Do-it-Yourself Bass Traps
By Ethan Winer
There are several ways to create a bass trap from raw materials. The simplest and least expensive is to place thick, rigid fiberglass panels straddling the room corners or flat on the walls spaced away with an air gap. Rigid fiberglass four inches thick and spaced well away from a wall is very effective to frequencies below 125 Hz. But many rooms have problems far below 125 Hz, and losing a foot or more all around the room is unacceptable to most studio owners and audiophiles. Since bass builds up most in the corners of a room, this is an ideal location for any bass trap. Mounting two-foot-wide rigid fiberglass panels straddling corners as shown in Figure 19.4 loses only a small amount of space.
Figure 19.4 shows the corner viewed from above, looking down toward the floor. When rigid fiberglass is mounted straddling a corner, the large air gap behind the panel helps it absorb to fairly low frequencies. For this application 705-FRK is better than 703, and panels four or even six inches thick are better than thinner panels. You can either absorb or reflect higher frequencies by facing the FRK paper backing toward or away from the corner to control the amount of liveness in the room. Note that stacking two adjacent two-inch panels absorbs the same as one piece four inches thick, so you can double them up if you’re unable to find the four-inch-thick type. You don’t even need to glue them together. However, if you’re using FRK fiberglass, you should peel off the paper from one of the pieces so only one outside surface has a paper facing. Also note that rigid fiberglass bass traps straddling a corner must not have a solid backing made of plywood or similar. The fiberglass must have both sides exposed to take advantage of the natural air gap behind the panel.
One nice feature of the bass trap in Figure 19.4 is that the air gap behind the fiberglass varies continuously, so at least some amount of fiberglass is spaced appropriately to cover a wide range of frequencies. If you cover a wall-wall corner from floor to ceiling, you’ll also have absorption at the tri-corners where three boundaries meet, which is especially effective. Understand that rectangular rooms have 12 corners, not just 4 where two walls meet each other. So besides wall-wall corners, bass traps are equally effective in the corners at the tops of walls where they join the ceiling and at the bottoms of walls where they meet the floor.
Filling a corner fully with rigid fiberglass is only a little better than using a four-inch-thick panel straddling the corner. If you can afford only a limited amount of material, it’s better to have more panels straddling additional corners, rather than fewer corners filled solid. Since the material in the deepest part of a corner is near to the wall boundaries, there’s less wave velocity for the material to act on. But when performance matters more than cost, filling a corner fully does maximize absorption. A good compromise is to place rigid fiberglass panels four inches thick straddling each corner, with the cavity behind each panel filled with less expensive fluffy fiberglass.
Using two traps adjacent in a corner as in Figure 19.5 works well, too, and takes even less space away from the room. This method is great when a room has a door in a corner, because one of the traps can be mounted directly onto the door. Spacing the panels two to six inches off the walls as shown improves their absorption further. As with bass traps that straddle corners, it’s best to cover the entire corner from floor to ceiling.
When using 705-FRK rigid fiberglass, you’ll achieve more low-frequency absorption if the paper side faces into the room. However, that reflects mid and high frequencies somewhat. Corners are not usually at reflection points, so it makes sense to use FRK panels with the paper side toward the room. This lets you put a lot of bass traps into the room, without making it too dead-sounding at mid and high frequencies. For traps mounted flat on the walls, away from reflection points, you can alternate the panels so every other panel has the paper facing toward the room, again to avoid making the room too dead.
For a typical unfinished basement ceiling, you can take advantage of the gap between the support beams and the floor above by placing rigid fiberglass between the beams. Short nails or screws can support the fiberglass, making it easy to slide each piece into place. Then cover the fiberglass by stapling fabric to the joist bottoms as shown in Figure 19.6.
Another method is to pack the entire cavity with fluffy fiberglass one foot thick. Any part of the ceiling that’s at a reflection point should not use FRK-type fiberglass or should have the FRK paper facing up toward the floorboards above. The same goes for parts of a live room ceiling above where you record drums and other instruments: It’s best to absorb mid and high frequencies rather than reflect them. But for the perimeter of the room, near where the ceiling meets the walls, FRK fiberglass gives the most bass trapping.
If you have a drop ceiling with standard office-type tiles, you should replace the tiles at reflection points with rigid fiberglass or attach rigid fiberglass or acoustic foam under those tiles. Most office tiles absorb speech frequencies only and are too reflective at high frequencies to use at reflection points. Do the same for any parts of the ceiling above where instruments and microphones will be placed, especially if the ceiling is low. Then lay batts of fluffy fiberglass insulation as thick as will fit (up to 12 inches) above the tiles for additional bass trapping. If you don’t want to cover the entire ceiling above the tiles with fluffy insulation, at least do that around the perimeter, where bass traps are most effective.
Another popular type of bass trap is the tube trap, invented and sold by ASC. There are DIY plans on various websites, though most plans wrongly claim that the top and bottom end caps of the tube must be sealed airtight. Tube traps are made from rigid fiberglass, which is porous, so sealing the tube ends is pointless. Tube traps work very well if they’re large enough—at least 16 to 20 inches in diameter. But like typical foam corners, small versions a foot or less in diameter are simply not large enough to absorb the lowest frequencies very well. The larger sizes work well in corners, in part because the tube’s diameter serves to space much of the fiberglass away from the corner boundaries.
Yet another type of bass trap is the Helmholtz resonator. Unlike acoustic foam and rigid fiberglass, a Helmholtz resonator can absorb very low frequencies. This type of trap works on the principle of a tuned cavity, and it can be very efficient if designed properly. Think of a glass soda bottle that resonates when you blow across its opening, and you have the general idea. Although a Helmholtz bass trap can absorb well, it works over a narrow range of frequencies, and like all bass traps, it must be large to be effective. The frequency range can be widened by filling the cavity with fiberglass or by creating several openings having different sizes.
One common Helmholtz variation is the slat resonator. This comprises a sealed box filled with fiberglass, with a large front opening partially covered by a series of thin, separated wood slats. Another design also uses a sealed box filled with fiberglass, but with a cover made of pegboard containing many small holes. These traps are tuned by adjusting the number of holes and their sizes or the spacing between the slats. Since all rooms need broadband absorption, Helmholtz traps are best used to target a single problematic lowfrequency mode, in conjunction with plenty of broadband traps for the rest of the bass range.
Another type of tuned bass trap is the membrane absorber, also called a wood panel bass trap because many designs use plywood for the front panel. Wood panel traps are a mass-spring system, where the panel is the mass and air trapped inside a sealed box serves as a spring. Figure 19.7 shows a cutaway view of a typical wood panel membrane trap, built directly onto a wall. When a wave within the effective range of frequencies reaches the front panel, the panel vibrates in sympathy. Since it takes energy to physically move the panel, that energy is absorbed rather than reflected back into the room. Even though the fiberglass doesn’t touch the plywood front panel, it damps the panel so it doesn’t continue to vibrate. Were the panel allowed to continue vibrating on its own, less energy would be needed to keep it moving, so it would absorb less. Further, a panel that continues to vibrate after the source sound stops adds resonance into the room rather than removes it, which obviously is not desirable.
One advantage of wood panel membrane traps is they don’t need to be thick to absorb very low frequencies. Like all pressure traps, they work best mounted directly onto a wall or ceiling, rather than spaced away as benefits porous absorbers. The center frequency absorbed by a wood panel bass trap is a function of the panel’s mass and the depth of the air gap, which serves as a spring. A trap four inches deep with a 1/4-inch-thick plywood front absorbs 100 percent at around 90 Hz, which is more than the same thickness of rigid fiberglass at that frequency. The audible bass range spans several octaves, and panel traps absorb only part of the bass range. Therefore, a mix of traps is required, with some tuned to absorb the lower bass frequencies and others for the higher bass range. Besides absorbing low frequencies very well, the wood front on a panel trap is reflective at higher frequencies. So installing enough of them to make a real improvement at low frequencies will not make the room too dead-sounding.
Wood panel bass traps are an older design, and more modern thinking prefers porous absorbers as thick as needed, with air gaps as large as needed, to target the lowest frequencies. However, panel traps work very well, and Figure 5.5 from Chapter 5 shows the pro studio I built in the 1970s using these on both the control room and live room walls. The stripes visible in the live room, through the control room glass behind the console, are wood panel bass traps alternated with rigid fiberglass behind fabric to absorb mid and high frequencies.
The last type of trap I’ll mention is the active bass trap, because it uses active electronics rather than porous materials or mass-spring resonance. The only commercial version I’m aware of is the E-Trap sold by Bag End, a company well known for its low-frequency loudspeakers. This is no coincidence because an active bass trap is at heart a subwoofer, plus a microphone that senses the sound. When bass waves reach the microphone, the speaker responds by sending a countering wave back into the room. One big advantage of the E-Trap is it can target extremely low frequencies—much lower than would seem possible given the 10-inch driver’s size. However, it can be tuned to absorb only one or two frequencies, and setting it up properly is much more complicated than hanging a passive bass trap across a corner. Further, the E-Trap’s countering output must be able to keep up with the volume from the main speakers. If your main speakers play louder than the E-Trap can counter, the trap will no longer function.
The always colorful, widely followed Ethan Winer has, at various times, worked as a studio musician, computer programmer, circuit designer, recording engineer, composer/arranger, technical writer, and college instructor. He’s had nearly 100 feature articles published in audio and computer magazines including Mix, PC Magazine, Electronic Musician, EQ Magazine, Audio Media, Sound on Sound, Keyboard, Pro Sound News, and Recording. In 2002 he started the company RealTraps to manufacture bass traps and other acoustic treatment, which he continues to this day. Ethan is also the author of the book The Audio Expert, recently published by Focal Press. Above is an excerpt from Chapter 19.