Car Audio Aperiodic, Horn-Loaded, Outboard D/A and Other Exotic Components

This chapter is admittedly about car audio components that most enthusiasts would never have a need for but I wanted to cover them briefly. I think it's important that you know your options and also be familiar with some of the high end components.

Aperiodic Enclosures

Aperiodic enclosures enjoyed some popularity during the early to mid 90's but are not as popular now due in part to the much smaller enclosures required by subwoofers today. An aperiodic enclosure is a subwoofer mounted in the smallest enclosure that it will physically fit with an aperiodic mat on one side. This mat will be a combination of various acoustic materials sandwiched between two mesh grills. Think of a house air filter and you'll get a rough idea.

Aperiodic Enclosure Design

This mat allows the subwoofer to operate as if it were in an enclosure much larger due to the resistive effect of the aperiodic mat. The advantage of this design is a very small enclosure while maintaining the sound properties of a larger, properly sized, enclosure subwoofer system. The disadvantage is the design complexity. If you are considering using this design then I would recommend choosing a pre-manufactured mat such as those available from USD Audio. There is also an installation tutorial available at the USD Audio website.

Horn Loaded Compression Drivers (HLCD)

Rising to popularity around the same time as aperiodic mats, horn loaded compression drivers are a means of achieving the equal pathlengths talked about in the chapter on kickpanel speakers. In fact the midrange speakers that are used in conjunction with horn loaded compression drivers are often placed in the kickpanels. Horn loaded compression drivers feature a tweeter type speaker coupled to a tapered horn. See the image below.

Horn Loaded Compression Driver
Photo Courtesy of Image Dynamics

The large, round device in the foreground is the compression driver and the long, tapered piece is the horn. The HLCD is attached under the dash of the vehicle. To see a step by step tutorial of the installation you can check out the USD Audio website. The advantages of HLCDs are better staging and much higher efficiency as compared to conventional speakers. Disadvantages are a slightly different sound and a more involved installation as compared to conventional speakers. They also tend to be more expensive. For more information about horn loaded compression drivers check out this article at the Image Dynamics website.

Outboard D/A Converters

Inside a CD head unit is a digital to analog converter or D/A. The D/A is responsible for converting the digital information on the CD back to analog information (music) and the quality of the D/A effects the sound quality of the entire system. Now most listeners cannot tell the difference between D/A converters, especially at normal driving speeds. However they can be advantageous in the sound off circuit when you are dealing with judges that have highly trained ears. These outboard converters are very expensive and are only made by a handful of manufacturers worldwide. The most commonly seen for the vehicle is the Apogee unit but any number of other units could be converted for 12 volt use. In most cases you are better off buying a high quality CD head unit than bypassing the internal D/A and using an outboard. The cost vs. benefit is just not there for most enthusiasts.

Impedance Matching Transformers

Another item that has lost its popularity is the impedance matching transformer. Impedance matching transformers offer a way for car audio competitors to lower the impedance load on their amplifiers to the minimum impedance they are capable of driving, regardless of the actual speaker impedance. This is useful for squeezing the most power out of high current amplifiers capable of driving load impedance loads while still remaining in the lower power class. For the average consumer it is much more cost effective to simply buy a larger amplifier. Running an amplifier at lower impedances will often cause distortion and premature failure of the amplifier. Unless you have a specific need for impedance matching transformers I would recommend against using them.

Noise Gates

A noise gate is an electronic device that is designed to eliminate any hiss heard in a car audio system when music isn't playing. A noise gate works by sensing a user set level threshold for music. For example, as the end of a song nears the music will often start to fade away rather than stopping abruptly. The CD continues to play as the music gets quieter. In the background a hiss may start to appear as the noise that was being covered up by the music begins to exceed the music signal. When this threshold is reached the noise gate will completely shut off the low level signal going to the amplifier, eliminating any hiss that could be heard from components upstream of the noise gate. These are useful if you have a head unit or signal processor that has an audible noise floor. Again, this is a competition driven component and not needed by the majority of car audio enthusiasts.

Jacobs Accuvolt

Some amplifiers have an unregulated power supply and can produce more power when driven with higher voltages (up to a limit). There is a product by Jacobs Electronics called the Accuvolt that is designed to increase the voltage up to a maximum of 16.25 volts. Using this device in conjunction with an amplifier that has an unregulated power supply can give you an advantage in car audio competitions. You will be putting out more power using the same amplifiers because your supply voltage will be higher (power is the product of voltage and current). Non-competitors would be better off buying a larger amp since they do not have the power class restrictions and do not have to use devices such as this to increase power output in their system.

Car Audio System Planning

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