Bass trap

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.

General description—types

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.

Examples of resonating type bass traps include Helmholtz resonators, and devices based on diaphragmic elements or membranes which are free to vibrate in sympathy with the room's air when sound occurs. Resonating type bass traps achieve absorption of sound by sympathetic vibration of some free element of the device with the air volume of the room. Such free elements in a resonating device can come in many forms such as the air volume captured inside a Helmholtz resonator - or a thin wooden panel held only by its edges [frequently called: "panel absorbers", a style of diaphragmic absorber] . Resonating absorbers can be made from just about any material that can either form a stiff walled vessel [a glass bottle for example] or any membrane stiff enough to be susceptible to being induced to vibrations by impinging sound.

Porous absorbers are most commonly made from fiberglass, mineral wool or other fibrous materials, and function through the existence of interstices [little holes] in the medium which present small captured pockets of air to the room which when excited by sound pressure waves in the room's air volume are themselves induced to vibrate like small springs. The fibers which make up a porous absorber are typically loosely bound together so that the "air springs" within the interstices created by the fibers are free to vibrate one against the other. In a manner of speaking, a porous absorber is a concatenation of many thousands of small Helmholtz resonators, of varying sizes, which are free to interact by vibrating against each other. It is this enormous variety of potential vibrational actions both inside each discreet air cell formed by the interstices, and between the myriad cells, one to another as allowed by the loosely bound fibers, which causes a porous absorber to function efficiently over a broad range of sound frequencies.

Design concepts for building bass traps

Resonating bass traps

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 1x4 frame 3.5" deep are:

*1/8" plywood = 150 Hz
*1/4" plywood = 110 Hz
*3/8" plywood = 87 Hz

Other common resonating bass traps are form of the Helmholtz resonator—such as either a stiff walled box with a hole in one side [a port] , or a series of slats over-mounted across the face as a stiff-walled box forming narrow openings in the cracks between the slat members.

Resonating bass traps will absorb sound with high efficiency at their fundamental frequency of resonance. Resonating absorbers can be broadened in the frequency range of efficacy to some degree by either introducing porous absorptive material to the interior of the vessel, by constraining the vibrations of the panel or membrane, or by installing an array of resonating devices each tuned to adjacent frequency ranges so that collectively the array functions over a broadened range of sounds. Such devices can be enormously effective over their tuned range, but can take up a great deal of space, especially when installed in arrays, and thus are sometimes not a practical solution. See the external links section below for several calculators useful for designing resonating bass traps.

Porous absorber bass traps

The keys concepts to efficiently building a broadband porous absorber are to choose the right material for the mineral fiber core, using a sufficient thickness of core material, choosing an appropriate fabric, and not overbuilding the frame work so as to yield an unwieldy [too heavy] device which cannot be easily installed for best performance. These fundamentals can be achieved by following a few simple rules:

# CORE: Use the best priced mineral fiber material your local market can provide—with some consideration given to material handling properties. If available at reasonable cost, semi-rigid fiberglass insulation boards in the 3 lb/ft³, or 48 kg/m³ density range such as Owens Corning 703 are an excellent choice this material will provide you a medium to work with that has good handling properties, durability, modest cost, and excellent acoustic performance. Mineral wool materials such as Rockwool in the 6 lb/ft³ or 100 kg/m³ range, or greater, will also work well, though this material is generally more floppy and less durable than fiberglass based alternatives [tends to crumble a bit on the edges] . Use naked, panels, not foil, or paper scrim. These boards are typically easiest to source in 2" thickness, just stack them to achieve the desired overall size [a minimum of 4" thickness is recommended] . However, thin paper or plastic facings increase bass absorption and reduce mid and high frequency absorption. This is often desirable in small rooms where substantial bass trapping is required, to avoid over-absorbing outside the bass range. See the Density Report link below for a comparison of various absorber densities with and without a paper facing.
# FABRIC: Any breathable fabric will work—literally put your mouth on the fabric and blow—if resistance is modest, the fabric will work fine, so just pick something that looks good to your eye and can be had at a reasonable cost. But also, to the extent appropriate for the location you intend to use the units, give some consideration to ease in cleaning/stain resistance, durability against wear and tear, and fire treatment. Upholstery and drapery fabrics work great or even muslin, craft grade felt or dyed burlap [Hessian or jute] will work and are very cheap. If needed, most fabrics can be treated for stain or fire resistance, or breathable fabrics already having these properties can be found.
# FRAME: Here is where most do-it-yourself bass trap builders go overboard. Any material will work provided it affords you a means of attaching the upholstery, and mounting hardware. Depicted to the right is a frame that has all the characteristics needed—note that the lumber is quite modest in size, but that it has corner gussets for reinforcement—you could also add a cross brace or two, but don’t over build and add unnecessary weight, or occlude the sides or backside of the panels unduly.

Keep the unit as light as is practical, and leave as much of the mineral fiber core exposed as is practical. Best practice would be to both glue and screw the frame together [plain old yellow carpenter’s glue is fine] and a counter-sink pilot bit will help keep your screws from splitting the wood. As to hanger hardware, standard picture frame wire and eye hooks will do, or you can use light weight decorative chain as might be appropriate for the mounting position. For “hover” mounting on a wall (with a gap behind the panel) you can use a long metal hook for the hanger wire, and add screw-in door stops for stand-offs for the lower back side edges or place a spacer made of cardboard tubing or a small block of packing foam behind the panel and out of sight.

Another very effective porous absorber design is to stack triangles of mineral fiber in a corner to build a floor to ceiling wedge— frequently known as the StudioTips SuperChunk. See the external links below for more on various porous absorber bass trap designs.

Mounting

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.

References

*Everest, F. Alton. "The Master Handbook of Acoustics", McGraw-Hill, 2000 (ISBN 0-07-136097-2).
*Kinsler, Frey, Coppens and Sanders, "Fundamentals of Acoustics, Third Edition", John Wiley & Sons, 1999 (ISBN 978-0471847892), Section 10.8: “The Helmholtz Resonator”.