Read along as ASC founder, president and TubeTrap inventor, Art Noxon, PE shares his experience with us this week! *Article edited for brevity.
Earlier in this paper, we looked at the conceptual mechanical side of the TubeTrap. There is also the acoustical circuit model of that subject, rather like an electrical circuit. The TubeTrap itself is comprised of a compliant volume (the electrical capacitor) surrounded by a resistive surface (the electrical resistor). Their combination forms the acoustic equivalent of a series RC circuit. As such, the TubeTrap is in effect a high pass filter with its own distinct time constant. The cutoff frequency is determined by the ratio of acoustic capacitance to resistance. Air motion in the wall of the tube is restricted at a rate of 6 dB per octave below that cutoff frequency. Larger TubeTraps operate to lower frequencies.
Acoustic impedance is defined as air pressure divided by bulk air velocity. Bulk air is in effect a distributed impedance transmission line. However, when a wall or corner is involved, this impedance becomes very large because the wave-propagative material has become acoustically stiff. The corner of a room usually has high-pressure fluctuations and little air motion. When the TubeTrap is in place, pressure is reduced and air motion is allowed at the surface of the tube, thus the impedance of the corner is reduced by the presence of the trap. This is equivalent to installing an impedance matching termination circuit to the end of an open-ended transmission line. Energy continues to be transferred down the line, not because of continued radiation but rather due to resistive dissipation. This process accounts for the name ‘acoustic window’ given to the model of TubeTrap we manufactured for Monster Cable (ca. 1985).
Another feature of the trap
…has yet to be mentioned. Limp mass diffusion panels are installed, permitting low-frequency pressure to pass into the resistive wall, but reflecting sound of the midrange frequencies and above. In general 100% of the TubeTrap surface is absorptive to low frequencies and 50% of its surface is reflective to mid and high frequencies. This crossover panel brightens the sound of the room.
The crossover rate is 6 dB per octave, appropriate to limp mass, and begins at ~320 Hz for the 11″ diameter units. The complete acoustical circuit of the tube trap has finally evolved into a series LRC circuit. The design, however, is deliberately so leaky that no LC resonance is possible.
The front half of each TubeTrap is midrange reflective, and the back half is broadband absorptive. To create a 100% dead corner one installs the type so that its absorptive side faces the room. We usually find that experienced listeners choose to aim the reflective side towards the room; they hear the difference and prefer the brightness.
Notice that when the reflective insert faces the room it does not reflect sound directly into the wall because the space between the TubeTrap and the wall is not bordered by two reflective surfaces facing each other. Because of this detail tube/wall sonic interaction is prohibited and any potential tonal resonance due to the TubeTrap installation is avoided.
The ease of adjusting brightness by rotating the tube seems to entrance some users. But–careful listening for undue coloration is in order. A safe way to have the adjustable brightness feature without risk of coloration is to install flat wall panels in the corner behind the tube, thereby canceling any potential for interaction between the crossover panel and the wall surface.
The bottom end absorption of the TubeTrap is controlled by the RC time constant. Then there is the crossover panel that chokes its absorption some 3 octaves higher. There is one other physical factor: the volume of the room. A corner-loaded trap pulls energy out of the fluctuating pressure zone in which it is located. The rate of energy drain from the room is proportional to the ratio of the number of trapped zones to the total number of zones in the room. The bigger the room, the more zones (n) there are for any given frequency.
A decay formula based on discrete 1/2 wavelength absorption is derived for resonant RT-60 decay times. It is simplified into a linear approximation good to 10% for the first two and a half octaves of major room resonances. A 2000 ft^3 concrete chamber with only 8 TubeTraps, one in each tri-corner will have a RT-60 of 0.4 seconds at 113 Hz. This demonstrates the strength of low-end trapping for major room resonances.
The basic items involved in our technique of corner loaded bass trapping have been reviewed and the remaining problem area for controlling room acoustics has been bared by implication: upper bass (250-400Hz). There are too many pressure zones for the field to effectively be depleted. Parallel wall standing waves can exist for a long time even if all the corners of the room have been cut out. For this reason, the standing of 9″ diameter trap columns on 3-foot centers along the perimeter of the wall has been one of the many ways the TubeTraps have found use in non-corner applications.