Polk Audio EX652a owner manual Called room gain or transfer function that gives

ACOUSTIC SUSPENSION ENCLOSURES FIGURE 1.

The acoustic suspension cabinet is a sealed airtight box, and is the simplest, easiest box to build. It is a very predictable enclosure with easily calculated parameters and a smooth natural sound. Properly built acoustic suspension cabinets have a reasonably flat frequency response that begins rolling off at 12 dB per octave at the frequencies below its cabinet resonance. This works very well inside a car because of a natural phenomenon

called “room gain” or “transfer function” that gives

you a 12 dB per octave increase in bass frequencies. You can roughly calculate at what frequency this gain begins by using the

equation F= 565 / L. F is the frequency at which bass gain begins, and L is the longest dimension of your “room”. If, for example, you measured the longest dimension in the interior of your car as 5.65 ft., the room gain begins at 565 / 5.65 or 100 Hz. If your goal was perfectly flat frequency response, you would design your cabinet for this particular car to have a resonance frequency of 100 Hz. Since most people want more bass than a flat frequency response yields, tuning the cabinet at a lower frequency, say 50 Hz, would give you a gain of 12 dB per octave between 100 and 50 Hz and flat response from 50 Hz down. The larger the cabinet, the lower the resonant frequency, and the lower the efficiency. Two identical systems will sound very different in a Honda vs. a Cadillac. The bigger the car, the lower the frequency at which room gain begins.

INFINITE BAFFLE	FIGURE 2.
Also known as free air enclosures, this essentially means mounting
your woofers in the rear deck of your car, and not building a box
around the speakers. At first glance, this would seem to be a very easy
way to install your new speakers, but many problems can occur if this
is not done exactly right. In the acoustic suspension design described

above, the box is providing two major functions: first, the air that is sealed within the box acts as a spring to help support the suspension of the speaker. As the woofer cone moves in towards the box, the air in the box pressurizes; and as the cone moves out a vacuum forms. This helps to

keep the cone from traveling too far and destroying itself. An Infinite Baffle design

has no box; therefore, it has no trapped air behind the speaker to act as a spring. If your speakers are not specifically designed for infinite baffle use they can be damaged. Your new EX Series II woofers are designed for use in enclosures and are not recommended for infinite baffle use. The second thing that a sealed box provides is sound wave isolation created from the front of the speaker cone due to the waves that are created from the back of the speaker cone. If the two wave fronts are not isolated from each other they will mix

together and cancel themselves out, making no bass at all. If you plan on building an infinite baffle system you must plan on sealing all of the air passages between the trunk and the passenger compartment (not an easy job). Also the metal used on the rear decks of cars does not make for a very good baffle board; the metal will tend to resonate and again cancel out the bass frequencies. You should plan on mounting your woofers on a non resonant wooden baffle. A properly designed infinite baffle enclosure will roll off at 6 dB per octave below cabinet (speaker) resonance, and will have a very smooth frequency response. The infinite baffle is the least efficient box design.

TUNED PORT ENCLOSURES	FIGURE 3.
The tuned port enclosure is similar to the acoustic suspension box
except that it uses a port to take the back wave from the
speaker and “tune” it to reinforce the front wave. A
system becomes more efficient when the
frequencies of the port and the front wave are in
phase. It is absolutely critical that the dimensions
of the box and the port are perfect, or the back and
front waves will cancel each other out, reducing
your bass output. Tuned port enclosures tend to be
peaky in the bass, and many people (particularly
Bassheads) like the punchiness of the sound. Tuned port
enclosures roll off at a very steep 18 dB per octave below cabinet

resonance, which means that the room gain of the car will not help to extend the lowest of frequencies. In addition, the woofer becomes “unloaded” below the tuning frequency of the port and very loud musical information below these frequencies can damage a woofer.

BAND-PASS ENCLOSURES FIGURE 4.

Band-Pass boxes can be built in a near infinite amount of configurations. They incorporate at least two individual chambers and are by far the most complicated box to model and build. The theory behind band-pass boxes is to limit the frequencies produced by the subwoofer, thereby making it more efficient in the frequencies that it produces.

A by-product of this efficiency is that it is often possible to build a band-pass box much smaller than its equivalent acoustic suspension box and get

the same quantity of acoustic output. If you have a very small car with limited room for a subwoofer box, a band-pass box

may be a good choice. The downsides of the band pass box are numerous. First, the construction must be flawless, any airleaks or rattles will seriously degrade performance.

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