About speaker boxes (enclosures) Part 3 the Ported (or vented) Box

Continued from part 2:

Ported and Vented are two terms that describe the same thing. Another less common name is “bass reflex”. All of these described a box with one or more ports or vents of a calculate length and width. The box still separates the rear wave from the front wave of the driver, controlls the cone movement (above tuning frequency), but the port adds it’s own element to the system.

The port contains a volume of air which will resonate at a calculate tuning frequency. Simply put it is the same thing happening as when you blow across the top of a coke bottle. The column of air in the port vibrates and acts as another sound radiator. The way this works is the air in the port vibrates in phase with the speaker and acts as essentially another speaker.

In practical applications this not only adds efficiency to the system (more output with the same power input), it also helps the system better produce lower frequencies. The tradeoff is that the box quickly loses control of the woofer below the tuning frequency, and improperly built ported boxes can sound really awful. Ported boxes also take up as much as double the volume of sealed systems for the same woofer.

When considering a ported box it is very important to either use a good box design software, or refer to the manufacturers reccomendations for volume and port size. Keep in mind, some woofers just aren’t engineered to perform well in ported applications and it just isn’t an option.

Area of the port is typically at least 1/4 of the area of the woofer. Ports can either round or rectangular though round with fluted openings is considered ideal to avoid turbulance (chuffing noise) as the air moves in and out of the port. The greater the port area, the less risk of port turbulence, and the greater the output of the system. Of course port length raises dramatically as area is increased at which point space becomes an issue. The shape of the port is irrelevent, the opening can be any shape, and the length can be bent, or curved to fit a box whose depth wouldn’t accomodate a straight port.

The factors that effect port tuning are the area and the mass of air inside the port. Another highly attractive alternative is a “passive radiator” This is simply a woofer cone (generally of larger area than the driver) with an adjustable mass that can be tuned to whatever the mass of air in a port would equal. This sysem performs the same task, vibrating along with the active woofer, and is considered a very good option as it eliminates port noise and requires very little added space (the volume of the port is subtracted from box volume for calculations).

Since the box has little to no controll of the woofer below tuning where the port “unloads” the speaker can be damaged without use of a “subsonic filter”, a type of crossover that filters out ultra low frequencies from your signal. It is generally a good idea to use one in any subwoofer application, and almost always reccomended with ported applications.

Again, great care must be taken in building this sort of box. They often get a bad reputation as sounding “sloppy” or “peaky”. This is typically because someone just throws any woofer in a pre-fab ported box. The sub MUST be designed to operate well in a ported box, and the box MUST be designed to compliment the woofer to be used or results will be dismal.

Calculations must be accurate in volume of the box and port system. Care should be taken to keep the port away from obstructions both inside and outside of the box (a port width minimum from walls, or other obstructions is suggested). Also very important is to build the box extra tight and solid. Contrary to popular urban myths, the port does not “let preasure out” of the box. It acts like another woofer and actually creates greater preasure in the system. The box must have strong airtight seams and additional bracing should be considered for high powered applications.

In practical applications, a ported box is often a great way to get the greatest efficiency and low frequency response out of your woofer. If your equpment is designed for the job, you have a little extra space, and you are careful with design, there is no reason that a ported system can’t achieve very pleasing musical results. For DB drag competitions, ported boxes are also useful in creating enormous peaks at precisely calculated frequencies. If you want to just throw ports in any old box, or throw a random woofer in a pre-fab ported box, odds are you’ll be in for a bit of a let down.

Next, the bandpass.

To be continued……