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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.
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 open cell foam, 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.
Design concepts for building bass traps
Resonating bass traps
A knowledge of the frequencies of resonances which require damping is needed before designing and constructing a resonating bass trap. This can be done by calculation of the room's modes or by direct measurement of the room itself.
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' × 8'] plywood panel resonances when mounted on a 1×4 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.
Porous absorber bass traps
A bass trap generally comprises a core absorbent damp material, a frame and a covering for aesthetic reasons.
- Core: Semi-rigid mineral-wool insulation boards with a density of 3 lb/ft³(48 kg/m³)such as Owens Corning 703 or Rockwool with a density of around 6 lb/ft³(100 kg/m³) will work well. 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]. Absorption type bass traps are broadband devices and will also absorb mid and high frequencies. HF absorption can be reduced somewhat by wrapping the mineral wool in a plastic film.
- Frame: A wooden batten skeleton frame is common to house the absorbent mineral wool and allow a breathable fabric covering to be attached to the exterior.
- Covering: Any porous fabric will be suitable. As will a slatted wooden frame.
Since low frequency resonances in a room have their points of maximum or minimum pressure in the corners of the room, bass traps mounted in these positions will be the most efficient. Bass traps are typically used to attenuate modal resonances and so exact placement depends on which room mode one is trying to target. Bass traps typically combine structural mechanisms that can work at both positions of high particle velocity/low pressure (thick fiberglass) and high pressure/low particle velocity (membranes).