To get an idea how a port in a loudspeaker cabinet works think of blowing air across the top of a bottle. At some point, the air pressure and bottle volume combine to make a noise. That’s the Helmholtz Resonance. As the volume changes with increased or decreased liquid in the bottle the “note” you hear changes in frequency.
Considering the Helmholtz Resonance in loudspeaker design, ports make loudspeakers more efficient by venting out the un-used sound energy that comes from the rear of the speaker. This allows the drivers to move less which improves efficiency and lowers distortion. Ports also lower the cut-off frequency of a loudspeaker, allowing even a smaller cabinet to produce much lower frequencies. A port truly lets a loudspeaker give more bang for the buck. Through calculation and simulation, the optimal spot and diameter of a port is determined, this is where the “tuned port” moniker comes in. It’s also the reason why you’ll notice the ports in KEF loudspeakers are not found in the center of the rear surface.
Surfaces behind the loudspeaker enhance bass response, which can cause the lower frequencies to lack detail or become too prominent. When placed near a wall or a corner, the added bass response from a ported loudspeaker can sometimes be too much. That’s why KEF provides customizable port bungs to attenuate some of the extra bass the port provides.
As you dial in your loudspeakers, experiment with the port plugs to find the response that is most pleasing to you. To help your ears along, start with the bass ports completely plugged and remove the amount of plug in each subsequent playback. You’ll hear the bass open up as you do this, and eventually you’ll find the optimal integration between the lower, mid and high frequencies.
By Jack Sharkey for KEF
