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The need for low-frequency absorption, combined with the back scattering of mids and highs, has been around for a long time. A different solution was developed early on and became a standard in studio design for forty years. “Membrane traps” utilize thin sheets of plywood, 1/8 inch typically, that are bent into a sequence of curved surfaces around the perimeter of the room. The airspace between the membrane and wall ranges from inches to feet and is packed with building insulation batt. This technique provides low frequency absorption with the important benefit of continuously curved surfaces creating lots of mid and high frequency diffusion. Rooms with membrane traps are lively, diffuse and well-damped. The efficiency of this technique is only fifty percent at best. This means that twice as much surface area is needed, but we end up with twice as much sound-scattering power. All in all, it’s a reasonable tradeoff. These rooms are expensive, but not too different than building a giant acoustic guitar. Their concave curve sections produce local sound focus effects, a problem for mic setups especially in a smaller studio.

Perimeter Traps

Another style of big room acoustics that has been used in control rooms is to lay up row after row of lightweight building insulation along the walls, but angled out from the walls. The hanging batt curtains occupy the outer two-foot to three-foot perimeter of the room. This technique is acoustically comfortable and stable. As the entire room surface has been converted into a great ball of fuzz, there will always be erosion of even the deepest bass energy. The depth of these fuzzy walls can vary depending on the location of the kinetic energy zones for certain problematic modes. The actual volume of room is about twice that of the apparent room. It is somewhat like a welter-weight anechoic chamber. This room can be successful in a downtown designer/contractor studio, but is not an option in the limited floor space of the home or project studio.

Pressure Zone Traps

Yet another version of deep bass absorption utilizes the sound pressure-zone concept. The fiberglass batt used in a ¼ wavelength trap is compressed by ten to twenty times into a medium density fiberglass board (commonly referred to as 703). This board is then ‘furred out’ a number of inches from the wall to produce a very effective sound trap. The major difficulty with this technique is keeping the fiberglass from vibrating as air moves in and out. When the fiat sheet of fiberglass moves, it shorts out the bass trap. Its response curve is spotty, and some frequencies are absorbed while others are not.

The trap design can also be outfitted with spaced slats to back scatter the mids and highs, and if properly made can develop high acoustic efficiency while staying close to the wall. The most common mistake in slat/pressure zone traps is that the slats are set flush against the fiberglass. This chokes off the bass breathing ability of the trap. There needs to be at least a ½ inch air gap between slats and the face of the fiberglass.

The pressure zone trap is a different type of sound trap than those mentioned. It uses lumped parameter acoustics while typical fuzz type absorption uses distributed parameter acoustics. Lumped parameter devices are designed like an electronic circuit with discrete items such as resistors, capacitors and inductors, and can be quite small; The distributed acoustic devices use the wave-guide approach to design and are sized directly to the wavelength of the note. For example: the pan pipe (¼ wavelength) and a soda bottle (lumped parameter) can both sound out the same note and equally loud, but the pan pipe will be many times longer than the soda bottle.

Improved Quarter-Wavelength Traps

Rather than a loosely packed fiberglass batt, which always settles, we can glue it to sheets of sound board which can be suspended by wires inside the closet. Nothing much new here; the same response curve as for the “ball-of fuzz” ¼ wavelength trap. The fiberglass does not settle out and so the trap keeps working for years.

Sympathetic Resonance Traps

The sympathetic resonance or panel trap is a creative cousin to the sound board and fiberglass trap. Often suspended in, supposedly, random overhead positions, these panels are each tuned by trimming to size and adding weights. Particular frequencies set these panels into sympathetic vibration motion, and the incident acoustic energy is converted to vibrating panel energy.

Dissipation of the energy occurs with the air moving back and forth across the face of the panel as it “twangs.” Its own internal friction also dampens its motion. These panels have to be ¼ wavelength in size, otherwise they would not be able to interact with the sound wave. An 8-by-8-foot panel would function at 40 Hz, if it was correctly tuned. Panel traps work best if aligned to meet the sound wave face on (like a ribbon mic) to engage action. The flat of the panel needs to face the wave front. Too often it is physically impossible to set up a real room with these panels because of size constraints.

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