tuned ports, how to?

i always hear everyone say or write so and so cu ft. box with port tunned to so and so hz. what does that mean and how is it achieved? like i know there is a connection between the size of ur box and the size of the port, but just want a little clarification on the tunning. thanks

p.s. does port location matter?

Port to the same side as the driver if you can design it that way.  Some people design a slotted port when a round port won't fit.  This will allow all frequencies to follow roughly the same path to your ears.

Tuning for car audio takes into consideration the relatively small confines of an auto along with the main music choices of the primary listener.  The small air volume of a car creates a large amount of cabin gain, which is an acoustic magnification of certain frequencies.  Since those frequencies are primarily in a range above 40 Hz, it creates a bump in most music's sub bass.  You can either tune the box so that it creates a further increase in that range (SPL), or you can tune it so that the lower frequencies can receive a boost of their own.  Tuning low tends to even out the response curve a bit so that you don't end up with too "boomy" of a sound.

Music choice has its influences on this.  Prefab vented boxes are generally tuned at about 40 Hz.  So are a lot of the box building plans that you find at the subwoofer manufacturer's sites and manuals.  This is a middle ground, so to speak, which might appeal to listeners in general.  It sounds loud because the frequency range coexists with the cabin gain range and widens it.  It is a somewhat boomy sound.  Listeners of popular hip hop and rap find that the strong beats (already amplified in the mixing of the recording) are further boosted in the car.  Boom.  Listeners of other genres and those who prefer a more natural sound would enjoy tuning at a lower frequency.  I tend to tune in the 20's if possible for my tastes.  But tuning is dependent on your choice of sub driver!  You have to choose a driver that will allow you to tune to your taste.

The volume of air in the port and slightly extending out each end gets excited at or (and near) the frequency the box is tuned for.  At higher frequencies, that chamber of air acts as a damper for the driver, creating resistance to its movement.  This damping resistance is a necessary function of the driver; without damping the driver loses control.  When the air is expelled through the port at tuning frequency the sound is primarily that of the box.  Below tuning freq, the chamber of air doesn't damp the driver;  the woofer has no controls on it.  Damage can occur if your box is tuned high and no subsonic filter is applied at the preamp.  So, since music has little (or very diminished) content in the low 30's and below, it is a safe bet to tune to these low freqs and keep a healthy subwoofer for a long time.

It's a long learning curve to design a vented enclosure.  I suggest spending the time to learn WinISD and use it to help you with the design.  Use my sticky subject on this forum to help you get started, and be sure to read kfr01's followup in that thread.