Here's what TermPak has to say about it: "When designing a vent, you should try to keep the vent air velocity value as low as possible. Otherwise, vent turbulence and other audible artifacts may be heard. Ideally, you would use a vent with a very large surface area to keep the vent air velocity low. This would work because vent air velocity is directly related to the area of the vent being used. Unfortunately, the larger the vent area, the longer the vent length needs to be for a given tuning frequency. In reality, large area vents usually need to be so long that they’re just not practical. Because of this, many designers are forced to trade-off vent air velocity for vent length. Typically, audiophiles recommend keeping the vent air velocity below 5% of the speed of sound. This is not very realistic in autosound installations where enclosure volumes are usually pretty small. At Wayne Harris Enterprises, we have determined that a vent air velocity as high as 12% is acceptable for most installations. With an Aeroport, the vent air velocity can be as high as 20% without running into significant problems." I hope this may help to explain it.