Light bounces. So does sound.
Videomakers can reflect or absorb light for creative effect. Same with sound.
Reflected light, if not controlled, can destroy a scene. Likewise, reflected sound.
Most videomakers spend lots of time fine-tuning their lighting. Not so their acoustics. This benign neglect sabotages the overall quality of your video. However, with a little planning and practice, you can make sure your acoustics-and in turn your audio-is the best it can be.
It’s natural to worry about visuals when you shoot. The placement of background elements, the quality of the lighting, the location of props-we all fine-tune these for realism arid continuity between shots.
But what about the acoustics?
Every enclosed space you’ve ever been in has a different combination of objects and surfaces; each adds reflected sound to the room’s unique acoustic signature. Our minds constantly process these sound reflections, which tell us almost as much about our surroundings as our eyes do.
Try a test some time: have a friend lead you blindfolded through an unfamiliar building. You’ll know almost instantly upon entering each room how large it is, especially if you clap
your hands or shout.
We’ve all seen movies or ads the dialogue-every word of been re-recorded in a studio.
To the eye, everything matches up precisely. To the ear, however, the resulting audio proves too sterile, flat, uninvolving.
Thit’s because the controlled acoustics of the recording studio add nothing to the soundtrack.
Our ears expected a potpourri of reflections and reverb; what they heard instead were the acoustics of a foam-lined room. It’s the aural equivalent of shooting every scene without a background.
Like it or not, the acoustic signature of a room becomes an integral part of your soundtrack. You would not shoot without first removing distracting visual elements from the background. Likewise, you should make sure your aural backdrop enhances- rather than hinders-the clarity and realism of your soundtrack.
The sound we hear is actually a complex mix of countless different frequencies. Our ears are sensitive to a broad spectrum of these frequencies, from down around 20 cycles per second (Hertz, or Hz) to up around 20,000 cycles.
No big deal, you might think- you’ve probably heard the term “20 to 20K” numerous times. With acoustics, however, this range is a big deal. Sounds at opposite ends of the human hearing range react with our environment in markedly different ways.
The reason is simple: though you may not think of audible frequencies as coming in different sizes, they do indeed. In addition to cycles-per-second, we can also describe sounds in terms of wavelength. This measurement indicates the length of one cycle of a given frequency as it moves through the air.
Here’s the big difference. Frequencies at the top of the human hearing range are just over half-an-inch long per cycle, while sounds near the bottom of our hearing range have a wavelength of a whopping 50 feet or more.
In the same way that it’s easier for you to move a matchbox car than a monster truck, shorter (higher) frequencies are much easier to push around. They tend to be more maleable, and more at the mercy of their environment.
Let’s take the matchbox car analogy one step further. If you zip your toy car over a full-size speed bump, it goes careening off in another direction entirely.
The monster truck, known to roll over other automobiles for fun, isn’t zoing to notice the speed bump.
Ditto for sounds. Low frequencies roll right over (or through) obstructions that send shorter wavelengths scattering. Whether a given frequency bounces off an object, dissipates into it or goes right around it depends primarily on the object’s size relative to the sound’s wavelength.
Composition, texture and mass of objects and surfaces also affect acoustic properties. The imperfections in a brick wall are enough to break up and scatter short wavelengths; long frequencies remain intact.
A thin blanket does not have enough mass to affect low frequencies, though it absorbs high frequencies. A sheet of lead, having considerable mass, reflects both high and low frequencies.
To sum up: videomakers exercise more acoustic control over high frequencies than low ones. Thankfully, the same properties that make midrange and high frequency sounds the greatest acoustic problem also make them easiest to control.
Addition is Easy, Subtraction is Hard
As you’ll see in a later section, adding reverb or ambience is easy. Reducing the acoustic effects of a room is another story. In fact, turning a massive echo chamber into a good recording set may be one of the biggest problems facing the audio-minded videomaker.
Large, empty rooms with smooth walls are the toughest to tame. Sounds of all frequencies are free to bounce around, turning your audio program into an echo-fest. Why? Because low frequency sounds have enough room to stretch out and resonate, while the smaller high frequency sounds find the smooth walls to be perfect reflectors.
Large, soft objects like mattresses, heavy drapes or cushions will help control mid-range and high frequency reverb. Place them on or near the walls, concentrating them in the corner of the room you choose as your “studio.”
Low frequency reverb is almost impossible to control without major time and materials, because of the long wavelengths involved. You may just have to live with some low-frequency rumble in your audio track.
Though it seems obvious, the simplest method for reducing reverb is to move the mike closer to the sound source. This changes the balance between direct and reflected sound, resulting in clearer up-close audio. Switching from an omnidirectional to a directional mike will produce similar results. Place a highly directional mike a few inches from the sound source and you can virtually eliminate the acoustic effects of any room.
Luckily, videomakers don’t usually have to deal with cavernous, empty rooms. A more common problem: small rooms with smooth, parallel walls.
These rooms can generate annoying repeating echoes, especially on sharp sounds like handclaps or footsteps. The facing walls bounce sound back and forth in a sort of sonic ping-pong match.
The solution: place sound-absorbing materials on or near two adjacent walls. Sound then bounces just once in either direction before hitting the absorptive material. Odds are such a room will have carpet, so you may not need to treat the floor. Low frequency reverb is usually not a problem in small rooms, as there simply isn’t enough space for long wavelengths to develop fully.
These same materials-cushions, couches, drapes and rugs-will help control high-frequency reverb in smaller rooms as well as larger rooms. Come to think of it, when your shooting location looks like a living room, it’ll probably sound about right. The presence of plenty of soft, absorptive materials makes the average American living room function as a great sound stage.
For more permanent sound treatment, numerous companies make foam products designed to trap and eliminate unwanted reflections. Usually glued to walls and ceilings, these products effectively reduce high and mid-frequency reverb. Markertek of New York City, Illbruck of Minneapolis, Acoustical Solutions of Richmond, Virginia, and National Foam of Woodland Hills, California, all make easy- to-apply acoustic foam sheets. Prices range from around $1 per square foot to over $3.
Don’t Kill the Reverb
You can go overboard with acoustic treatment, killing reverb in a room almost entirely. If you want to capture the feel of real-world locations, this type of acoustically sterile environment is not the way to go.
studios of the ’60s and ’70s used this type of acoustically “dead” room, created by piling shag carpet and egg crates on the walls. Today’s audio engineers recognize the added realism of natural ambience when recording sounds or music.
Still, sometimes an acoustically dead room is exactly what the videomaker needs. When doing voiceovers, any reverb with the voice can prove distracting. If you’re trying to simulate someone whispering in the viewer’s ear, less ambience makes for a more convincing effect.
Pros often use small, foam-lined rooms called iso (isolation) or voiceover booths for recording narration.
Free-standing iso booths turn any room into a controlled acoustic environment. They’re available in a wide range of sizes, and break down quickly for transport. WhisperRoom of Morristown, Tennessee markets portable iso booths on wheels, complete with ventilation systems.
If you plan to add ambient effects during editing or dubbing, you can get away with recording dialogue or sound effects in a reverb-free room. This will give you added control over the final sound, but requires more equipment and time in the end. When you add reverb during post-production, be careful to choose an effect that matches the visuals. If a group of people are chatting in a small room, adding coliseum-sized reverb will ruin the illusion.
For good examples of ambience not matching the accompanying images, catch a couple re-runs of The Brady Bunch. Listen closely to scenes supposedly shot in the Brady backyard. If you concentrate, you’ll hear that they have a definite “indoor” ambience to them. You can’t miss the telltale acoustic signature mingling faintly with the voices.
This is because the sound from the actors bounced around an indoor sound stage, one that needed more acoustic treatment. The resulting ambience isn’t really reverb; it’s more like a handful of very short echoes. The effect is subtle, but realism does suffer.
If these scenes truly had been shot outside, sound would have radiated away unhindered. A clever set may fool your eyes, but your ears know the difference between indoor and outdoor acoustics.
Because the ears are such sensitive instruments, maintaining continuity in a soundtrack is not easy. Unfortunately, many videomakers don’t think about audio continuity until it’s too late.
Say you’re shooting a scene with two young girls sitting against a gymnasium wall, complaining about being cut from the school basketball team. You hide a wireless mike near them to capture their dialogue while shooting from a wide shot. Their voices echo convincingly through the empty gym, increasing the sense of separation and rejection. The microphone picks up just the right amount of ambience, while preserving the integrity of the girls’ voices.
You go ahead and shoot the whole dialogue from the wide shot. You plan to shoot close-ups at a later date, in front of a similar-looking wall in your garage. You’ll simply match
quality of the lighting, position me girls in the same way and cut the ew close-ups of their dialogue in to the wide shot. Easy, right?
Wrong. That luscious reverb you admired while shooting in the gym be gone, replaced with a hollow noise from your near-empty garage. Thee gym and your garage have very different acoustic signatures, and the viewwer’s ears will recognize the lack audio continuity right away.
You do have a few options in this case: to shoot the closeups back at the gym, or to enhance the sound of your garage with an electronic reverb unit. Almost all types of audio post- production use artificial reverb, to fake everything from huge caves to the metallic maw of a spaceship. Most reverb units will also create discreet echoes, as well as pitch-based effects such as chorus and flange.
Setting up a reverb unit is simple. Most allow you to select the type of sound space you want to simulate, from small or large room to chamber or hall. After picking the room, you set the length of the reverb. Normal settings fall between 0.5 seconds (a living room) and five or more seconds (an aircraft hanger).
High-frequency decay also affects the reverb. This controls how bright the reverb is, and tells the ear reams about the surface material and construction of the “walls.” Spend some time with these controls, and you should be able to mimic the reverb heard in the empty gym.
In a pinch, you could use the gym itself to add ambience to the footage you shot in your garage. Here’s how: Route your soundtrack to a portable stereo sitting in the empty gym. Play back the audio at moderate volume, and capture the resulting reverb with a stereo or mono microphone.
Mix the reverb back in with the original sound, and you’ll have a convincing effect that smoothes the continuity of your cutaways. You may have to haul a lot of gear to the gym, but it will be worth it. Tricks like this worked well in the years before electronic reverb, when many studios had special chambers designed just to create reverb effects.
Whether you’re cutting out ambience with drapes or adding it with electronics, your videomaking will benefit from a clear understanding of the whys and hows of acoustics. When left to run amok, reflected sound can turn brilliant dialogue and creative sound effects to mud.
But learn to control it, and the world of acoustics will reward you with powerful, gripping audio.
Loren Alldrin is Videomaker‘s technical editor.