CHAPTER 7

BOXES

Enclosures for Woofers:

Because woofers move a lot of air they generate a back wave behind them. If you mount a woofer in free space without an enclosure you will get almost no bass because the back wave will cancel out the sound from the front of the woofer. There are many 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. Another method employs mounting the woofer (which needs to be a free air type in this case) to the rear deck of the car and using the trunk as a big box. This method is subject to many variables but can work well if done properly. Another benefit of this method is that you do not lose space from a large enclosure box. 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). Plywood is more impervious to water and probably the strongest and lightest, but Particle board and MDF being much denser have (possibly) better sonic properties, and MDF makes the nicest looking boxes and is easy to work with… The box material should be considered for the application by the system designer and chosen for the individual application.
Many folks seem to think MDF is the best and anything else will blow apart when you fire up your sound system, that's totally incorrect. All of these woods have places of honor in the car audio field.

Box Construction

Typically, for car audio, custom boxes are usually made from MDF and are glued together with wood glue, the box builders usually tap these boxes together with a brad nailer or screw it together. The brads, screws or nails are basically to hold the box together till the glue dries, the glue is the real strength, but we don't want to waste time clamping it together and waiting all day for the glue to dry. The screws, brads or nails allow the box to be completely assembled. upholstered and installed while the wood glue is drying.

Also, most installers will seal the inside seams of the box with some kinda goop or caulk, I have used clear or black silicone for decades and I like it the best.
Recently some folks have been claiming the fumes from silicone is bad for loudspeaker surrounds.. I say bull !!! I have not only been using silicone since the 1970s with no bad effects on the speaker boxes I have also used silicone to repair small tears in speaker cones or torn surrounds 100s of times, whoever is telling these lies must have some playdoe fetish or something???

Bracing the box

When the woofer moves in and out while in the speaker box a fair amount of internal air pressure is present when the speaker moves in and a negative air pressure occurs in the box when the speaker moves out, plus a whole lot of vibrations occur from the speaker moving and these pressure changes… 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 cause hearable distortions. Bracing the box can help minimize these distortions. In a box with two woofers a center divider can also act as a brace… Making internal bracing (or external) is pretty easy and makes the box a lot more sturdy… One trick I seldom see anymore is using the circle you cut out from the woofer hole as a brace elsewhere in the box, it can be cut in quarters and used as brace material or used to thicken the center of your largest panels which could vibrate.
 
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. 

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) allow 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)….
Some folks seem to think the box should be absolutely airtight! Some folks go so far as to seal the inside of the woofer box with fiberglass to ensure an absolute airtight seal! Some folks seem to be worried about the way some wood allows air to seep through it! I got news for those guys, most woofers are much more porous to air pressure than any wood we will be using!
Well, it wont hurt anything to be airtight, but I get the impression from a lot of folks that they put way too much effort into trying to achieve air tightness… To be honest though, the main problem with air leaks is high frequency sounds caused by air rushing though any small holes.. The air pressure changes inside the box can force air in and out of any small leaks and cause whistling sounds! DISTORTION! The distortion is what were trying to avoid… Generally, 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 to do 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!
And most importantly (to me anyway)
b) it prevents the speakers from interacting inside the box…
Let me explain, we know that 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??? Lets say one speaker is working fine and the other speaker is not playing at all, the working speaker will push the non working speaker (through the box) allowing out of phase air pressure to come out of the box through the non working speaker, causing the good speaker to sound really bad! If there was a divider, this wouldn’t happen, if there was a divider the good speaker would still be pumping nice clean sound to your ears, while the non working speaker just sits there… Even if the speakers are both playing but one is playing less loud, the sound from the loud speaker averages DOWN with no divider, but having a divider will leave the loudest speaker still playing loud, and even the less loud speaker will reinforce it somewhat making it even loader….
So basically, with no divider even a small problem with one speaker will make the whole box sound bad, but with a divider it is common for one speaker to completely go silent and the working one still sounds great!
In conclusion, my advice is to use a divider! Unless your too damn lazy…

How to figure box size

To figure out how big a box is measure the Height, Width, and Depth of the box (in inches) then HxWxD / 1728 will give you the cubic ft. of the box. Usually you will be concerned with the airspace inside the box so be sure 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. and use the formula (HxWx(TD+BD)/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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