Sealed, bass reflex or passive radiator?
Three common enclosure concepts solve the low-frequency problem in different ways. The right choice depends on the driver, target bandwidth, available volume, excursion and practical construction.
Sealed
A sealed box uses the enclosed air as an additional spring. It is mechanically simple, predictable and usually offers a smooth low-frequency roll-off. The trade-off is that deep bass often needs more cone excursion and amplifier power.
Bass reflex
A vented enclosure adds a tuned air mass in a port. Around the tuning frequency the port contributes strongly to acoustic output and cone excursion falls. Below tuning, however, the driver becomes progressively unloaded, so a high-pass filter can be important at high levels.
Passive radiator
A passive radiator replaces the air mass of the port with a moving diaphragm. This is useful when a low tuning frequency would require an impractically long port. Added mass, radiator excursion capability and enclosure losses still need to be considered.
The enclosure is part of the electro-mechanical system; volume and tuning should be simulated from the actual Thiele/Small parameters rather than selected by rule of thumb alone.