When sound bounces uncontrolled in an enclosed space, it can come back to haunt you. Heres how to control it.
Anyone who has clapped their hands or shouted in a large, empty room knows what bouncing sound–"reverb"–sounds like. Anyone who has tried to shoot video in such a room knows that too much reverb can turn your audio to mud. If you find your videos plagued by reverberant sound, youre not alone. Read on for answers to some of the most common questions about reverb and acoustics.
What causes reverb?
Like light bouncing from bulb to reflector to subject to lens, sound shoots off from its source and careens off anything in its path. As with light, every surface absorbs some sound, while the rest bounces off in a new direction. When all the original sound energy is finally absorbed, the bouncing stops.
Your ear may pick up the first few bounces as discrete echoes. As the sound scatters through the room, youll hear these closely spaced echoes as a smooth, decaying blur of sound. We call this phenomenon "reverb" (see figure 1).
Why do rooms sound so much different from one to the next?
To explain, lets consider another comparison between light and sound. As white light falls on a colored object–blue, for example–the surface of the object absorbs certain frequencies of light and reflects others. In the case of a deep blue object, it absorbs all but the blue frequencies of light. A white object bounces all frequencies equally, while a black object absorbs equally across the visible spectrum.
So it is with sound. Certain surfaces absorb low frequencies and reflect higher ones; others absorb high frequencies and let lower ones pass. Every room has a unique mixture of these surfaces, which contributes to the character of the reflecting sound. A gymnasium has smooth surfaces all around, which reflect a large amount of higher frequencies. A church with plush pews and thick curtains kills high frequencies almost instantly.
The strength of the reverb and how long it takes to die away also differ, both controlled largely by the size of the room. An empty closet has a dramatically quieter, shorter reverb characteristic than an aircraft hanger. Larger rooms also create a longer delay before the earliest reflections, as the original sound has farther to travel to get back to the ear.
Why does reverb sound good sometimes, and bad other times?
"Good" and "bad" room sound really depends on the application. In general, rooms best suited to shooting video have relatively short decay times (with reverb disappearing completely in about a second or less) and an even balance of bouncing frequencies. Hear some examples of good and bad reverb by downloading the MPEG video at www.videomaker.com/edit/other/mpegpa.htm.
Rooms with very long reverb times tend to muddy up the sound. Rooms with lots of very hard, flat surfaces keep high frequencies bouncing around long enough to create a hissy, bright reverb sound (like in a racquetball court). Rooms with more high-frequency absorption than low-frequency tend to have a boomy, dark reverb thats not very pleasing to the ear. When all the audible frequencies decay at roughly the same rate, our ears perceive "good" reverb.
Is less reverb better?
In almost every case, yes. Usually, the goal for your video soundtrack is the most crisp, intelligible dialog possible. Too much reverb can make this very hard to achieve. Too little reverb (a very "dead" room) is almost never a problem. If you want to add some room ambience back in, you can always do so at a later time with an effects processor.
How do I know if I have too much reverb in my audio?
The surest sign that reverb is overpowering your audio is when everybody (and everything) sounds very distant. One way our ears perceive how close we are to a sound source is by analyzing how much reverb is along for the ride. Sounds washed in reverb seem distant and indistinct; sounds that are crisp and free of reverb seem very close. If your audio sounds like it was recorded in a big tin can, thats a sure sign youve got too much reverb.
How do I control reverb when it gets in the way of my audio?
If youre planning to make permanent changes to a place you shoot in regularly, there are several things you can do. The first is to put down carpet, if theres not already some in the room. If you cant carpet your whole shooting area, try putting throw rugs or other small pieces of carpet on the floor. These will go a long way toward absorbing excessive reverb.
The second technique is to bring in a few pieces of soft, stuffed furniture. A couch or loveseat is like a black hole for sound, absorbing almost everything that hits it. Recliners and futons will help as well.
Finally, drapes or other pieces of heavy fabric will kill a lot of reflecting sound. Suspending packing blankets a few inches from one or two walls will cut down on reverb dramatically. You dont need to cover all of each wall. In fact, youll get better results staggering your absorption across two adjacent walls than piling everything on one wall (see figure 2).
If it seems that your shooting space is starting to look like a living room, its no accident; living rooms with soft furniture, carpet and drapes rarely suffer from reverb problems.
When shooting somewhere other than your house or garage studio, excessive reverb becomes a much greater challenge.
What if Im recording away from my house, say in a school gym? How can I control reverb in a huge, echoey room?
In most cases, you cant. Its impractical for most videographers to haul around acoustic screens to block sound, and you wouldnt want them in the shot, anyway. Laying 1,000 square yards of shag carpet is an even less attractive option.
Your first recourse against runaway reverb is to find a new location. Do you have to shoot in the gym, or will an outdoor playground shot suffice? If the gym shot is a must, try setting your action up in one corner. Reverb may be slightly less noticeable there, or you may be able to find a corner of the gym with something absorbent already hanging on the wall.
The best solution is to use an external mike and get it as close to the action as humanly possible. This wont actually make the reverb any quieter, it will just make your talent louder in relation. The end result is the same–crisp, up-close sound.
A lavalier will often give adequate isolation of the voice, though many lavaliers pick up sound equally from all directions and may still capture too much reverb. For the ultimate in reverb rejection, give your talent a directional handheld mike. Not only will the directional mike reduce the reverb hitting it from behind; it will be just inches from your talents mouth. This will make your talents voice much louder than the competing reverb.
Ive seen studios with egg crates and carpet on the walls. Does that help cut down on reverb?
Contrary to popular belief, egg crates do almost nothing to stop reverberation. Absorbing sound generally requires mass, something egg crates have very little of. Carpet will help to some degree, though it tends to absorb only the highest frequencies. This will give you less reverb, but the ambience thats left may be an unnaturally dark and dull.
Foam meant for sound treatment of studios provides equal absorption of all but the lowest frequencies. Its not inexpensive by any stretch, but its the best way to tame troubled acoustics. Thankfully, you dont need to cover every inch of every wall with acoustic foam to make a noticeable improvement in your sound. Try spacing the squares of foam around the room, covering about 25% of your wall surface area. The results will amaze you. Important note: though it may be better than nothing, the foam used for hospital beds looks similar to acoustic foam but is nowhere near as effective.
What if I want lots of reverb to make a sound seem distant?
Simply use your camcorders built-in mike and shoot away! Seriously, the easiest way to get a purposeful "distant" sound is to move the mike a good distance from your sound source. The rule is simple: the greater the distance between mike and sound source, the greater the amount of ambient sound and reverb recorded.
Using a less directional mike will also help, with an omnidirectional (non-directional) design picking the greater proportion of reverb. For a really distant effect, try pointing a directional mike away from the sound source. Aimed this way, it will pick up almost no direct sound at all.
Lastly, you can always capture your audio as cleanly as possible with plans to add reverb at a later time. A digital reverb processor costs less that $200, and can add convincing ambience to even the driest sound. This method has the added benefit of giving you complete control, allowing you to try many different types of ambience before you settle on the effect thats right for your shot.