Lee
07-14-2002, 07:38 PM
4.9 What is an "aperiodic membrane?" [CD, DK]An aperiodic membrane is one part of a type of subwoofer enclosure. It is an air-permeable sheet which has frequency-dependent acoustical resistance properties. The original design goes back to Naim, for use in home systems, but has been applied by several individuals and companies in car audio.
The completed system will be aperiodic, which means it will prove to be over-damped with a Q well below 0.7. In contrast, the most commonly used sealed enclosures have Qtc's in the range of 0.8 to 1.1 which are considered, by definition, to be underdamped. When improperly used, a high-Q system may have poor transient response, nasty peaks in frequency response, and high rates of roll-off. Aperiodic systems will feature excellent Aperiodic systems are characterized by better transient response, flatter frequency response and somewhat extended low frequency response.
Another benefit of the system is that you can pretty much choose whichever driver you'd like to use, as long as they are big. The Thiele/Small parameters (which would normally determine what kind of box would be used) are taken into consideration by the membrane designers so that the response is extended and overdamped, regardless of the characteristics of the driver.
Physically, the aperiodic membrane isn't for every car. It requires sealing the trunk from the passenger compartment in an air-tight manner, as well as sealing the trunk from the outside for best results. The drivers are then mounted into the baffle between the passenger compartment and the trunk, as would be standard in an infinite-baffle/free-air set-up. The aperiodic membrane is then placed either in front of the driver or behind the driver, depending on the type. When mounting behind the driver, the membrane is used as the rear-wall of a very small box which the driver sits in (as in Richard Clark's infamous Buick Grand National). So, in short, it's not suitable for trucks, jeeps, R/V's, or hatchbacks.
You should probably only get an aperiodic membrane if you've got money to burn, lots of amplifier power, some big subs, a sedan, a desire for trunk space, and no wish to boom. If your tastes lean towards bass-heavy booming, as opposed to well-recorded acoustic instruments, you're not going to be pleased with the result.
The completed system will be aperiodic, which means it will prove to be over-damped with a Q well below 0.7. In contrast, the most commonly used sealed enclosures have Qtc's in the range of 0.8 to 1.1 which are considered, by definition, to be underdamped. When improperly used, a high-Q system may have poor transient response, nasty peaks in frequency response, and high rates of roll-off. Aperiodic systems will feature excellent Aperiodic systems are characterized by better transient response, flatter frequency response and somewhat extended low frequency response.
Another benefit of the system is that you can pretty much choose whichever driver you'd like to use, as long as they are big. The Thiele/Small parameters (which would normally determine what kind of box would be used) are taken into consideration by the membrane designers so that the response is extended and overdamped, regardless of the characteristics of the driver.
Physically, the aperiodic membrane isn't for every car. It requires sealing the trunk from the passenger compartment in an air-tight manner, as well as sealing the trunk from the outside for best results. The drivers are then mounted into the baffle between the passenger compartment and the trunk, as would be standard in an infinite-baffle/free-air set-up. The aperiodic membrane is then placed either in front of the driver or behind the driver, depending on the type. When mounting behind the driver, the membrane is used as the rear-wall of a very small box which the driver sits in (as in Richard Clark's infamous Buick Grand National). So, in short, it's not suitable for trucks, jeeps, R/V's, or hatchbacks.
You should probably only get an aperiodic membrane if you've got money to burn, lots of amplifier power, some big subs, a sedan, a desire for trunk space, and no wish to boom. If your tastes lean towards bass-heavy booming, as opposed to well-recorded acoustic instruments, you're not going to be pleased with the result.