Enclosures for Car Audio Woofers:
Because woofers move a lot of air they generate a back wave behind them. Mounting a woofer in a free space without an enclosure will cause a loss of bass as the back wave cancels the sound from the front of the woofer. There are many varieties of types of enclosures for woofers to handle this backwave.
A popular one is a ported box.
This enclosure has the woofer mounted in box with a hole in it and a port (tube) attached to the hole. The port is made a specific size and depth to cause a "bump" or rise in the frequency response at that point. This makes the overall system more efficient but can cause the bass to be somewhat "boomy" or less "tight" depending on how its done.
A newer technique is a bandpass enclosure. The woofer is mounted inside the box and fires into another chamber within the box that is ported to the outside. Again, this increases efficiency greatly but only at a certain frequency. This effect can make the system very loud and boomy.
The oldest and most popular type is a sealed enclosure. This method simply has the woofer firing into the car and the back wave is suppressed inside the box. This method usually produces tight accurate bass but is not as efficient.
Also this method typically requires a large box to work well. Finally because of the lower efficiency of this design more powerful amps and woofers are needed to play loudly. When any of these enclosures are created using the specs of the woofer as a guide you can create the type of bass response that you desire.
BOX STUFF
Box Material
Speaker boxes can be made of anything rigid enough to not vibrate and cause losses or distortion; the most common materials are Plywood, Particleboard, and MDF (medium density fiberboard). The material used should depend on the needed application.
Common belief is that MDF is superior to other materials, which is not true.
Box Construction
Usually, for car audio, specifically designed boxes are usually made from MDF and are glued together with wood glue, the box builders typically tap these boxes together with a brad nailer or screw it together. The brads, screws or nails are there to hold the box together till the glue dries, the glue is the real strength.
Also, most installers will seal the inside seams of the box with some kind of goop or caulk, Black silicone is excellent from personal experience.
There has been recent talk that fumes from silicone are bad for loudspeakers, but this is a false claim.
Bracing the box
Pressure changes when the car audio play cause vibrations. The wood the box is made out of can vibrate also, using a thicker wood or denser wood will minimize these vibrations.
If the box panels are large (or thin), they can resonate enough to make hearable distortions. Bracing the box can help lessen these distortions. In a box with two woofers a center divider can also act as a brace. Internal or external bracing is simple to make and makes the box more sturdy. Folks rarely go to these extremes to brace a box used in car audio, but some go so far as to use double panels and fill between them with sand or concrete! Its up to you. 
About getting an Airtight seal

A sealed enclosure is designed to (a) keep the back wave of the speaker from coming in contact with the front wave of the speaker. (b) allows the air pressure in the box to add a suspension component to the speaker (in other words the air pressure of the box keeps the speaker from moving too far as it plays)….
A 100% airtight box is hard to achieve. However too many little leaks can cause distortion. Typically, a few small air leaks wont change the performance of the woofer like folks seem to think, in fact an air leak would have to be huge to greatly affect (a) and (b) above… The main thing we want achieve is avoid air leaks because of whistles!
Common Enclosures

There seems to be some confusion about using a divider in a woofer box with two woofers… Many installers seem to be telling their customers that by having no divider the box will make more bass! I think those installers either don’t know anything about speaker boxes or are making excuses because they are too lazy to build the box correctly…!!!
If both woofers are exactly the same, the box will sound the same with and without a divider! There is no increase at all by not using a divider…
But there are some good reasons for having the divider!
a) It braces the box!
b) It prevents the speakers from interacting inside the box…

If the woofers are putting out the exact same sound and level then having no divider wont matter. But what happens if one speaker is playing LESS than the other speaker??? If one speaker is playing and the other is not playing at all, the lack of a divider will allow air pressure to come in from the non functioning speaker causing a drop in the quality of sound from the functioning speaker.
This would not be the case with a divider. Even if both speakers are functioning there would still be an issue if one plays less sound. The sound from the loud speaker averages down with no divider. This also can be avoided with a divider.
So basically, without a divider even a little problem with one speaker will make the sound quality suffer, but with a divider it is common for one speaker to completely go silent and the working one still sounds great!
So the wise choice is to use a divider.
How to figure box size
To figure out how large a box is measure the Height, Width, and Depth of the box (in inchees, then Height * Width * Depth / 1728 will give you the cubic ft. of the box. Usually you will be concerned with the airspace inside the box so be certain to get the inside measurements and not the outside measurements for your calculations.
How to figure angles
To calculate a truckbox or one with a simple angle, measure Height, Width, Top depth and Bottom depth use the formula (Height x Width x (Top Depth + Bottom Depth)/1728)/2 to convert it to cubic Ft.
For more than one angle you will have to measure and calculate sections and then add the sections together for your total.

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