The length of the port is usually determined by what frequency the cabinet is tuned to. Port length is usually a multiple of the sine wave that represents the frequency, for example, 35 Hz. The wave length of that particular frequency is divided usually by 4 or 8 to give port length. That is, the port length is 1/4 or 1/8 as long as the sine wave of 35 Hz. Hope this helped, it might just be confusing.
