Bass reflex

This app can be used for vented speaker box design (bassreflex tube length calculation) or for reverse analysis of existing solutions.

Wikipedia: A bass reflex system (also known as a ported, vented box or reflex port) is a type of loudspeaker enclosure that uses the sound from the rear side of the diaphragm to increase the efficiency of the system at low frequencies as compared to a typical closed box loudspeaker or an infinite baffle mounting.