A bass-reflex port is an opening in a loudspeaker cabinet designed to help the speaker reproduce low frequencies more efficiently. It works together with the woofer and the air inside the cabinet to increase bass output over a particular range of frequencies.

When a woofer moves backwards and forwards, it produces sound from both the front and rear of its cone. In a bass-reflex loudspeaker, the sound energy produced behind the woofer interacts with the air contained within the cabinet and port.

The port is normally a tube or duct connected to an opening in the loudspeaker enclosure. Its dimensions and the internal volume of the cabinet are carefully chosen so that the system is tuned to a particular frequency. Around this tuning frequency, the movement of air through the port contributes significantly to the speaker's acoustic output.

This allows a bass-reflex loudspeaker to produce greater low-frequency output than a similarly sized sealed enclosure in the region where the port is effective. It can also allow a manufacturer to extend the useful bass response of a loudspeaker without simply increasing the size of the woofer or cabinet.

Bass-reflex ports can be positioned on the front, rear or occasionally the bottom of a loudspeaker. The location can influence practical speaker placement, particularly with rear-ported designs. Positioning a speaker close to a wall can change its perceived bass response, so manufacturers often provide recommendations for placement.

The shape of the port is also important. Poorly designed ports can produce unwanted turbulence or audible airflow noise at higher listening levels. Many manufacturers therefore use flared or specially shaped ports to help air move more smoothly through the opening.

Bass-reflex designs are extremely common in both standmount and floorstanding loudspeakers, although not every speaker uses a port. Sealed-box loudspeakers use a completely enclosed cabinet, while other manufacturers employ passive radiators or more complex loading systems to control low-frequency performance.

A bass-reflex port is therefore more than simply a hole in the speaker cabinet. It is a carefully tuned part of the loudspeaker system that works with the woofer and enclosure to improve efficiency and extend low-frequency reproduction.