Bass traps are acoustic energy absorbers which are designed to damp low frequency sound energy with the goal of attaining a flatter low frequency (LF) room response by reducing LF resonances in rooms. They are commonly used in recording studios, mastering rooms, home theatres and other rooms built to provide a critical listening environment. Like all acoustically absorptive devices, they function by turning sound energy into heat through friction.
General description—types
There are generally two types of bass traps: resonant absorbers and porous absorbers. Resonant absorbers are further divided into panel absorbers and Helmholtz resonators.
Both types are effective, but whereas a resonant absorber needs to be mechanically tuned to resonate in sympathy with the frequencies being absorbed, a porous absorber does not resonate and need not be tuned.
Porous absorbers tend to be smaller in size and are easier to design and build, as well as less expensive overall than resonant absorbers. However, the deep bass attenuation of a porous absorber is generally inferior, so its usefulness for attenuating lower frequency room resonances is more limited.
Resonating absorbers tend to absorb a narrower spectrum and porous absorbers tend to absorb a broader spectrum. The spectrum of both types can be either narrowed or broadened by design but the generalized difference in bandwidth and tunability dominates their respective performance.
Examples of resonating type bass traps include a rigid container with one or more portholes or slots (i.e. Helmholtz resonator), or a rigid container with a flexible diaphragm (i.e. membrane absorber). Resonating type bass trap achieves absorption of sound by sympathetic vibration of some free element of the device with the air volume of the room.
Resonating absorbers vary in construction, with one type of membrane absorber using a springy sheet of wood that attaches to the enclosure only along the edges/corners, and another using a more floppy sheet of thin material stretched like a drumhead. A Helmholtz resonator can have one port tuned to a single frequency, or several ports tuned to either a single or to multiple frequencies, with round port, slotted port, or even perforated construction. Resonating absorbers often incorporate porous absorption internally to simultaneously lower the resonant frequency and broaden the spectrum of absorption.
Porous absorbers are most commonly made from fiberglass, mineral wool or open cell foam that resists the passage of air molecules through the interstitial space. Porous absorbers often incorporate a foil or paper facing to reflect frequencies above 500 Hz. Facing also improves low bass absorption by translating the physical compression of air at the facing into physical compression of the fibers that are in contact with the facing while also maintaining the resistive loss of air as it is driven through the bulk of the fiber by the facing.
Design concepts for building bass traps
Resonating bass traps
Resonating bass traps will absorb sound with high efficiency at their fundamental frequency of resonance. As such, a knowledge of the frequencies of resonances which require damping is helpful before designing and constructing a resonating bass trap. This can be attained by calculation of the room's modes or by direct measurement of the room itself.
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.
Panel absorber
A simple panel resonator can be built to hang on a wall by building a wooden frame, adding several inches of mineral wool to the inside, and fitting a sheet of plywood over the top attached only at the edges. A small gap should be left between the panel and the acoustic insulation so that the panel is free to resonate. Panel resonance 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' × 8'] plywood panel resonances when mounted on a 1"×4" frame 3.5" deep are: