Aperiodic Membrane
Aperiodic Membrane is one of the latest successful methods in car audio. The membrane material is a resistive element that the woofers play through. The membrane assists the mechanical performance of the woofer improving not only the woofer’s transient response (the control of the woofer) but also the amplifier’s power transfer. It is very noticeable when using a membrane, that the woofer's behavior is less affected by the confines of the automobile when compared to either a infinite baffle application or other conventional enclosure application. When compared to an infinite baffle application, the aperiodic system greatly improves the mechanical power handling of the woofer. Of course, with any benefit there are always tradeoffs. The price for this is reduced efficiency and, typically, increased cost of membranes when compared to conventional enclosure designs. In simple terms, you're not going to win any SPL competitions with an aperiodic system. For this price you get improved mechanical power handling, excellent transient response and trunk space.
There are two different ways to construct Aperiodic enclosures: front loaded and rear loaded. There are good examples of both designs. The front loaded membrane has a performance advantage of filtering out the high frequency output of the woofer. This reduces the audible effect of the second and third harmonic distortions of the woofer.
In both examples, the woofer is coupled to the membrane. In the front loaded design, the membrane is directly coupled to the woofer. The amount of air between the woofer and the membrane is negligible. In the rear-loaded design, the woofer is coupled to the membrane via a small sealed enclosure. Ideally, the volume of air between the woofer and the membrane should be minimized for best performance. There are some arguments from membrane suppliers that varying the volume of air between the membrane and the woofer can achieve the desired performance goals. Some of the recommendations from the manufacturers of rear loaded membranes call for the volume of air between the woofer and the membrane to equal the volume of air required by the woofer to achieve a closed box "Q" of 1.1. (More on sealed enclosure "Q" in the Sealed Enclosure section) While this may be a good guide for their particular applications, similar performance can be achieved by changing the resistance (air porosity) of the membrane material.
In reality, there is only one type of Aperiodic enclosure design. The difference between the different applications is how the woofer couples to the membrane and how resistance affects the total "Q" of the woofer’s operation. Listed below are the performance and characteristics of the different types of applications for the Aperiodic design. Table 1
Mechanics of Membranes
The most notable change to the woofer’s operation is the reduction and flattening of the woofer's impedance. At resonance, Fs, the impedance is greatly reduced when the woofer is used in conjunction with a correctly designed membrane. The result is the woofers overall impedance curve is absent the impedance peak that defines the resonant frequency Fs of the woofer. This change creates what is called a "critically damped" system. A critically damped system is one that when subjected to an input signal, the woofer will move; when the input signal is removed, the woofer will immediately stop. The woofer's movement will very accurately track the input signal. The result of this is low frequency system with an awesome transient response.
Designing Aperiodic Membranes
Designing Aperiodic Membranes is a difficult task. There are formulas to calculate the resistance (porosity) of the material and the thickness of the material needed for membrane construction. This area goes beyond the scope of this article.
How Resistance Affects Performance
In many applications a base resistance for the membrane is chosen based of the Qtc range for a particular driver. This membrane is then fine tuned for the woofer through measuring the impedance and changing the thickness of the membrane material. If the material is too thick, the efficiency of the system is needlessly reduced. If the membrane is too thin, the resistance will not be great enough and the goal of a critically damped system will not be achieved. Although the design can be a time consuming process, patients are well rewarded.
In rear mounted applications, adjusting the amount of air that couples the membrane and the woofer will have a great affect on the performance (transient response) of the system. As the volume of air between the woofer and membrane increases, the greater possibility changing performance at different power levels due to the compressibility of the air.
When selecting a woofer for an Aperiodic System, several different performance criteria apply. In Table 2, these criteria are
listed in descending order of performance.
| Parameter |
Importance |
Fs of the woofer |
Determines the low frequency cut-off of the system. Lower is better |
X-Max |
Determines the maximum output capability of the system with unlimited power |
Qts |
Determines the relative efficiency of the system. The lower the Qts the better. Best result are achieved with the Qts of the woofer lower than 0.35. This factor is more important on rear mounted membranes. |
Maximizing Performance
Maximizing the performance of a membrane design is very similar to maximizing an infinite baffle application. With the exception of...
 The coupling between the woofer and the membrane must be airtight.
The membrane mounting must clear the peak excursion of the woofer.
To maximize the performance of an Aperiodic system, a high-pass high "Q" active filter is recommended. This filter affects two difference performance characteristics of the Aperiodic operation: excursion and frequency response.
Because the sound quality of the Aperiodic Membrane system is so different, it is often difficult to hear when the woofers suspension limits are reached. To best protect the woofer, the excursion of the woofer needs to be limited below the resonant frequency, Fs, of the woofer. To accomplish this an ordinary high-pass filter centered at the Fs of the woofer can be used.
High-passing the woofer only solves one problem of the design. The other part of the design that can benefit from electrical assistance is the lower end frequency response of the woofer rolls off. Equalization in the lower frequencies can solve this situation. These two needs can be solved with one filter. This filter is called a high "Q" filter. A high-pass filter with an electrical "Q" of 2 will generate approximately 6.0 dB of boost at the center frequency. The result is a low frequency system that offers excellent output down to the resonant frequency of the woofer. It is important to remember that asking your amplifier to produce 6 dB more output is asking your amplifier to make four times the power at that particular frequency.
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