Recording

Bass-ic Instincts

Managing Low Frequencies in Any Room 6/01/2004 8:00 AM Eastern

A few members of the Lissajous family that were "phased" by this experiment

Initially, my spare bedroom-turned-control room was an abysmal environment. Now, clients remark about the stereo image and their ability to hear new things in “familiar” material, proof that small spaces can be rehabilitated. It's not exactly a 20Hz room, but it's finally usable.

There are many off-the-shelf acoustic solutions, but the challenges are determining what is needed and where to put it. This month, I'd like to present a procedure that provides instant feedback: You'll be able to see — and feel — what a bass trap does in real time and what the solutions are and how they work. The caveat? Every room is different, so I can't tell you the universal fix for every environment.

IN ONE EAR

Last year, I used Steinberg's WaveLab to “impulse” the room and capture the response. An echo of the impulse revealed a physical reflection, the calculated distance that corresponded to the width of the room (where an acoustic panel's position had not yet been optimized). Such reflections smear the stereo image and show, by example, how high-frequency behavior can be visualized and how easily anomalies can be tamed.

Your ears are the most sensitive from 3 kHz to 4 kHz, become less so around 300 Hz and are steadily de-sensitized by about 6 dB for every octave below, so it's only natural that we try to pump more bottom into a room and a mix. Compared to headphones, it takes lots of air movement to make bass happen in a room — that's why woofers are bigger than tweeters. The resulting energy increase below 400 Hz is enough to stimulate walls, floors, ceilings and cavities to resonate, which can be a good thing, a bad thing — or a little of both.

THE EYES HAVE IT

Surely, you've had a single tone up on the monitors and noted the obvious: Sound waves are anything but consistent as you move about the room. Stay in one place and slowly sweep through the spectrum and the room will tug at your ears. I successfully documented this sonic weirdness using a closely spaced pair of cardioid mics connected to an oscilloscope.

If you are not fluent in oscilloscope operation, there are two basic modes: Sweep and X-Y. The latter generates a family of patterns also known as “Lissajous,” named for a 19th-century scientist who literally used mirrors and tuning forks to prove his theories. The X-Y designation also refers to the scope's horizontal and vertical axes, and should not be confused with the X-Y term that refers to stereo miking. In the 'scope's X-Y mode, connect the left mic preamp output to scope input 1 and the right channel to input 2. A left-only signal makes a vertical line, while a right-only signal makes a horizontal line (line width = amplitude). A mono signal routed to both 'scope inputs will create a 45-degree diagonal line; acoustic information will never be that perfect. Reverse the polarity of one channel and the line will flip direction. (See the figure.)

Using Adobe Audition's tone generator, I created a bass sweep from 40 Hz to 480 Hz, while my NTI Minilyzer monitored frequency. Audition looped until the troubled spots were noted and narrowed to three primary regions, progressively zooming in until the center frequencies were found: a null at 81 Hz, a bump at 93 Hz and a phase shift at 317 Hz. Then, by monitoring the phase of a single frequency, it was possible to see the effect of a bass trap and fine-tune its position.

Note: A while back, I made an imprecise generalization about MiniTraps. They are not a solid piece of 703 or 705 Fiberglas, but a multidensity sandwich. Still oversimplified, a true bass trap absorbs sonic energy through friction, and, while you may not believe this, heat is generated as a by-product. For more bass treatment solutions, see the sidebar.

Starting with the 93Hz bump, I monitored the 'scope while positioning the MiniTrap until the Lissajous pattern changed from a less-than-45-degree oval to a diagonal line. The narrow end of the 2×4-foot traps was a few inches off the ceiling at a 45-degree angle and very close to the listener's position. Not the most aesthetically desirable location, but the energy that transferred from the panel to my hands was nothing short of amazing! Most bass traps are intended for corner applications. This single sine wave technique is helpful when trying to localize problems. Just place some acoustic panels and you begin discovering some things you may not have thought of.

THE DOORS

Once satisfied that the X-Y test had potential, I moved from acoustic panels to the space itself; specifically, the doors. Slowly opening and closing the entrance door turned out to be an effective phase manipulator at 81 Hz and 93 Hz. (I placed tape on the floor beneath the door for repeatability.) At the opposite side of the room, the door to the under-stairway computer closet also had an effect, with both items behaving as a Helmholtz resonator. (See sidebar.)

THAT DIRTY LOW DOWN

I hope this technique proves useful to those struggling with low-frequency issues. Taking the time to investigate your control room acoustic issues will pay off, sometimes instantaneously; other times, it is after giving the matter much thought.


Visit Eddie Ciletti at www.tangible-technology.com for more fun.

Bass Traps and Related Acoustic Solutions

Audio behaves similarly to light at frequencies above 400 Hz (think “rays”). That's oversimplified, but the typical treatment options are more placement-critical than composition-specific — almost anything will do the job — hence, the popularity of foam-based products that are easy to work with. It's not about how much area is covered, but how efficiently the solution can be applied. An anechoic laboratory is unnaturally dead and not the goal.

What happens below 400 Hz is more challenging, because low-frequency waves with sufficient energy don't just reflect, they stimulate prone surfaces (walls, ceilings, windows, floors) to resonate. In addition, sound travels through solid materials faster than through air. For this reason, loudspeaker systems should be mechanically de-coupled from their environment by using a mechanical absorber such as neoprene (synthetic rubber).

TREATMENT

Acoustic foam works for high-frequency absorption, but as frequency decreases to the low mids (about 500 Hz), higher-density Fiberglas is better-suited. Increasing thickness extends the low-frequency coverage down to about 100 Hz, below which conventional treatment becomes impractical, both physically and economically, even for Fiberglas. For example, a “true” ¼-wavelength 94Hz bass trap requires a 3-foot depth of Fiberglas — space that does not exist in modern project studios. Worse yet, there are still one to two lower octaves to treat, depending on room size. Treating low frequencies in a large room is easier than in smaller spaces, because the wave's energy has time and space to diminish before slamming into a surface.

I = 1/d2

Note: A sound wave dissipates energy as heat in the simple act of stimulating air molecules. As per the Inverse Square Law (Physics 101), the relationship between intensity (I) and distance (d) is such that doubling the distance from one foot to two feet would reduce the intensity by ¼ (Thank Newton!) rather than ½.

SCIENCE FRICTION

To control very low frequencies, a stubborn wave must be “tricked” into submission by using more space- and cost-effective solutions such as diaphragmatic or resonant absorbers. One of the fundamental mechanisms for sound absorption is friction, exactly what happens when sound travels through air. Think of the air that is pushed and pulled through a speaker system's tuned bass port. Now, expand the surface area and the number of “ports” and you've got…

THE HELMHOLTZ RESONATOR

We've all blown over the top of a soda bottle to make them “toot” and this is exactly how a Helmholtz resonator works: a defined amount of air space behind a perforated panel (with holes of specific spacing and diameter) made of hard board or some other rigid material. The idea is simple, but the implementation is not, as these units tend to be highly frequency-selective.

A simple closed box with holes resonates with so narrow a bandwidth, or Q, that it is only useful to “notch” out a single frequency. Like an equalizer, a trap's bandwidth performance can also be manipulated, in this case by how much the cavity is “broken up” using “batts” of high-density Fiberglas. More Fiberglas equals wider bandwidth. Other factors affecting performance are cavity depth and construction materials.

Spaced slats can be substituted for the perforated panels, but again, there is considerable calculation required because the slat thickness, width and space in between all conspire against or contribute toward the final result. Although equations are available to determine the resonant frequency of Helmholtz units, the results must be looked upon as merely guidelines, because any number of factors can shift the results to unknown values, a potentially expensive failure. For the amateur, it is best to investigate one of the following solutions.

LE LOUVRE ET LE MEMBRANE

A variation on the Helmholtz theme is to replace the holes with adjustable “louvers” that can be fine-tuned after the fact — an obvious advantage. The louvers serve a dual purpose as diffusers, allowing the room to retain its liveliness.

Another solution is the so-called membrane “trap” that uses thin sheets of ⅛- or ¼-inch plywood placed on boxes that are mounted on surfaces around a room. The airspace between the membrane and wall is packed with damping materials designed to broaden the bandwidth of low-frequency absorption. The full-sized Real Traps work on the same principle.

Combining a membrane trap with a curved surface adds diffusion of mid- to high frequencies to the low-frequency solution. Such curved units are called poly-cylindrical or functional traps, such as those made by Acoustic Science Corporation under the trade name Tube Traps. Although their LF effectiveness is limited by size, the ability to treat two almost inseparable problems at once — low-frequency modal buildups and the lack of diffusion inherent in small rooms — makes them a remarkably efficient solution.


Eddie Ciletti adapted and condensed this sidebar from a longer piece written by D. T. Hazelrig, www.diyacoustics.com.