Acoustic Sciences Corp. Tube Traps
While most audiophiles today know that proper acoustical treatment of the listening room is essential for good sound reproduction, few are aware that the usual wall treatments have practically no effect at low frequencies. A treated room can be almost completely dead at middle and high frequencies yet suffer from severe standing-wave interference at low frequencies (footnote 1).
Recording studios use wall "traps" to control low-frequency reflections (footnote 2), but, since these must be built into the walls during construction, the're only of academic interest to the majority of audiophiles. Most of us have had to accept the inevitabiiity of standing waves, and, by judicious placement of the loudspeakers, try to minimize their detrimental effects. Now at last, there's a better solution.
Pressures and Waves
In order to understand how the ingenious devices from Acoustic Sciences Corp. work, it's necessary to understand a little bit about how sound waves behave in an enclosed space. We all know that sound waves are created b any vibrating object, be it a violin or a shrieking infant. Less generally known is that sound waves produce only slight changes in pressure in the surrounding air.
As the vibrating object moves, it presses against the surrounding air and causes a pressure rise at the surface of the object. Because the surrounding air molecules are unconfined, they immediately relieve the pressure by simply moving away from the soundsource. Likewise, as the source produces a surface rarefaction by moving away from the surrounding air, the air molecules are drawn towards it, again maintaining the air pressure at a relativelv constant level (see fig.1).
Fig.1, How the movements of air molecules (dots) transmit sound waves. The white dots show the position of air molecules in the absence of sound. The black dots show the molecular shifts during transmission of a sinewave.
As each of these molecular shifts occurs, it is passed on to adjacent air molecules; the shift travels outwards in all directions from the vibrating object at a speed of about 1100 feet per secondthe velocity of sound in air. It is important to note that these air-molecule shifts do not cause a wind (which is the acoustical equivalent of direct current). The shifts are only temporary, with all molecules returning more or less to their original positions in space after the sound wave has passed.
The easiest way to visualize sound transmission through air is to think of a falling row of dominoes. When the first one falls, it knocks over the next, which in turn knocks over the next, and so on. Each individual domino moves only an inch or sO, but the state of "fallen-ness" traverses the entire line of dominoes. Each domino behaves like an air molecule, while the "fallen-ness" is analagous to the sound wave.
Because sound transmission in open air consists of molecular shifts and pressure changes, when sound waves impinge on a boundary like a room wall, the air molecules cannot move any further; they pile up On one another, causing a pressure rise at the boundary surface. If the surface has no absorptive qualities, the pressure rise will have no place to go except back out into the room, as a reflection. It will then cross the room as an acoustic wave until it encounters another surface, which may reflect it once again. This will continue until the wave dissipates as a result of friction in the air.
A room's reverberation is the result of multiple reflections between opposite boundary surfaces. The time it takes for the reverberation to die out is considered to be the room's reverberation time or RT. (For measurement purposes, "dying out" is defined as the point where the strength of the reverberation has fallen to 60dB below the strength ot the original sound; hence the term RT60.)
If it weren't for friction acting upon air motions of the wave, and sound transmission through the walls, the sound wave would be trapped in the room almost forever. Such is the case in marble-faced reverberation chambers.
When a room dimension equals half the wavelength of a sound, the reverb does not die out smoothly and predictably. bccause the wave acts to reinforce its own reflections. When the boundary zone at one wall surface is pressurized, the opposite one is subjected to a rarefaction, and when these reflect hack into the room. they just exchange ends. Each reflection feeds the other, and the result is a storage of energy between the opposite wall surfaces. The reverb therefore takes longer to dissipate than it normally would. This is what we call a "standing wave"; its effect on reproduced sound is to emphasize a certain frequency and add hangover to it, impairing both the smoothness and the detail of LF reproduction (not to mention the spectral balance).
Further aggravating the situation, every standing wave also supports lesser standing waves at multiples of the lowest one's frequency. Thus, a 24Hz standing wave will also cause weaker room resonances at 48, 72, and 96Hz, each of which will be stimulated by any of those frequencies in the program material.
And that's only a third of the problem. Standing waves occur between all parallel room boundaries, and every room has three such pairs detining height, width, and length. If any of these dimensions are the same, or multiples of each other (such as an 8' ceiling in a 16'-long room), the harmonic progressions will coincide across the hoard, and the standing-wave problems will be at least twice as had. It is not difficult to see how this can can wreak havoc with an audio system's LF performance.
Footnote 1: Only very thick panels3" or 4"have signifiant absorption down to 500Hz. The thinner, more commonly available stuff doesn't help until 900Hz, which is pretty well up in the nmidrange.Larry Archibald
Footnote 2: Wall traps are cavities filled with absorptive material and faced with irregularly spaced slats that serve to pass low frequencies and reflect high frquencies. The low-frequency energy is then dissipated in the absorptive filter of the cavity.J. Gordon Holt