Speaker Cabinets Tips
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Dead / Non-Resonant Speaker Cabinets
Today, most mainstream speaker manufacturers strive for a 'dead' sound from their speaker cabinets. They want the speaker box to interact as little as possible with the speaker itself, and they use a number of techniques to accomplish this. If building speaker cabinets yourself, you can use these ideas too: start with some beefy, high-grade 3/4 inch plywood and use internal bracing in the seams of the speaker cabinet whenever possible (some speaker designers actually use two layers of plywood for the front of the speaker to cut down on resonances further).
To cut down on internal standing waves, some speakers have no parallel walls. A cheaper and easier way to deal with standing waves is with internal sound absorption (usually in the form of 1 to 2 inch thick fiberglass batting on the inside back and sides of the speaker box for ported designs and poly-fill material to stuff the box if it's a sealed design).
*If you REALLY want dead speaker enclosures there's always concrete.
The Open Baffle
The open baffle speaker cabinet is the most basic design. It is simply a speaker mounted to a board, or baffle. This design was popular in the early 1900's when theaters began experimenting with amplified sound systems. These early systems usually consisted of a 12 or 15 inch woofer mounted to large piece of plywood, accompanied by a large horn tweeter.
Although not seen much today in the commercial speaker market, there is a large audiophile and Do-It-Yourself community that praise the open baffle speaker design as the purest, most accurate sound attainable with cone speakers. Open baffle speakers are inherently dipole (that is they project a front wave of sound into the room, and also a back wave from the rear of the speaker, which is out of phase with the front wave). In the right listening room, the dipole affect gives an added sense of spaciousness. There are many great resources online for open baffle speaker designs if you feel like experimenting.
The Golden Ratio
The Golden Ratio is based on the 'Golden Number' from ancient Greece, Phi. Phi (1.618033988749895...) is considered an irrational number like Pi (3.14159265358979... you probably remember this concept from geometry class). Phi is present throughout the natural world. The growth pattern of many organisms, bucky balls, and Quantum Physics are just a few examples.
The Golden Ratio refers to the ideal ratio for height, width, and depth of a speaker enclosure to reduce internal standing waves which is 0.6: 1.0: 1.6 (Phi). The details are very mathematically specific and have to do with sound wavelengths in relation to the lengths of the internal walls of a speaker cabinet. Regardless, however, if you're a math genius or not, if you have a speaker cabinet that follows the 'Golden Ratio', internal standing waves will be greatly diminished.
Speaker Cabinet Theory
Most cone speakers require some type of cabinet, or enclosure to optimize their sound quality. The main purpose of a speaker cabinet is to cut down on rear output that creates sound cancellations which can have a detrimental affect on a speaker's performance.
In the audiophile world, there are three basic schools of thought when it comes to speaker cabinets. The first, and most primitive, is the Open Baffle Design. This is simply a speaker mounted to board, in a round cutout. The board or baffle is cut to a particular size to optimize the speaker's performance (many old theater speaker designs used this principal). The second ideas (that gained a foothold in the 1950's) was the use of 'lossy', or resonant cabinets. A lossy speaker cabinet will attenuate rear sound waves, making them less noticeable. The final basic theory is to make the speaker cabinet as non-resonant, or 'dead' as possible. When knocking on the side of this type of speaker cabinet, it should feel, and sound solid and very dull.
Form Vs. Function
Once you begin hunting for your next set of speakers, you'll find most speakers found in chain stores look very similar (if you were to cut them in half, you'd see the similarities continue within the internal speaker cabinet as well). Lesser speaker cabinets will have MDF (Medium Density Fiberboard) cores, and be covered in vinyl to look like your favorite wood or to blend into your black home theater. These lower-end speakers rarely have internal bracing, or high-quality crossovers.
As you climb the ladder of speaker cabinet craftsmanship, you'll start to see some changes. Thick plywood cores, real wood veneers, heavy internal bracing, and better quality crossovers are the norm. Climb higher still and you'll see the use of exotic materials and advanced cabinet construction. Some high-end manufacturers will even build custom speaker cabinets to match home decor. The best speaker for you can be decided by assessing what's important to you in a speaker (some people are willing to take a hit in the performance department if it means they'll save a few bucks.
*Be aware that if a speaker manufacturer is skimping on cabinet materials and crossover design they're likely cutting corners in other areas too.
Acoustic Suspension Speaker Cabinets
Have you ever wondered how Acoustic Suspension Speaker Cabinets work?
• A speaker mounted in a small box will compress the air inside when the cone moves inward
• The air then pushes on the cone harder than the air on the outside
• The speaker will expand the air in the box when it moves outward
• The outside air will be pushing harder than the inside air
By using this pressure to restore the speaker cone to neutral, a much smaller box can be used to get deep bass. The trade off is efficiency (it is lower). Acoustic suspension dominated the speaker designs of the 1960's and 1970's as big, solid, state amplifiers were being developed to provide the needed power.
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