Room Resonance
Almost everyone can read about “room acoustics,” which actually discusses the midrange and high frequency, the upper three octaves of the keyboard. Now, the domain of low frequency acoustics in small rooms is to be explored. This article will provide an overview of the theory, history and practice of bass trapping with an eye towards home and project studios.
Without proper decay times, mic work or listening in an audio room is hampered by excessive reverberation. Resonances color the acoustic signature because they are a group of specific tones that overhang longer than the others. Excessively sustained overtones cover over, blur and mask out the low level musical inner detail. The control of decay times in the audio room means controlling the resonances, and giving the room a neutral voice.
Resonant frequencies are not always the same; they will vary depending on speaker position. With a walking, talking person, the position of the sound source changes, stimulating different resonances. The loudspeaker however, is fixed in position. It stimulates the same group of resonances over and over again, The coloration is fixed; it penetrates and stains all recorded and playback material. Instead of capturing the “infinity” of musical variations that create evanescent luster in audio recordings, resonance forces a redundant tonal emphasis which renders music essentially boring–no matter how much talent is applied.
Electronic upgrades in the studio should develop enhanced performance. The need for any improvement springs from some dissatisfaction with the present system. The room acoustic is the first and last link in the audio chain. It is staggering to consider how many pieces of electronic gear have been purchased out of frustration with a system whose real problem was not electronic at all, but was driven by the colorations due to room resonance.
There are only two ways to get residual low frequency sound energy out of a room. The first and most common is leakage. Unlike the downtown recording studio, deep bass leaks out of most home and apartment construction. Leakage paths can be direct transmissions through the walls, ceiling, floor, doors and windows. The heavier the surface, the less leaky it becomes. Other leakage paths are through openings such as under the door.
Absorption is the second method by which acoustic energy is removed from a room. Downtown recording studios are heavy-walled and sealed airtight to keep unwanted sound out. This is called isolation. If sound is kept out, it is also kept in, and so studio builders have developed a variety of low frequency sound absorbing techniques. Hopefully, most of these will be reviewed in this article. The designer/contractor-built studios usually have bass traps built in. The rapid expansion of MIDI equipment has resulted in many serious home-based project studios that are virtually without acoustic control.
The single most important result in a properly bass-trapped room is that it has more bass, deeper punch and smoother extension. This sounds contradictory–that bass trapping a room gives more and not less bass. Actually, what you get is the bass you always had; you just could not hear it because the resonant colorations covered it.
Once the basic concepts of room resonance and bass traps are developed, the practical matter of setting up a room needs to be discussed. This is broken into two sections. Trapping the front or driven end of the room requires special considerations because of its proximity to the loudspeakers. The back of the room is more intuitively obvious and belongs to the world of deep bass traps.
Virtually every downtown recording studio uses some type of bass trap to control distortion and coloration of the frequency response in the room due to low-end build-up. Bass traps in these studios can be found hidden above the ceiling, inside the walls, below the floor and sometimes even in adjacent rooms. The nagging problem for home and project studios is that most engineers cannot consider contractor renovations as an option for an acoustic upgrade of their living rooms.
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