A-Periodic Membranes

27th of January 2009

A-periodic membranes are not a new concept. Though rarely seen in the mobile audio industry, they have actually been around for decades. Dynaco was the first known company to introduce an enclosure alignment of this sort. Dynaco used a damping material in the vent which differs from the applications used today in the automobile environment.

A-periodic 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 compressed/spun fiberglass or duct board. The job of the membrane is to control the excursion of the driver and adjust the Q of the system for tuning. 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 we’ll 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 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. Others fire the drivers through the back of a seat, as in a sedan.  What can you do if you own a hatchback? Easy! Use the world’s atmosphere as the rear enclosure by venting outside of the vehicle with or without an enclosure. I built a system that had four 10” subs behind the seat in my truck with no box whatsoever.  They just fired through the back of the bed via the a-periodic membrane.  Should you opt for the hatchback or truck idea, be sure to put a louvered piece of steel or a Gortex cover over the a-p mat for protection from road elements and weather.

A-periodic membranes will never be big in the car audio industry because they are seldom understood.  I believe that this "rediscovered" method of subwoofer loading is one of the finest sounding alignments possible.  I know I’ll keep building them and enjoying them…

by Grizz Archer