Diffraction

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 BIIIIG 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.

Absorption/Reflection

This brings up the next issue about the long wavelengths. 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 to at least partially 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 either 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 ft. maximum dimension) rooms.

So, we need to have techniques to absorb low frequency energy in small rooms other than by simply sending it over to the neighbors. (Heh, heh!)

Absorption of Sound by Friction

The most obvious way absorb sound energy is by converting it to heat by friction. It is obvious, audible and axiomatic that you can do this by placing absorbent materials on the walls. Hence, egg-cartons, Sonex, curtains, fiberglass panels, 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).

Nodes and Antinodes/Pressure vs. Velocity

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.

Now, 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!

The Quarter Wavelength Rule

This leads to the quarter-wavelength rule. If a wall is a node, then the nearest other node at any frequency will be 1/2 wavelength away from the wall (trust me on this, because I don’t want to write a whole other article about standing waves and resonance right now). Given that this is so, and it is, then the antinode is midway between those two points, or one-quarter of the wavelength away from the wall, for any given frequency. So, if you want to filter out, say, 60 Hz., you can do a dandy job by finding the wavelength of 60 Hz. (about 18 feet), divide it by four (about 4’ 6”), and hanging a thin layer of frictional material (like fiberglass) at that distance from the wall(s), and floor and ceiling as well if you’re really serious about nuking 60 Hz. If you want to filter out all frequencies above 60 Hz., you can hang many such layers, or simply install a thick soft fuzzy boundary layer 4 1/2 feet thick! It’ll work wonders! It’ll also be expensive (have you priced 4.5’ layers of fiberglass lately? How about Sonex?)!

A four-inch layer of Sonex or other open-cell foam represents a quarter wavelength of 800 Hz., so such a layer will absorb virtually all sound energy striking it above 800 Hz in frequency. The absorption will fall off rapidly below that.

The Myth of Corners

There is a commonly held assumption that the best place to trap bass is in the corners of a room, because there is a “bass buildup” there. Unfortunately, the bass buildup is an increase in pressure variation, not molecular velocity. If you stick a “bass trap” tightly into a corner, it can’t do much absorption at frequencies lower than those whose quarter wavelength is equal or less than the dimensions of the trapping device. For instance a trap one foot in diameter touching the walls will only absorb frequencies down to about 300 Hz.

Absorption of Sound Through Resonance

There is more to this, however. There is another dimensional behavior of acoustics which comes into play that we can use to absorb sound. That behavior is called resonance, which is the sympathetic or induced vibration of a system (a solid, a membrane or an air space) in response to the presence of vibration in the air.

When a sound of a given frequency occurs in air, its energy may excite resonance, or secondary vibration. When that happens, one of the things that occurs is that the original frequency’s energy is dissipated by the generation of resonance. The resonating motion can be viewed as an energy sink at that particular frequency.

By itself, however, the resonating system simply displaces that energy over time, drawing the energy from the original sound and re-emitting most of it back into the space after the original sound has stopped. Hence, we have ringing and reverberance.

If we wish to use the resonator as an absorber, we have to allow it to resonate in sympathy with some frequency band of sound and then to frictionally damp the motion of the resonating element, so that the energy is not re-emitted into the air in significant quantities. Such absorbers can be very effective, in comparatively small spaces. This is particularly true, happily, for low frequencies, because of their diffracting qualities. Low frequencies go everywhere! If there is a chamber that will resonate at a given low frequency anywhere in the space, then that given low frequency emitting from anywhere in the space will ultimately dump energy into the resonator.

The Power of Corners

Now this is where corners come in handy. The pressure nodes for all frequencies build up in the corners of rooms, particularly at the intersections of the walls and floor, or walls and ceiling. The regions in the vicinity of such corners can be thought of as crudely resonating spaces, and energy can quite effectively be trapped by placing frictional absorbers (soft tubes, for instance) at the desired quarter wavelength out into the room from the corner. For instance if you would like to reduce that amount of energy in the space around 80 Hertz, a soft frictional tube (a 24” diameter canvas cylinder filled with fiberglass would do the job nicely), hung from floor to ceiling about four feet into the room from that corner will absorb a lot of energy in the 80 Hz. band.

The Tube

The tube trap is another variation on this, one that isn’t dependent on a corner placement. A solid-wall open tube will resonate at the frequency whose wavelength is equal to the length of the tube. If you build a solid-wall tube with one closed end, that tube will tend to resonate at a frequency whose wavelength is twice its length. The amplitude of the resonance will tend to increase with the diameter of the tube. Regardless of where you place it in the room, it will tend to resonate. If you pack it gently with fiberglass, it will damp that resonating frequency quite effectively, reducing the amount of that frequency energy in the room.

The Flexural Absorber

Another absorber based on this principle is the flexural absorber. Build a panel of something that is quite compliant at low frequencies, like masonite panels. Mount these in a frame near a wall and loosely pack the air space between the masonite and the wall with fiberglass to damp the motion of the panel as it flexes in response to the sound energy. Such absorbers can also serve double duty as high-frequency reflector/diffusers.