracks except your primary voice. This might be a mixture of dialog recorded on location, dialog or voice over recorded in a studio, sound on tape or perhaps in some cases even singing or other vocal performance. Even though you may be adding atmospheric sound and music to your mix, keep in mind that your main purpose is for your primary voice to be intelligible to your audience. If they can't understand what's being said, you'll lose them very quickly.

Decide what level you are mixing for in general. A common level is -14dB, although sometimes other levels are used. Your edit system may have a default recommended level: typically around where your level meters turn from green to yellow. Pick a level. Stick to it. Watch your meters. Use changes in volume judiciously. Remember this is art, not reality. If there are places where your sound gets much louder or much softer than your target level, be aware of why you are making this choice creatively. For example, a loud surprise, or encouraging the audience to strain to listen to create tension. Note that these are both exceptional situations, after which you would return to your target level for general listening. Your edit system should allow you to bring levels down as much as need be and give you a small amount of standard boost. Both Avid and Final Cut Pro also have separate tools for boosting a signal more than +12dB, should you need to do so.

It's often helpful to set rough levels in your source material. Well recorded sound should be at a consistent level, so once you set a level in your source it may be good for any time you use that clip. Also, prerecorded music may import into your edit system at an unusually loud level and need to be adjusted for any kind of use.

You should be able to set the volume on your monitor speakers to a comfortable level and then leave it for the entire time you are mixing. If you find yourself changing the volume on your monitors, it may be that the levels in your mix aren't consistent. Keep an eye on your level meters to make sure you're hitting your target.

You may also find that your ears become tired after you have been mixing for a while and that you'll be unconsciously making tracks louder to compensate. Your hearing will also tend to become duller as your ears tire. This will happen more quickly if you have your monitors very loud and may happen more quickly if you are using headphones. It's helpful to take a break every so often and get away from amplified sound to give your hearing a chance to recover.

Watch out for some common things that people do while talking. Sentences often start loud, level off for the most part, then trail away towards the end. People will also tend to get louder again if they take a breath during a sentence. It can be helpful to smooth this out a little. Often the changes you need to make will only be small ones between 3dB and 6dB. Be careful any time you notice yourself making a large change and make sure it is genuinely necessary because of changes in the source.

Next, unmute your foley, ambient or natural sound tracks. These sounds add a number of important factors to your mix. Natural sound, foley or sound effects add impact, or at least make sense of actions we see on the screen. They typically don't have to be very loud, although they may be for specific effect, particularly surprise. Most importantly should not distract from any dialog or other primary voice.

Ambient or natural sound tracks also help establish time and place and give flavor and depth to scenes. Like foley and sound effects they should be used selectively to specific effect. Ask yourself what each sound is adding that helps tell your story. If there is something you don't like in your track, don't just make it softer. Unwanted sound is "noise". Remove unwanted sound altogether or replace it with something you do like.

It's important to maintain continuity in ambient sound and make smooth changes in it. Abrupt changes will be particularly noticeable on high-end sound systems. Your sound operator may have recorded "room tone" (sometimes called "world tone" if you are shooting outside) which is typically around 30 seconds of ambiance that you can lay in areas of silence or under edits to help maintain continuity of sound. Think about the type of edits you are making and what may sound abrupt or discontinuous.

Once you have a fairly consistent mix of primary voice and your other sounds put together, it shouldn't be difficult to add music. Most prerecorded music will already have been mixed to a fairly consistent level. Classical music may change more dramatically and more abruptly, as might an original score. Frequently you can determine two basic levels at which to cut in your music - one level where the music is the point of focus and a "ducked" level where the music will be under your primary voice. The difference between these levels will often be between -6dB and -12dB. If you find yourself ducking more than that the levels of your primary voice may not be consistent. You may also need to bring the music down further if it changes, such as extra instruments being added.

It is sometimes difficult to balance music consistently with voice. Make sure your music is panned correctly if it is in stereo as this will help separate the two. Voice is usually mono and panned center. The balance will often sound different on different televisions or sound systems. If in doubt, favor your primary voice track.

If you find different parts of your mix competing remember your main purpose - to make sure your audience can understand your primary voice. In particular, watch out for sounds that compete directly with your primary voice, like other voices in the background, singing or instruments such as electric guitar which have similar frequencies to the voice. A common error is to allow your primary voice to become masked by other sound elements, but not to notice it because, as you've been editing, you have already heard it dozens of times and know what it says. Your audience, who hasn't heard it before, may have a harder time making it out.

When balancing different elements of your mix, keep in mind that it's relative whether one part is too loud or another is too soft and you may mistake one for the other. If in doubt, go back to muting all the tracks except your primary voice and make sure that it is consistent. Then, bring in ambient and other sound and make sure it is not unnecessarily loud. Finally, listen again with your music added back in. This will help make sure you are identifying the problem track.

Once you have your mix, there are several ways you can test it. One is by listening to it loudly, another softly. Let's call the first the "threshold of pain" test. Just kidding. Listening to very loud sound for extended periods of time will dull your hearing temporarily and may damage it permanently. Anyway, listen to a general part of your project and turn your monitors up to a level that is at the top of your range of comfortable listening. Now play the whole thing. Anything that is much louder than the first section you were listening to should be uncomfortably obvious and you can adjust it.

The opposite technique is to listen to a general part of your project and turn your monitor level way down, so that you can just make out what is being said. Now play your entire piece. Anything that is significantly softer than the first section you listened to, you won't be able to make out, and can be adjusted.

The final test is a simple reality check. Make a viewing copy of your project and try it on different pieces of equipment and in different environments. Try it on a cheap TV. Try it on a home theatre system. Try it on a laptop. Have other people listen to it. If you're having a screening, listen to it on the system that will be used for the event. The response of both consumer and event sound systems will most probably be different to those of your studio monitors. The only reliable test is real world environment.

Note: Craig Tollis is an Atlanta-based editor and Filmmaker with ten years of experience. He will gladly answer your editing questions. Just email: craig@screenreport.com.