This week we jump into the middle of an early AES paper written by Art Noxon, TubeTrap inventor and President of ASC. The paper first explains why corner loaded bass traps are needed in small rooms to achieve satisfactory musical articulation, then shares a “new” invention that fills the role wonderfully. This snippet shares the physics behind your favorite room acoustic treatment. No, it’s not a simple porous absorber – but it shares some features. No, it’s not a tuned pressure trap – it works much faster. Where it shines is in the corner, and what it delivers is rapid decay of transient-smearing bass range reflections, all without wrecking your small room’s ambient reverberation.

To make a room sound “good” requires paying attention to more than absorption coefficients and frequency response. Small rooms dedicated to music should not be treated the same as gyms, offices, or restaurants. A different approach is needed, one which TubeTraps uniquely serve.

Read the entire paper

We’ve now covered…

…problems and remedies involved in accurately tracking both tone burst transients. We can combine the remedies to control both the attack and decay transients of the tone burst simply by locating a good bass trap in the tri corner behind the speaker. We can simultaneously damp the strength of the first reflected wave, thereby correcting for attack phase distortion, and also damp the resonant modes of the room, thus correcting for decay distortions. We see that both transients of the musical tone burst can be cleaned up through bass trapping in the corner behind the speaker.

Well, so far everything seems to be getting better. The traps are to be located in zones of maximum pressure fluctuations, the tri corners of the room. Unfortunately, few devices extract energy directly from pressure changes. The most common method for sound absorption is friction — friction due to the air-motion part of the sound wave that is scrubbing through some micro-porous piece of material, usually fiberglass.

Now, air-motion is very small in zones of pressure fluctuation, by definition. For example, at 100 dB, 100 Hz, it’s on the order of 1/10 the diameter of 5 micron fiberglass fibers. Prospects for developing friction look poor unless we first transform energy. We’d like to convert the pressure fluctuations into substantial air motion, and then dissipate acoustic energy by friction against the air motion.

A new device uses this approach with considerable success. It is tubular in form and is supplied in 3 foot sections, hence its generic name: TubeTrap. It is comprised basically of two distinct elements: an internal air chamber and a porous wall. The ends of the tube are sealed. The TubeTrap is, in fact, a sealed chamber with a resistive opening to its interior void. Its length is incidental and now functional to its operation. Air pressure fluctuations outside the tube impart motion to the air in the porous tube wall where friction operates.

It is interesting to note the pressure distribution associated with the operation of the TubeTrap. When pressures outside the tube are higher than those inside, a pressure gradient across the wall of the tube results from friction as air is driven inwards through the wall. The difference in pressure across the wall is the measure of the force that is being transferred into frictional energy. The thickness of the wall tells us the distance over which that force is developed. Their combination tells us how much work is being done. We like as much force to occur over a large distance to get as much work out of each half cycle pressure fluctuation as possible.

If for example, the tube has a thin but highly resistive wall, the pressure drop would be very steep — but the distance of the action would be too small for any real work to be done. Conversely, if the tube were simply full of loose fiberglass, the gradient would be too small, though the distance of the action would be large; again, the work would be minimal. The variables of wall material bulk flow resistance and the wall thickness, along with the air chamber volume can be manipulated to access any low frequency with optimal efficiency.

We’ve been looking 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, for example is 37 Hz for the 11″ diameter tube trap. Air motion in the wall of the tube is restricted at a rate of 6 dB per octave below that cutoff frequency. A 20 Hz model will soon be in production and custom traps have been built to 5 Hz.

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 manufacture for Monster Cable.

One 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 tube 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 tubes.

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 TubeTrap has finally evolved into a series LRC circuit. The design, however, is deliberately so leaky that no Lc resonance is possible.