enclosures use a
tuned port or vent to increase output at lower frequencies.
Air inside the port acts
like a piston or motor that moves in phase with the woofer,
reinforcing the frequencies response.
are distinguished by a vent or duct in the structure. This
vent allows the rear sound wave of the woofer to interact
with the front sound wave. The coupling of a vent to the air
inside the enclosure reinforces the low-frequency response
of the subwoofer system and can greatly increase the
efficiency. By changing either the length or surface area of
the port, the resistance to motion of the column of air
within the port changes its resonant characteristics, thus
causing the tuning frequency of the enclosure to change.