Ah ... got itIt is a physical port (opening) in the sub cabinet which increases the output volume, but can reduce the transient response and make the bass sound somewhat muddy or boomy.
It works well if you want maximum volume for rock music or movie sound effects, but not so good if you want accurate music bass sound.
It can be in the form of a tube into the cabinet.
It may be behind the speaker grill.
Thanks
Port is really obvious rectangular holes at front of speaker ground level
Should I have bought fully sealed no-port speaker?
Will that give me "punch"
Sorry for dumb questions, but I am willing to learn
I already learnt a lot from this thread
Many thanks
Link to subwoofer is below
Behringer VQ1800D 18'' Active PA Subwoofer at Gear4music