Welcome back to Sound Advice on Acoustics! Last time we began a detailed look at problems and solutions in controlling bass frequencies in the studio, learning about how long wavelengths equal trouble. Dave Moulton continues to rack up the issues we need to deal with....
The next issue with long waves is that they tend to diffract (bend) around most objects in the room. The ability of a wave to diffract around an object is dependent on the relative sizes/lengths of the wave and the object. Small objects tend to be transparent in the face of long wavelengths, and vice versa. So a 50 Hz wave (22 feet long) will tend to diffract around any object smaller than about, well, 22 feet—which is to say it will diffract around any object in a typical room except the walls. It will not reflect off the console (unless you have a truly huge Neve), nor the light fixtures, nor the RPG diffusers on the wall. Those things are irrelevant to its progress. It is only reflected by the boundaries of the enclosure.
And then there’s absorption vs. reflection. Low frequencies can be absorbed by a wall by passing through it via diffraction (i.e., escaping through fissures, holes, and small air gaps), or by causing the wall to flex (and thereby converting sound energy to heat generated by the flexing motion).
Most residential building techniques involve wall construction that doesn’t reflect low frequency energy all that well. Concrete, stone, or very massive solid particle board construction will tend to contain low frequencies and cause them to reflect back into the room. Other more typical building materials, like sheet rock and windows, tend at least partially to absorb and pass low frequencies.
The primary issue with low-frequency room acoustics in studio design is isolation of the studio’s various rooms from each other and the outside world. This involves construction techniques to block the passage of low frequencies through the walls by leakage or by flexing. When you do this, you also increase the amount of low frequencies sent back into the room as reverberant energy, which is generally undesirable in small (i.e., less than 30 foot maximum dimension) rooms. So we need techniques to absorb low frequency energy in small rooms other than by simply sending it over to the neighbors. (Heh, heh!)
The most obvious way to absorb sound energy is by converting it to heat, through friction. It is obvious, audible, and axiomatic that you can do this by placing absorbent materials on the walls. Hence Sonex, Auralex Studiofoam and other open-cell acoustic foam products, curtains, fiberglass panels, egg cartons (for the desperate), etc. But there are limits to this, based on the behavior of sound waves.
First, we have to think a little bit about what sound consists of. Sound is air molecules jiggling back and forth in small areas of space and banging into one another. The sound itself is actually a pressure front moving through the air molecules at 1130 feet per second, a zone of high pressure followed by a zone of low pressure. The molecules themselves move only tiny distances as the density of molecules becomes great (high pressure coming through) and then small (low pressure coming through).
When boundaries are involved in this, some interesting things happen. The boundary wall becomes what is known as a node. As the wavefront approaches the wall, the amounts of molecular motion become smaller and smaller, while the pressure differences become greater and greater. This is because the wall resists the motion of the air molecules. As the wave travels away from the wall, the pressure variation becomes less and less while the molecule motion becomes greater and greater. The point where the molecule motion is greatest, and pressure variation the least, is called the antinode.
Along with the molecule motion being the greatest at the antinode, by the same token, the velocity of the molecules is greatest as well, while at the node, the velocity of the molecules approaches zero.
This is important to know because sound absorption by friction works best where the molecule velocity is greatest, and it works worst where velocity is least. Interesting, eh? You wanna absorb a sound wave? Don’t bother trying to damp the node (i.e., the wall), but instead put some frictional material in its antinode. That’ll stuff it every time!
Next time we’ll learn how to apply this idea in a practical way, with the quarter-wavelength rule. See you then!