About speaker boxes (enclosures) part 2 The Sealed box

Continued from Part 1:

The sealed or closed box is the simplest, most forgiving, and probalby the most common sort of speaker enclosure. It has the benefits of good power handling, linear response, small space requirements, and is the easiest to build.

Very simply put, a sealed box is just that, a rigid airtight box into which a speaker is mounted. The function of the box (to review) is to isolate the front wave from the rear wave of the speaker, and provide more stiffness to the cone. To elaborate a bit on the latter, when a speaker is playing music and the signal stops, the speaker will tend to keep moving (Newton’s first law of motion). The added compression or vaccuum of the air trapped in the box will tend to resist this continued motion after the signal stops. The volume of air inside the box also helps protect the speaker from moving in and out past it’s mechanical limits and damaging itself. This isn’t to say that you can’t tear a woofer apart by overpowering, but across the board, sealed box will have the greatest power handling at all frequencies in it’s range.

The drawbacks of the sealed box are that it will have the least efficiency, and not the best low end extension.

As a rule, smaller sealed boxes will be “tuned” higher. In other words, it will do better playing higher frequencies (with potential for high peaks in volume at certain frequencies). One thing to look at is a box’s “Q”. This is basically how fast the level of the sound rolls off below the FS (resonant frequency of the speaker in the box). A high Q indicates a steep rolloff and above 1.0 it will yield a peak. A low Q predicts a gradual roll off (better handling the low notes) at the expense of loss of output. Most box builders shoot for a Q of .707 (considered linear, low frequency roll off is 3db per octave) to 1.0 (will be a bit louder at the expense of output along the lower frequencies). Higher than 1.0 and you will get into large noticable peaks. This should be avoided unless you have a really good reason to do otherwise.

Size of the box is mostly dependent on the woofer. Many speakers are specifically designed for certain applications. If the manufacturer doesn’t suggest one, you should refer to a box design calculator program (or do the math longhand) to decide if a sealed box is right for your speaker. Car audio woofers designed for sealed box applications will generally require a volume of .25 to 3.5 cubic feet. If your calculations are off by as much as 10% of the volume when building you should notice little difference so these are considered very forgiving.

We’ve discussed how the very low frequencies are not very efficient with sealed design, fortunately due to the accoustics in a car we typically gain quite a bit of output at the lower frequencies anyway, and it often will provide in practice a very linear response (each frequency being about the same volume as every other with the same applied power).

There are many prefabricated sealed boxes on the market, many are as cheap as building yourself and very convenient if you don’t have access to tools and the size of the box will be a proper match for your speakers.

If you are building yourself, take care that the box is rigid and airtight. The most common materials these days are MDF (medium density fiberboard) or fiberglass, though some are made with Lexan, or other more exotic (read: expensive) materials. Pretty much anything dense, rigid, and airtight will do. Depending on the size of the speaker and the amount of power you plan to use, the thickness of the box should be increased and internal or external bracing should be considered for very high power applications. For most people 5/8″ MDF is adequate, on the extreme end, people have used a sandwich of 1.5″ MDF with a layer of concrete between the layers! The important thing is that the walls of the box don’t flex, the box is built with a material which won’t vibrate (the denser the better!) and that it is completely airtight.

Real world results are usually very satisfactory. A good even response with a “tightness” or “quickness” often described by the listener. In cars, the low frequencies are generally boosted up enough by the car’s accoustics to be acceptable, and they are easy to build and take up the least ammount of space. Infinite baffle will take up less space, but that will be another subject!

Next, the Ported/Vented Box

To be continued…….