Frequency ports are an important design element of loudspeakers. They affect the frequency response of the speaker significantly. These ports are small holes inside the enclosure of the speaker as well as a tube which is inserted into the hole and extends towards the inner of the speaker.
Through the frequency port mostly low-frequency waves are emitted. The diameter and the length of the plastic tube of the frequency port affects the frequency response. Usually the lower the diameter of the port the more linear the frequency response of the speaker. During the design of a loudspeaker, you should experiment with the location as well as the diameter and length of support. The location of support is not that critical. However, the diameter and length of support will affect the speaker quite a bit.
Some loudspeakers don’t use a frequency port. Those speakers usually have a fairly large air volume and stolen require ports for shaping the frequency response. Some people say that speakers without support sound more clear the speakers that do have it. The reason why most manufacturers incorporate a frequency port is that there is a trade-off between size and frequency response. Obviously many customers prefer having small speakers. Small speakers are more easily transported and take up less space. Also they are more aesthetically pleasing than bulky speakers.
However, for achieving best sound quality the size of the speaker has to be large. That is why almost 90% of high-end loudspeakers are fairly big. The large size of the speakers is also marketing instrument. Obviously having a large product justifies a large price tag.
Some loudspeakers incorporate multiple frequency ports. There is some advantage of having multiple ports however manufacturing of the speaker becomes more complicated. Also, the tolerance of frequency ports is usually fairly low. Some manufacturers use plastic inserts. However, these inserts can shift and thus affect the response. A better solution are plastic tubes which are glued into place.