Many videomakers, even serious beginners, will take a lot of time to properly place their lighting
equipment for the best possible look. They’ll use scrims and flags and all manner of lighting products to
eliminate or create even the slightest of shadows. And then, once the tape starts rolling, they’ll stick a
microphone in the scene seemingly without concern for the quality of audio they’re recording.
Perhaps this is because so few videomakers understand acoustics, which is the nature (and study) of how sound moves. Like light, sound moves and bounces around the room where you’re shooting. If you
understand how sound behaves in the various spaces and places you shoot video, you’ll be able to control it
nearly as well as you control your images.
Every room or enclosed space has a different effect on sound, sometimes called an “acoustic signature.”
Sound travels in waves, almost like ripples on a pond. Lower bass frequencies have longer waves (greater
time and/or distance between peaks) and higher treble frequencies have shorter waves. When you make
sound, the waves actually bounce around and reflect off of a room’s surfaces for a time before they decay.
Every time a sound wave bounces it gets both partially absorbed and scattered in different directions.
When the decay takes long enough that your ears can distinguish it from the original sound, you hear this as reverberation. If the sound decays quickly, your ear may perceive a sterile, lifeless quality.
In either case, the size and contents of the room you are recording in will determine whether it is a “live” room or a “dead” room. Sound engineers call an empty room with hard walls a “live” or “hard” room, because sound can easily bounce around in it. In such rooms, especially very large ones, sound may bounce around for several seconds before being completely absorbed. For example, consider the long, ringing echoes a basketball makes on the floor of an empty gymnasium.
A room with soft furnishings, soft drapes and carpet (especially a smaller room of this type) is a “dead” or “soft” room. Here sound waves travel for just a short time and get absorbed more quickly by the room’s surfaces. There is little or no reverberation. An extreme example of this is the muffled sound inside of a closet full of clothes. For a graph of how sound waves decay in live and dead rooms, see figure 1.
We mentioned the wavelengths of sound earlier. Because sound waves radiate out in straight lines from the sound source, the shape of a room will influence sound bounce. A perfectly square room will cause certain frequencies of sound to reverberate with more intensity. A room with unequal length walls will results in a smoother decay. Recording engineers build studio rooms at specific sizes to eliminate these
We can think in terms of specific conditions of reverberation and decay time to fit any given room. Examples include:
- Strong reverberation with long decay. You’ll find this combination of acoustics in concert halls, large
stone- or cement-walled rooms, gyms, churches or cathedrals, subways, and large warehouses.
- Strong reverberation with short decay. You’ll find this acoustic signature in tiled bathrooms, kitchens
or medical rooms, open office rooms and open metal rooms such as on ships.
- Minimum reverberation with short decay. Here’s the living room with furniture, drapes, and carpets.
You’ll find the same effect in bedrooms, stores and most any small, furnished room.
- No reverberation. Very wide open spaces outdoors, or a small room with virtually no hard
Whatever the case, the acoustics you capture with the sound of any scene will be a permanent part of your soundtrack for that scene. It’s important to note these acoustic conditions when recording indoors.
Remember–if you want to convince the viewer that any two sounds were recorded in the same locale, their
acoustic signatures have to match.
Acoustics affect the ambient sound (the natural sound) of any given location as much as they affect sounds you create. Professionals know this, and they go out of their way to record at least a few moments of natural sound or “room tone” at any location. You should, too.
You can use this recorded ambient sound later to enhance parts of the soundtrack where there is no dialog. Or you can lay it down as a base for dialog to add continuity to cutaways. Such natural sounds as running water, fans, or traffic noises in the background are the key to aural continuity whether dialog is
present or not. The smooth, continuous sound behind different edits tends to tie your production
One of the easiest ways to control not-so-good acoustics, and therefore your resulting audio recordings, is
to get the mike as close to your talent as possible. If your mike is close to your talent, direct sounds (sounds
traveling directly from the talent to the mike) will be much stronger than reflected sounds. The end result is
crisp, clear audio even from highly reverberant rooms.
The rich, natural reverb of a large church may lead you to believe you’re getting great audio in your recordings. However, if the reverb is strong and your mike is positioned too far from your talent, the reverb will make your talent unintelligible.
Likewise, even smaller rooms can give you enough reverb to make clear recordings nearly impossible. If choosing another location to record is no option, you’ll need to take control of the acoustics of the room. You do this by absorbing or blocking sounds before they have a chance to bounce around the room. This isn’t as easy as it sounds.
Earlier we mentioned that high frequency sound travels in short waves (sometimes less than an inch in length) while low frequency sound travels in long waves (which can reach over twenty five feet). Whether a given frequency bounces off of, dissipates into, or goes right around an object depends on the object’s size relative to the sound’s wavelength. High frequency sounds, for example, will never make it past a soft, fluffy recliner. Bass frequencies will wrap right around the chair and keep on going.
This is why low frequency reverb and echoes are very difficult to control. You’d need a great deal of material to absorb the large, low frequency sound waves. This may be too difficult for the videomaker to
control and you will have to resign yourself to having some low frequency reverberation in your
Shorter mid and high frequency sound waves are easier to control. The solution is to place sound
absorbing materials on or near two opposing walls just outside of the scene. The material absorbs the sound
waves within the first or second bounce. The absorbing material need not be elaborate. Large, soft objects
like mattresses, carpets, cushions, throw rugs, or any other soft and large materials work great. Simply tape,
tack, or otherwise place the materials on the walls. You may want to make hangers out of scrap lumber for
this purpose so you don’t have to mount directly on your walls. For an example of controlling acoustics
with sound dampening frames or “gobos,” see figure 2.
Rooms with carpeting, sofas, and drapes will work too. In fact, the average American living room
makes a perfect sound stage.
Should you want to go farther in controlling your acoustics, numerous companies offer foam products designed to absorb and eliminate unwanted reflected sound waves. These products will effectively reduce high and mid-frequency reverberations. Foam sheets are available from several vendors including
Markertek of New York, Illbruck of Minneapolis, and National Foam of Woodland Hills, California.
Do It The Right Way
Its pretty easy to reach overkill when controlling acoustics. A completely dead room sounds just that way,
dead. If you don’t want your video soundtrack to sound dead, then remember that a little reverb is a good
thing. A little reverb adds realism and a natural ambience to sound, dialog, and even music. Leave some
life in your room when setting up your acoustic absorbers.
When editing, don’t try to merge sounds recorded in an extremely dead room with visuals from an larger indoor setting. Everyone knows what large rooms sound like, and the edit will not ring true with your viewers. Likewise, don’t use sounds from a reverberant room with outdoor scenes.
Take the time before setting up your scene to study your location and listen to the way it sounds. Then make a few test recordings to get an idea of how sound will record there. Then you will know if extra
measures for good audio are necessary. The simple tips given here should help you to know what to expect
from a given location, and what you can do to control the acoustics, if necessary.