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Intro to Enclosures

Many things should be considered before building a speaker cabinet. For example, the type of music the customer listens to might determine whether you use a sealed or ported enclosure, or possibly a bandpass. The customer will need to have an idea of how much amplifier power and what size woofers he wants in the system.

The amount of space that a stereo system occupies must be known and approved during the design stage of that system. The salesman must make it known how much room will be sacrificed in order to get the performance level desired. The more sound a person wants, the more useable space must be given up. Midrange and tweeter drivers don't usually take up much space and are not considered at this time. Amplifiers, subwoofers, and to a certain extent the midbass drivers all need their own space in the interior of the vehicle. If woofer enclosures are desired, they can take up the most space of all. Larger woofers naturally require larger enclosures. In addition the larger woofers require more amplifier power to maximize them. This all takes up space.

One efficient way to get the customer the most performance is to measure that part of the car that he is willing to give up. With this total space in mind, subtract the room required for sufficient amplification, then subtract the thickness of the wood to be used. This will give you the net airspace avail¬able. If for example the net airspace came out to 3 cubic feet (85 liters) then you would be able to choose the woofer(s) that would work best in this situation. The choices would include a single C15a or F15a, two C10a or F10a's, or even four C8a or F8a's. Of course the Solobaric woofers could be used in much less airspace. The Solobaric woofers are normally re¬served for the customer that wants the best performance in the least amount of space with the understanding that more amplifier power is required.

An alternative to the enclosed woofer designs mentioned above would be to use either Freeair woofers on a baffle board or the self-contained Kicker Substations. While both of these alternatives take up a certain amount of space, they are a lot more compact, cost less, and take less time than the custom enclosure route.

Infinite Baffle (Freeair)
The infinite baffle system is very simple and very easy to install. A baffle board is a wall the speaker is mounted to that separates the front and rear sound waves. The most important thing is to make sure the baffle board completely seals the front and rear areas. If the areas are not completely sealed, cancellation will occur and bass performance will be reduced drastically. The Kicker Freeair series drivers are designed specifically for this type of application.

The basic difference between our sealed box (Competition) and infinite baffle (Freeair) woofers is the design of the cone suspension. The tighter suspension of the freeair driver provides the cone motion control in an infinite baffle that would normally be provided by the small volume of trapped air in an acoustic suspension enclosure.


Advantages of Infinite Baffle ( Freeair)
1 - Smooth rolloff on low end (12dB per octave ) for extended low frequency response.
2 - Simplified installation on a baffle board ( no full enclosure ).

Disadvantages of Infinite Baffle
1 - Limited power handling.
2 - Cancellation of bass if not done properly.

Sealed Enclosures
The most basic and simple of all speaker enclosures is the sealed box or acoustic suspension design.

The acoustic suspension design has several advantages; it is easy to build, easy to tune and offers high power handling, tight response and extended low end output. Acoustic suspension enclosures produce lower bass because they roll off at 12 dB per/octave. Cone motion is better controlled at all frequencies because of the constant pressure on the back side of the cone, which enables you to run more power to the woofer. The Kicker Competition and Solobaric series are designed specifically for sealed enclosures.

Advantages of Sealed Enclosures
1 - High power handling capability
2 - Extended low frequency response and smooth rolloff (12dB/octave)
3 - Excellent transient response.
4 - Tolerant of minor enclosure size variations.
5 - Easiest enclosure to build.

Disadvantages of Sealed Enclosures
1 - Requires a woofer with a longer voice coil.

Vented Enclosures
A vented enclosure is not much more complex than a sealed box. It consists, basically, of a box with a hole in it. However, despite its simple design, vented boxes are considerably harder to get good performance from than sealed boxes - although many times the extra effort can be worth it.

The vent in the enclosure interacts with the volume of air in the cabinet and the driver to help increase output and reduce cone excursion at and around the tuning frequency. In fact, at box tuning, almost all the bass is produced by the vent - not the woofer.

The trick in building a vented box is to get the right size enclosure and the right size vent. You can't be too far off on either of these factors or your speaker's performance will suffer. In particular, using a too-small box or a too-high vent tuning frequency can eliminate bass instead of increasing it. Porting a sealed box that is too small usually does nothing to improve frequency response. The vent's placement within the enclosure is also important. You must leave at least the equivalent of the vent's diameter between the vent and any inside wall. For example, you would not place a vent with a 3" diameter within 3" of any wall. The same is true for clearance between the vent opening and the bottom of the enclosure.

Advantages of Vented Enclosures
1 - Reduced cone excursion and reduced distortion around vent tuning.
2 - Increased output capabilities around vent tuning.
3 - Vented boxes allow for specialized sound qualities such as the "bump" often preferred for rap and rock & roll.

Disadvantages of Vented Enclosures
1 - Total loss of cone movement control below vent tuning, which can result in high distortion and driver mechanical failure.
2 - Midrange sound coming from inside the box through vent can produce unpleasant sound coloration.
3 - Vented enclosures are more sensitive to changes such as temperature, humidity and driver fatigue.
4 - Enclosure design is more complex and the enclosure itself must be more solidly constructed because internal pressure at frequencies around vent tuning can be nearly twice as high as a sealed enclosure.
5 - Vented enclosures usually don't sound as "fast" as sealed boxes because of the resonant effects of the vent tuning which is always slightly out of phase with the driver's output.

Bandpass Enclosures
A single reflex bandpass enclosure (sometimes called "fourth- or fifth-order") is a specialized cabinet design which uses a combination of sealed and vented box volumes to produce a shaped speaker response. We recommend Kicker Competition Series and Solobaric Series drivers for these enclosures.

Bandpass boxes are a critically tuned design and internal pressures can be very high. Small air leaks can seriously reduce performance, and panel flex can be much more of a problem than usual - particularly with enclosures using built-in acoustical gain.

Since all speaker output comes from the vent, air speed within the vent can be very high. This can produce some turbulence and noise (particularly with high gain enclosures), which can be reduced or eliminated by rounding the ends of the vent with a quarter-round router bit or a round file. The inside end of the vent must be at least one diameter away from any inside cabinet walls, or flow restriction will occur.

Advantages of Bandpass Enclosures
1 - Acoustical gain can be built in. This means that one speaker can be as loud as four or more (using the same power input) at a limited part of the frequency spectrum. Acoustical gain and low frequency extension are inversely related; a high gain box will not go as low as a low gain box. Experiment with gain and cut-off values to find the best compromise.
2 - Specific low frequency cut-offs can be designed, including lower cut-offs than would normally be possible with simple sealed or vented boxes.
3 - A reduction in cone excursion can be included in a design, which can reduce distortion and improve power handling.

Disadvantages of Bandpass Enclosures
1 - Box construction is considerably more complex.
2 - Enclosures which add acoustical gain can be "boomy". The higher the gain, the less crisp and detailed the bass is likely to be.
3 - Boxes designed for extended lowfrequency performance will have less SPL output than either vented or sealed enclosures.
4 - Generally requires 50 to 100% more total enclosure volume than vented or sealed types.
5 - Usually has less power handling than a simple sealed box. Designing bandpass enclosures for increased power handing can result in very poor deep bass response.

Compound Loaded (Isobaric) Principle
Compound loading (a variation on the Isobaric principle) refers to the process of coupling two identical drivers together to work as one. Isobaric means constant pressure in reference to the air trapped between the two drivers. One of the attractive features of this design is that while the Fs and Qts remain the same for a pair of compound loaded drivers the Vas is cut in half when compared to one of the drivers by itself. When these values are plugged into an enclosure design formula the resulting box size is 50% smaller than that for one of the same drivers in the same style box.

There are three Compound loaded configurations possible: cone to cone (commonly called clamshell), cone to magnet, and the design that mounts two drivers magnet to magnet.

Compound loading can be applied to any of the enclosure designs described earlier in this section with the benefit of greatly reduced box size. Note that when using these configurations we have to power up two voice coils while the radiating cone area remains the same as a single driver. This results in a 3dB decrease in efficiency as compared to one driver with the same power.

For car audio, where the space savings is the main reason for using this design principle, the most logical version should be the cone to cone design because it is the most compact. When actually building this design a spacer with a minimum of 1/2" thickness must be added between all but the C18a drivers to prevent catastrophic failures.

The cone to magnet and magnet to magnet designs have a coupling chamber that is as compact as is possible between the drivers. This still adds to the overall size and negates the benefit of the smaller enclosure design.

When wiring the cone to cone or magnet to magnet designs it is necessary to wire the driver whose basket faces the listening environment electrically out of phase to ensure that the drivers move in the same direction at the same time.

Compound loaded enclosures that require porting, including Isovent and bandpasses, can be difficult or impractical to build. Because of small box volumes the length of a port may be too long to fit inside the enclosure. This can be solved in two manners. One is to allow some of the port to reside outside the box. The other option is to use 90 degree elbows inside the enclosure. Both these situations can cause other problems with wind noise and organ pipe resonances.

Isobaric Configurations