We must be concerned about directionality of high frequencies from the enclosure with a single speaker. A 15" speaker sounds totally different when listening directly on axis than 45 degrees to the side.

PORTS:

I'll bet my descendants from a few generations ago in the mountains of North Carolina were playing music on washboards, combs, broom handles on tubs with string, saws and jugs. When blowing into the mouth of a ceramic jug, the sound of tubas and trombones could be imitated. The highfalutin term for this is a Helmholtz Resonator. The loudest tone occurs when the air suspended in the neck resonates. When a tube or port is placed in a loudspeaker enclosure, it acts like the jug and becomes a "vented" enclosure. This vent tunes the resonant frequency of the box such that the port vibrates in phase with the speaker cone. The cone driver becomes almost motionless and the sound comes mostly from the port.

It should amaze us all that the first vented enclosure designs were developed in the 1930s and were empirically derived. It wasn't until 1971 the Australian engineer Nevelle Thiele published his works in the Journal of Acoustic Engineering Society that the world had a useable mathematical tool for designing vented enclosures. Later in 1973, Richard Small added to his works. Their papers were a plea for all speaker manufacturers at that point to provide electrical and acoustic parameters with their speaker products that would be compliant with the world standard Thiele/Small parameters.

The vented enclosure is not a perfect solution. It has its problems as well as advantages.

ADVANTAGES

1. A vented enclosure can be 3dB more efficient that a closed cabinet of the same size.
2. The bass response of a speaker driver can be extended to one third of an octave lower.
3. Decreased cone movement at the resonance results in lower distortion.

DISADVANTAGES

1. The vented box is very sensitive to parameter variations. You must know the parameters of the speaker. Installing another speaker in the same hole can result in a dramatic difference in response.
2. The vented box falls off at 24dB/octave whereas a closed box falls off at 12dB/octave. This reduces transient response.
3. They have a more complex impedance curve, but that really doesn't matter too much with today's amplifiers with stellar damping factors.
4. They have more phase shift, group delay, and a more complex excursion response.
5. Last but not least is vent leakage. Some midrange frequencies come right through the vent out-of-phase with the driver radiation. This reduces the midrange output. Insulation of the cabinet with damping material will reduce this effect. This can also be eliminated by adding a 90-degree bend to the port tube.

ENCLOSURE MATERIAL

Pine is a good material to use since it is a somewhat homogenous wood. Medium Density Fiberboard (MDF) is one if the best materials. Much unlike typical particle board, it is dense with well glued fibers. It is hard on saw blades. Plywood is one of the worst since its layers of different woods and overall construction can setup a resonance of its own. Excitation from the driver can make the plywood structure rumble with its own set of tones. This is an awful thing called sympathetic vibration.