Sub Box Theory

SUBWOOFER ENCLOSURE THEORY
You can put a subwoofer in a car in almost any mounting fashion but there should always be some kind of enclosure to ensure a good bass output. Why pay good money for that sub and have inferior bass output?

An enclosure is necessary for a woofer to provide any kind of significant bass reproduction. Even a large woofer will offer disappointing bass output if you don't use an appropriate enclosure. The reason for the poor response is the ability of the rear wave to wrap around the woofer , due to the long wavelengths at low frequencies, and cancel the front wave. The solution for good bass is to isolate the rear waves and front waves using a "baffle". Unless you build the speakers into a wall, placing the woofer inside a box to prevent the rear wave from wrapping around and canceling the front wave is necessary.

Most OEM car systems take advantage of a "semi-infinite" baffle between the rear seat and the trunk by installing 6x9" drivers and using the 15 cubic feet of trunk space for deep bass response. Door speakers work similarly but with much less volume. Although infinite baffle design is very simple it is somewhat inefficient because of air leaks and poor seals that exist in most OEM installations; a rigid baffle must be fabricated.

SUBWOOFER ENCLOSURE TYPES
In the car audio world there are 5 types of subwoofer enclosures commonly used; Infinite Baffle, Sealed Box, Vented Box, Single-Vented Band-Pass, and Dual-Vented Band-Pass. No single design is superior because each has its own compromises in performance, power handling, and design/construction complexity. You need to make the choice based upon personal requirements.

Below is a summary of these 5 popular enclosures. More importantly their respective advantages and disadvantages are noted so an informed choice can be made regarding the best box for your needs.

INFINITE BAFFLE
("Free-air")
This is simply a woofer installed in the back shelf of the car that uses the boot as the enclosure.

Advantages: simple installation and adequate performance;

Disadvantages: only moderate sound pressure levels and poor power handling.

SEALED BOX
("Closed Box")
The simple sealed box offers one of the best compromise in performance, power handling, and tonal quality.

Advantages: simple box construction, high power handling, excellent transient response, and smooth low frequency roll-off.

Disadvantages: moderate to large box sizes, moderate efficiency, moderate sound pressure levels.

VENTED BOX
("Ported Box" and "Bass Reflex")
The vented box is simply a sealed box but with a specifically tuned tube inserted. It extends the low frequency response compared to a sealed box.

Advantages: low cut-off frequency, low distortion in its operating pass-band, excellent efficiency, good transient response, moderate power handling (in its pass-band).

Disadvantages: complex design and limited power handling if driven hard at very low frequencies (below port tuning frequency).

SINGLE-VENTED BAND-PASS
("4th Order Band-Pass")
The single-vented band pass is one of the best designs for the car. It is basically a "compromise" design between a sealed and vented box with a good compromise in performance and reliability. Probably the best type of enclosure for most applications.

Advantages: small box volumes, extended low frequency performance, and can be used with very high power amplifiers.

Disadvantages: slight drops in efficiency compared to a vented box and increased box design and construction complexity.

DUAL-VENTED BAND-PASS
("6th Order Band-Pass)
The dual vented band-pass box is unfortunately often used in the car as an acoustical "horn" - lots of high-SPL but marginal tonal quality if the enclosure isn't precisely constructed. Its performance is also limited to the typically very narrow bandwidth of frequency response.

Advantages: tremendous SPL's from VERY small boxes

Disadvantages: VERY complex construction and susceptibility to cone over-excursions for frequencies outside its operating region if a high quality electrical filter is not used.