A-periodic membranes are not a new concept. Though
new to the mobile audio industry, they have actually been around for decades. Dynaco was
the first known company to introduce an enclosure of this sort. Dynaco used a damping
material in the vent which differs from the applications used today in the automobile
Subwoofer systems have more "pros" than
"cons". The greatest benefit of utilizing one of these membranes is the drastic
reduction of enclosure size. The enclosure volume should be as small as possible.
Generally, .3 ft^3 or so for a 10", .6 ft^3 for a 12" and .9 ft^3 for a
15", or smaller.
The a-periodic membrane is simply a resistive element
consisting of a damping material such as spun fiberglass or duct board. The job of the
membrane is to control the excursion of the driver. Since the a-p mat is not a solid
material, it can "breathe" from the other side of the mat, I.e. the trunk. This
allows plenty of air "access" for low bass extension. Overall, this will exhibit
a flat impedance which makes response very predictable, unlike some traditional
enclosures. Achieving the right response comes through tuning the enclosure via the mat,
which well get to in a minute. "Quick" attack, lower cone radiated
distortion and smoother frequency response are a few other benefits. The transient and
phase response have similar characteristics to an acoustic suspension enclosure (sealed).
In essence, you have a very large enclosure, the trunk, with a resistive element that
controls and damps the energy transfer.
With all of these benefits, there had to be a sacrifice.
Efficiency is slightly lower than an acoustic suspension enclosure. However, most will
agree that this does not outweigh the advantages that the a-p mats have to offer. The
typical user of this system will be a sound quality competitor, not an SPL competitor, so
this is not an unbearable loss, especially since you get the sound quality benefits of a
large box with the power handling and damping control of a small box.
There are a few different applications possible. The most
commonly seen is the "fire through seat" method found in sedans. Bass enters the
listening area via the seat and uses the trunk as the "large enclosure". Another
place for the driver is in the rear deck. When installing the driver in this location, the
enclosure can be bypassed altogether by bolting an a-p mat to the front of the driver.
What can you do if you own a hatchback? Easy! Use the worlds atmosphere as the rear
enclosure by venting outside of the vehicle with or without an enclosure. Should you opt
for the hatchback idea, be sure to put a Gortex cover over the a-p mat for protection from
road elements and weather.
A-periodic membranes are still a bit new to the car audio
industry and seldom understood. I believe that this "rediscovered" method of
subwoofer loading will become more popular in the future.