|LoudspeakerBuilder.ca - (Speaker Enclosures)|
In every loudspeaker sound wave are produced at the rear of the speaker. Those waves will have their energy absorbed into the enclosure causing the enclosure to resonate (vibrate). This interaction between the standing sound waves and the enclosure is what causes the audible coloration and smearing of the loudspeaker's sound. In extreme conditions where the enclosure has no dampening materials the energy from the standing waves can approach the output of the drive unit itself. To control these waves we dissipate their energy through frictional losses with any material which offers high resistance to the sound waves such as a damping sheet or acoustical dampening fiber fill.
Another very effective way to control enclosure resonance is through internal bracing. The best way of bracing is to design it into the enclosure from the start and not just add some cross bracing latter. Better quality cabinets use diagonal or planar bracing which bonds all four sides of the enclosure together. A simple often used method is to add bracing which is basically a board with a hole cut in it. The hole space in the center of the board are needed to allow air movement in the enclosure.
The loudspeakers shape also is also a factor in limiting enclosure resonance. A cube is the absolute worse shape (unless your designing a sub woofer) that a loudspeaker could be due to the standing waves produced inside the enclosure. In general try to pattern your loudspeakers to the golden rule ratio of (Heigh 2.618 : Width 1.618 : Depth 1) This ratio will minimize these standing waves and the resulting enclosure resonance and distortion. You don't have to match the ratio exactly, but the closer the better.
Speaker Box Enclosure Mass Loading
Adding weight such as dry sand or lead shot into a special compartment at the base of the enclosure will benefit the loudspeaker by increase it's effective mass. By adding mass to the loudspeaker structure it will act as an efficient absorber of vibrational energy. The heavier an object is the more energy it takes to make it vibrate; so weight is a resonance dampener. By experimenting with the amount of the fill material it is possible to fine tune the enclosure's tonal qualities to better match your own tastes or to help control some unpleasant sounding resonance produced by your loudspeaker enclosures. If you are determined to extract the absolute best performance that your loudspeakers can deliver, it is recommend that you mass load your speakers.
Speaker Box Enclosure Material
Most speaker builders would agree that medium density fiberboard (MDF) or medium density overlay (MDO) board is the product of choice. This is due to it's relative low cost, and because the stuff is strong. Don't use particle board or cheap quality plywood. Higher quality plywood that have no voids in the laminations are acceptable; you can spot poor quality by checking for missing pieces of laminates that have fallen out along its edges. Three quarters of an inch is the minimum board thickness that should ever be used because a strong and stiff enclosure is needed to minimize enclosure resonance.
Speaker Alignment within the Box Enclosure
The tweeter, midrange and the woofer should be kept in a straight vertical line with one another; this is done to keep the sounds in-phase with the listeners ear. The woofer sits nearer to the floor and the tweeter should be near ear level with the midrange in between the two. Mount all the drivers close together; the sound quality is usually improved due to less phase shifting and better imaging.
Make the Speaker Box Enclosure Air Tight
Make the enclosure airtight (except for the port of course). Leaks will show them self as unwanted air noise and in more extreme cases the leaks could effect the proper operation of the drivers and change the port frequency of base reflex loudspeakers. Use caulking to seal around the inner joints and any other place that air could escape.
Yeh, Size Does Matter
It's only logical that if you want big sound with deep clean base you need big speakers. I have read claims by some builders that their 6.5 inch based or smaller loudspeaker can match or exceed the sound quality and quantity of a much larger driver based loudspeaker.
The laws of physics say higher frequencies don't require much air movement, but for every octave drop in the audio spectrum the driver has to be capable of moving 4 times more air to keep the same output level (loudness). Simply put, deep base requires you to move a lot of air, something a small driver cannot do so don't fight the real facts, just accept them.
So now your considering using a 15 inch driver in your loudspeakers but are concerned that your loudspeakers will have too much bass and they will sound unnatural; don't worry, they won't. They will sound very similar to those quality 6.5 inch speakers at low to mid volume levels. They will just be more capable of delivering much louder clearer bass than the 6.5 inchers when asked. They will only sound boomy if your enclosure is too small; remember keep the Qtc to 0.707.