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A classic never-to-be-forgotten sound trap is the Helmholtz trap, which carries the name of a great, old-time German acoustical scientist. Conceptually, the Helmholtz is little more than a jug, tuned with loose batt stuffed inside. However, it usually looks like a panel of ¼ in. pegboard behind which is a 1-3 in. air space fluffed with light building insulation.

The absorption curve illustrates the strong frequency selective property of this type of absorber. Two difficulties exist with using such a trap: It is a single-frequency type, and must be tuned to a known room mode, and The trap’s performance is strongly dependent on the amount of batting placed in the cavity and the rigidity of its wall, especially the perf panel. It is difficult to tune.

In the early 1950s, Dr. Harry Olsen, director of RCA Labs and a prolific masterful contributor to audio practice and theory, presented his “functional sound absorber.” It was especially unique because of its unprecedented one hundred and sixty percent efficient handling of low frequency sound. He envisioned its use overhead in large rooms and halls. But elsewhere in his literature he advises that low-frequency sound absorbers are best located in the corners of smaller rooms.

The “functional sound absorber” is a close cousin to the flat pressure zone trap. The density of the fiberglass for this type system is impedance matched to the radiation impedance of free sound waves in air. Essentially, if the fiberglass is too dense, sound bounces off; if it is too loose, sound goes right through. The resistance of the surface combines with the volume of the airspace inside to provide a very low frequency response curve for the trap, similar to an electronic RC circuit. By adjusting the value of R and C, the desired RC time constant can be picked for the trap’s roll-off characteristic.

Sound absorption is always a function of two factors: the surface of acoustic material exposed to the sound field and the efficiency frequency response of the surface. Dr. Olsen’s cylinder bass trap has just over three times the apparent frontal surface area. Secondly, it is very efficient into the lower frequencies because it is an acoustic circuit of RC time constant design, rather than the more traditional ¼ wavelength “fuzz ball” approach to acoustics.

As with all traps, midrange and high frequency partial reflectivity remains of value. Accordingly, today’s pro style functional-type bass trap is usually outfitted with a membrane section to back scatter mid-range frequencies (usually above 400 Hz). These traps are extremely efficient, and particularly when located in the corners of a room. To increase absorption in a selected frequency band or to extend the low frequency response curve, the interior volume can be fitted with a low Q Helmholtz resonator. It is particularly suited as a corner-loaded bass trap in small audio rooms because it is small, efficient, modular and easy to set up, more like studio equipment than a remodel construction project.

Home/project studios in rectangular rooms suffer from a malady that most designer studios do not have–head-end ringing. Speakers are usually located near the front of the room. From this location they easily stimulate room resonances along the length of the room. It takes about ten exchanges of sound between the front and back of the room to build up the condition of resonance, typically ¼ second.

Speakers may be far from the back wall, but they are very close to the side walls and floor/ceiling walls in the front of the room. Because of these short lateral dimensions, side to side and vertical resonances can build very quickly (within 1/20 second in the front end of the room), long before the entire room Can be engulfed in the resonance. This fleeting, quick resonance is called “head-end ringing” and because of the time scale, dramatically affects imaging and the color of attack transients.

Head end ringing is not a deep bass problem–it is a mid bass coloration effect due to a lack of bass traps in the front end of the room. Designer studios with the Reflection Free Zone (RFZ) cup shaped front end don’t have this problem. The raked walls and ceiling eliminate any opportunity for reflections to stay and build up in the front of the room. But with home and project studios set up in rectangular rooms, head end ringing is a major problem that near-field or mid-field monitors cannot even avoid. Typically, playback monitors are located about halfway between floor and ceiling, and about one-third in from the side walls. The classic head end ringing problem occurs at about 140 Hz. A substantial distribution of mid-bass traps on the walls and in corners of the front end of the audio room is the only way to control head end ringing.

Over the years bass trapping has matured unique to the recording industry. We don’t usually see them in press release photos because they have always been built in behind the walls of the designer/contractor studio. Nevertheless, bass traps are a tradition that is integral to the definition of a recording studio or control room. They are the primary acoustic consideration that separates recording rooms from regular rooms. Although many versions have evolved, one thing is for sure: bass traps have been, are now, and will most probably continue to be the cornerstones for the pro room acoustic.

But these are modern times and the availability of personally affordable studio-grade equipment is changing the face of the recording industry. Home and project studios are being set up at a ratio of ten to one compared to the traditional designer/contractor-built studio. This new and rapidly developing division of the recording industry may be wired like downtown studios, but their room acoustic is all too often set up with no more than a couple of pieces of foam tiles and particularly depleted of bass traps. Consistency is always important, and the first rule in studio design is that it must “look like a studio.” In this sense the topic of bass traps in the designer/contractor-built studio and the home/project studios do have one thing in common–no bass traps are visible.

There is only one reason that studios have to look like studios– to help establish client confidence. But this requirement for designer/contractor studios does not apply in the home/project studio.

To a large degree, the owner of the home/project studio is the client of the studio, The home/project studio may not have to look like a designer/contractor studio in order to do its business, but it certainly has to act like one. Since bass traps won’t be built in behind the walls of any home/project studio, they will have to be set up in front of the walls and corners of the room. For the first time, engineers will simply have to look at bass traps.

Essentially, bass traps are “coming out of the closet” in order to get back to work in the home/project studio. After all, any chain, even the home/project studio audio chain, is no stronger than its weakest link, and bass traps are critical to the last link of the audio chain—the room acoustic.

Go to AcousticSciences.com to re-read this article.