research.bass traps

Bass Traps are acoustic absorbers or sound baffles which have the ability to capture low frequency sound. Bass traps are used to provide acoustic absorptive treatment in rooms and are common tools used in architectural acoustics. Bass traps are particularly useful in the acoustic treatment of home theaters, recording studios, mastering suites, and other venues built to provide a critical listening environment. Bass traps provide a means to control room reverberations at low frequencies, a part of the audible bandwidth especially troublesome when critical listening venues are located in small rooms. Bass traps, like all acoustically absorptive materials, function by turning sound energy in a room’s air volume into minute amounts of heat through friction.

There are generally two types of bass traps: resonating absorbers and porous absorbers. By their nature resonating absorbers tend toward narrow band action [absorb only a narrow range of sound frequencies] and porous absorbers tend toward broadband action [absorbing sound all the way across the audible band – low, mid, and high frequencies], though both types can be altered to be either more narrow, or more broad in their absorptive action. Both types are effective though the porous absorber has certain practical advantages in application as porous absorber type bass traps need not be specifically tuned to match the job at hand, and they tend to be smaller in size and easier to build than resonation type devices. For this reason most commercially manufactured bass traps are of the porous absorber type.

The key to build a resonating bass trap is to choose a style of device that will best fit the available space, and then match the resonance of the device to the frequency range you wish to absorb. In designing, building, and implementing a resonating bass trap one requires knowledge of the frequency of sound which requires absorptive treatment and then the device’s dimensions and/or the vibrational properties of the panel or membrane—must be matched to these needs.
For example a simple panel resonator can be built to hang on a wall by building a wooden frame, adding a couple of inches of mineral wool to the inside and add a sheet of plywood over the top attached only at the edges. Leave a small gap between the panel and the acoustic insulation so that the panel is free to resonate. Panel resonation can be enhanced by reducing the point of connection between the panel and the frame by means of narrow spacer material such as a loop of wire or welding rod run along the edge of the frame so that the panel is perched on a thin edge. Approximate full sheet [4′ x 8′] plywood panel resonances when mounted on a 1×4 frame 3.5″ deep are:

The best place to mount bass traps is in corners. With porous absorber bass traps like the design described above, mounting the units straddled across the diagonal of a corner yields an extremely efficient unit both in terms of material costs, and space requirements. Slat type resonating bass traps are often installed on walls where they can be up and out of the way.
One can also use broadband porous bass traps in early reflection controls positions [see external links for an explanation of early reflection control and its benefits] and thereby accomplish not only a diminution of the early reflections but also add beneficial additional low frequency absorption.

Interior Space Acoustics; This is the science of controlling a room’s surfaces based on sound absorbing and reflecting properties. Excessive reverberation time, which can be calculated, can lead to poor speech intelligibility.
Sound reflections create standing waves that produces natural resonances that can be heard as a pleasant sensation or an annoying one.[1] Reflective surfaces can be angled and coordinated to provide good coverage of sound for a listener in a concert hall or music recital space. To illustrate this concept consider the difference between a modern large office meeting room or lecture theater and a traditional classroom with all hard surfaces.
Interior building surfaces can be constructed of many different materials and finishes. Ideal acoustical panels are those without a face or finish material that interferes with the acoustical infill or substrate. Fabric covered panels are one way to heighten acoustical absorption. Finish material is used to cover over the acoustical substrate. Mineral fiber board, or Micore, is a commonly used acoustical substrate. Finish materials often consist of fabric, wood or acoustical tile. Fabric can be wrapped around substrates to create what is referred to as a “pre-fabricated panel” and often provides the good noise absorption if laid onto a wall. Prefabricated panels are limited to the size of the substrate ranging from 2’x 4′ to 4′ x 10′. Fabric retained in a wall-mounted perimeter track system, is referred to as “on-site acoustical wall panels” This is constructed by framing the perimeter track into shape, infilling the acoustical substrate and then stretching and tucking the fabric into the perimeter frame system. On-site wall panels can be constructed to accommodate door frames, baseboard, or any other intrusion. Large panels (generally, greater than 50 square feet) can be created on walls and ceilings with this method. Wood finishes can consist of punched or routed slots and provide a natural look to the interior space, although acoustical absorption may not be great.
There are three ways to improve workplace acoustics and solve workplace sound problems – the ABC’s.
A = Absorb (usually via ceiling tile)
B = Block (via workstation panels, wall placement and workspace layout)
C = Cover-up (via electronic sound masking)
While all three of these are recommended to achieve optimal results, C = Cover-up by increasing background sound produces the most dramatic improvement in speech privacy –– with the least disruption and typically the lowest cost.

“If you walk into an empty room and clap your hands, you’ll hear a series of closely spaced echoes. Often these echoes also possess a discernable musical pitch, called ringing, especially if the room is small. Echoes and ringing are caused by sound striking the walls, and then bouncing back and forth between the opposite walls. Besides the obvious intrusion of echoes in a room designed for playing and mixing music, the ringing also causes certain frequencies to be emphasized. The time between the echoes and which frequencies are emphasized depend on the room’s shape and dimensions.

To avoid these problems, professional mixing rooms are designed to eliminate most reflections. Deadening the room helps you to hear any reverb and other effects being added to a mix, without being influenced by natural ambience within the room. It also kills the ringing along with the echoes, thereby minimizing the need for 1/3-octave equalizers. (See the sidebar Fine Tuning the Control Room.) But proper acoustic treatment involves more than just eliminating the audible echoes and ringing, which impact only the midrange and upper frequencies. Unless your recording is limited to voice-overs and narration, it is just as important to eliminate the reflections that occur at low frequencies.

Many home-studio owners install commercial acoustic foam on their control room walls, mistakenly believing that is sufficient. After all, if you clap your hands in a room treated with foam (or fiberglass or heavy blankets), you won’t hear any echoes or ringing. But these products do nothing to control low frequency reflections, and hand claps won’t reveal that. Basement studios with walls made of brick or concrete are especially prone to this problem – the more rigid the walls, the more they reflect low frequency energy. Indeed, simply building a new sheet rock wall a few inches inside an outer cement wall can help to reduce low frequency reflections. The wall vibrates, thus absorbing some of the sound energy instead of reflecting it all back into the room. But this alone is inadequate for a serious mixing room, and you’ll get much better results using resonating boxes designed specifically to absorb low frequency energy. These boxes are called bass traps, and they absorb the lowest frequencies where fiberglass and foam stop working. The bass traps I have found most effective are built from plywood panels, and designed to vibrate over a broad range of bass frequencies. Fiberglass is mounted behind the panels to damp the vibration, thus absorbing the bass energy from the room.

When bass frequencies bounce around in a room they generate standing waves. Standing waves are pressure nodes created when a sound wave reflected from a wall collides with the direct sound emanating from the loudspeaker. At some frequencies the reflections reinforce the direct sound, creating an increase in level at that location in the room. And at other frequencies the reflections tend to cancel the direct sound, lowering the volume or in some cases eliminating it altogether. (Standing waves can be reduced with non-parallel walls and an angled ceiling, but such construction is too costly for most home studios.) The variation in bass response caused by standing waves is perhaps the single biggest obstacle to mixdown satisfaction for home-studio owners. You create what you think is a terrific sounding mix in your studio, only to get complaints that it sounds either boomy or thin everywhere else. Standing waves can also occur at midrange frequencies, but they are less intrusive there because most musical material does not contain sustained single notes as much as in the bass region. Further, midrange wavelengths are short enough that moving your head even a few inches will bring back a canceled tone. However, it is possible for a sustained note on a flute, French horn, or clarinet to create a standing wave. For this reason, sine waves are never used when measuring the frequency response of monitor speakers in a mixing room. Instead, pink noise is played through the loudspeakers because no single frequency is present in pink noise long enough for a standing wave to develop.
Although acoustic foam products are useful for absorbing midrange and high frequencies, they are relatively expensive: Sculpted foam two inches thick costs about five times more than type #703 one-inch rigid fiberglass board which is just as effective. (Rigid fiberglass is similar to the fluffy type used for home insulation, but it is much denser. A sheet of #703 one inch thick is equal in sound absorption to a much thicker batt of regular fiberglass.) Likewise, pre-built commercial bass traps are readily available, but they too cost many times more than the raw materials needed to build your own.

The plans provided here use sheets of rigid fiberglass one inch thick, covered with fabric for a better appearance, to absorb the mid and high frequencies. These are complemented with two types of bass traps made from plywood and one-inch rigid fiberglass to handle the low frequencies. One type of trap uses 1/4-inch plywood to absorb the deepest bass frequencies; the other is built with 1/8-inch plywood and handles the upper bass range. The rigid fiberglass is made by Owens-Corning, and can be purchased in boxes of two by four foot panels from a commercial insulation supplier. Nearly any porous fabric can be used on the mid-high frequency absorbers to cover the fiberglass and make it more attractive. I chose an off-white dyed burlap because it is inexpensive and acoustically transparent, yet it also looks good.”


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