Acoustics Without Voodoo
by John Reynolds
john@reynoldsaudio.com

I am no physicist, but...
I was fortunate to have a (fairly menial) job at the University of Pennsylvania in Philadelphia in the late '70s. One of the perks was that I could take courses at this excellent Ivy League school for free.

In the physics department there was a professor who was also a musician. To merge his passions, he offered a course called "The Physics of Music", which was presented as a "non math" physics course. I didn't know when I signed up that "non math" to a physics major meant nothing more complicated than calculus. So I struggled with it, but got out with a B+. Point is, the stuff I learned there, like all physics, holds up over time. It has served me well and proved it's worth over and over, as I've worked in many studios and hundreds of venues since then.

It really isn't hard to understand. Ok, the math is complex if you want to calculate the intersections of standing waves, in three dimensions, in an irregular room (which we did). But avoiding those waves is fairly simple. Here's a synopsis of what I learned at U of P, as well as some tips I've picked up working in the real world of audio.

For this discussion, I'll focus on concepts that pertain to obtaining a good environment for performing or recording music, using relatively inexpensive treatments. Sound proofing (isolation), that is keeping loud sounds in or out of a room or building, can be both difficult and expensive, so I will not go there for now.

All materials or objects in your room are one or more of the following
1- Transparent- no affect on sound (or absorptive to only very high frequencies), thin fabric or paper materials (including the egg cartons once thought by many to be a good low cost treatment).
2- Absorptive (soft, porous), heavy fabrics, soundboard, acoustic foam, upholstered furniture. Note that when sound waves are absorbed, the energy turns into heat.
3- Reflective (hard, dense), painted sheetrock, windows.
4- Diffusive (irregular shape or density and/or convex reflective), loaded bookshelves, convex curved surfaces.

The rule is that the lower the frequency (longer the wavelength) the more material (mass) is required to cause 2, 3 or 4. Very loud and low sounds will penetrate all but the densest and/or thickest materials.
So layered materials of differing densities and thicknesses are most effective for broadband results in any of the applications described below.

Clap your hands loudly in a relatively empty, small, box shaped bedroom.
You'll hear a short, twangy midrange reverb that you probably don't want added to your vocal or acoustic guitar sounds. These are standing waves. Notice that as you move around the room, the pitch and loudness of the echoes will change.

Standing waves are caused when there is an integral relationship between the wavelength (lower frequency = longer wavelength) and the distance between parallel surfaces. These waves will reflect back on themselves at the node (the place in the sound wave where it crosses from positive to negative air pressure), causing resonance (boosting that frequency). Or if the length is such that it reflects at the anti-node (that is the peak of positive or negative air pressure in the sound wave), cancellation (the out of phase reflection causes that frequency to be less audible).

In my studio in Santa Cruz, which was quite small, I actually built new interior walls which slanted in a few inches towards the ceiling, to avoid standing waves. It looked a little like the Mystery Spot (folks who've been to the Central Coast tourist scam will understand), but did the job without eating up too much floor space. Short of that, placing absorptive and diffusive materials on those walls helps, or you can even place a sheet of plywood covered with foam or moving blankets at an angle (any angle) to disrupt the parallels. The fewer reflective parallel surfaces, the better.

Don't be afraid to experiment with your room treatment. If you must work with parallel walls, go ahead and dampen them with absorbers. It can do no harm. The easiest thing to do with a box shaped room, (any box shaped room will have undesirable standing waves), is to deaden it. You'd be amazed what a few heavy blankets, carpets or curtains can do.

The other thing you want to do is diffuse. That means to reflect the sound in multiple directions. This makes the kind of musical reflections associated with a nice reverb, because they bounce around randomly and don't accentuate any particular frequencies. Book shelves loaded with books make excellent diffusers due to the random variety of sizes and densities. The wall that your monitor speakers face should have lots of 'em.

Another problem is low ceilings, or walls that are too close to the mic and source. The inverse square law says that the sound energy will decrease proportionally to the distance from the source. The closer the surface, the louder the reflections will be in the mic. This is why large rooms with high ceilings are coveted by studio owners. Get in the habit of placing your source and mics away from reflective surfaces when possible, unless you're trying to create a slap back echo (if so, figure approximately 1 foot=1 ms, in both directions).

Another thing to watch out for is arched or cathedral ceilings (or bay windows), which will reflect back towards the center, causing a build up of focused reflections, the same way a satellite dish focuses radio waves on an antennae. Hanging layered materials of different densities such as plywood, sheetrock, soundboard, foam (not the acoustically transparent kind of foam used for packing or sleeping, but the denser acoustic foam) from these ceilings will absorb and diffuse the reflections in different directions. Concave surfaces will focus reflections. Convex surfaces will diffuse, which is why large tube (or half tube) shaped materials are helpful.

Ceilings can be particularly problematic. I find that acoustic "clouds", layered absorptive materials suspended from the ceiling, especially in the direct line from monitors to ceiling (above the "sweet spot") and above the performing space, help tame reflections before they start bouncing around the rest of the room.

Walk around the room with a drum stick and a wood block or other percussion with a fast decay. Listen for those places where you get flutter (multiple) echoes, and dampen and/or diffuse the walls or ceiling closest to those spots. Those flutters will always blur and color the sound of an instrument in that position. So if you can't treat them effectively, avoid placing instruments or mics in those places, and try to find more natural sounding positions.

Dealing with low frequencies gets a little trickier, mostly due to the mass needed to affect long wavelengths. Measuring and calculating dimensions for tuned bass traps (targeting specific problem frequencies) isn't simple, but generally what you want is lots of absorption positioned where bass is likely to build up (corners and wall to floor or wall to ceiling lines). Big thick sofas or couches can be helpful, placed against a wall. The big foam wedges that companies like Auralex sell are also effective for taming bass build ups inherent in corners.

I was able to build an effective bass trap in my new studio by placing all the empty boxes my gear came in (multiple layers of thick cardboard, including the absorptive foam inserts) under a wooden counter that runs the length of the far wall. The result is a 3'x3'x24' absorber made up of different densities of materials (including the wooden counter and drawers spaced along it and the dead air inside the boxes), that lets the bass go in but not bounce back to the room.

Too much absorption, of course, will kill all of the room sound, which for really small or bad sounding rooms (such as an isolation booth), may be the best solution. But if you want to use the sound of the room, a balance of absorption, reflection and diffusion is desired, along with low frequency control, to keep remaining reflections scattered and free of resonance.

All of these concepts will hold true in any room or venue. Acoustics is not voodoo. The principles of wave behavior apply not only to sound, but to ripples in water, light, radio, or any other wave in nature, and can be observed in everyday life. Remember sound waves will always reflect just like billiard balls. So it's pretty easy to follow the line of sight just as you would when lining up a bank shot, and know where the sound is going to go.

Play with moving some materials around your room. Like anything in music, if you don't like the results, keep changing stuff and trust your ears.

Finally, since we're talking about art here, sometimes wrong is right. A particular resonant or focused reflection that would normally be avoided, might be just the right thing to bring out a desired musical effect in certain contexts. So it helps to allow for some flexibility to modify or remove your acoustic treatments, and not be too locked in to always doing things the "right" way.
Use your ears and musical taste.

Of course there is much more to the science and physics of sound than this, but I've found these principles to be reliable and effective for creating good sounding spaces for performing or recording, and you don't need magic potions or secret incantations to understand and use them.

c.2004 John Reynolds