A1: Many new models of speakers use a
ported enclosure, to allow the air in the enclosure to be refreshed continuously. The air
is exchanged whenever a frequency corresponding to the tuning frequency of the port is
played, and if you put your hand in front of the port, you can feel the air exchange in
progress. That the port helps the speaker to produce more bass is a by- product of its
main role in the design.
A2: This is one area in which the DIY`er
can definitely improve on mass-manufacturing practice.
The best solution I've found to date is the model
Aleph-Null controlled diffusion inducer, by Scott & LaForge Ltd. It mounts inside the
cabinet, out of sight (they even thought to use a heavily-damped self-adhesive mounting
pad, so the device actually helps suppress cabinet-wall resonances!).
The inducer's effect is to increase the osmotic
permeability of the cabinet wall, thus allowing a slow exchange of gasses through the
cabinet. The effect is designed to be self- limiting and fairly modest - as I recall, a
50% turnover of the air in the cabinet takes about a week with normal levels of use (2-3
hours per day at standard listening levels). The osmotic exchange rate is deliberately
kept low enough that the cabinet continues to act as a sealed box at any frequency of
conceivable interest to the human ear (unlike e.g. a Dynaudio "variovent", whose
flowthrough rate is a good deal higher and which acts as a resistive damper at bass and
infrabass frequencies).
It's powered by the audio signal - you hook it up in
parallel with the woofer, on the low-pass side of the crossover circuit. Its impedance is
in many respects complementary to that of common woofers... typically about 50 ohms, but
dropping to about 10 ohms near resonance (which is tunable). The woofer- and-inducer
combination presents a nearly flat impedance load to the crossover. It's efficient enough
to be used even with low-power "single ended" amplifiers such as the Zen.
It's a well-thought-out, and well-engineered device, and
at the price of only 87 quatloos in quantities of 2 or more it's quite cost-effective.
The only problem with it, in practice, is that it won't be
invented for another 217 years. I had a hell of a time getting engineering samples.
A3: Hah! This shows that you've failed to
read Dorkinson's "Loud Woofer Designer's Checkbook". The whole point of sealing
the boxes is to keep the air from leaking into the box. Don't you know that air has oxygen
in it? If oxygen is bad for speakers wires, imagine the havoc it would cause if it
actually got into the speaker! Most people use a nitrogen purge before sealing the
speakers, but you can substitute helium for a lighter sound. Some people like nitrous
oxide, but when asked to justify this choice, they just start giggling uncontrollably.
A4: I'm using a mix of 2/3 air collected
on a Cornish moor, and 1/3 from the Oregon Cascades. Harder to get, but a refreshing blend
of qualities, and it seems to last longer. I've been tempted to try a dry Australian, but
I am afraid of complications from it swirling in the wrong direction due to residual
Corleolis effect.
