Everyone has seen "ports" on a speaker. Those holes either in the back or front of speakers known as "bass reflex" speakers. These speakers allow air to flow out of the speaker and are designed to increase the bass response. How does the port give a small 6.5" or 5.25" speaker more bass? When the woofer moves back and forth in the speaker cabinet it moves air. The only place that air has to go is out of the port. Great, but how does that increase the bass?
A "tuned port"
Think of an empty bottle and the noise it makes when you press your lips against the side and blow down into it. Depending on the amount of liquid in the bottle and its shape the "pitch" of the sound varies. Speaker designers will change the size and shape of a port in a bass reflex to create a certain pitch caused by the air blowing out of the port. They will "tune" the port to be complimentary to the "roll off" of the natural bass limit of the woofer.
View Ported Speakers
See the chart below. The gray line is shows the woofer would naturally roll off, or diminish in bass around 250Hz. The red line shows what happens when a designer designs a port to play a note at a lower pitch than the speaker can create. Instant deeper bass. The blue line shows what you're ear will hear. Instead of the speakers bass dropping off totally at 250Hz it drops slightly then rebounds from the sound coming out of the port and diminishes from there.
When done correctly a tuned port on a bass reflex speaker can create wonderful noise. The only way to tell is listen to a few bass reflex speakers and find one you like.