In a bass reflex speaker (as opposed to a closed design) a tunnel (or: port, vent) is installed within the speaker’s cabinet, connecting the back of a speaker cone with a hole on the front or the back of the cabinet. Bass port and volume of the cabinet act as a Helmholtz-Resonator in this construction. With the help of the bass port, air can easily leave speaker, increasing the efficiency of the woofer while helping to extend the low end (lower cutoff frequency, less distortion). Bass port are often designed in rectangular or cylindrical form.

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