18" bass reflex cabinet. The most common type of bass speaker cabinet, using a front-facing driver with bass ports to extend low-frequency output.

For this type of cabinet, its best to try to match the resonant frequency of the original driver, as the cabinet will have been designed to get the best results from that driver through adjustment of the port size. Most common 18" bass reflex cabinets are tuned to around 32–40Hz, and its usually best to stick with a driver that has an Fs (resonant frequency) in this region.

If the port tuning does not align well with the resonant frequency of the driver, you lose much of the benefit of the bass port. This typically results in reduced bass output, encouraging the user to apply excessive low-frequency boost to compensate, which can lead to overloading and eventual failure. Poor alignment between the cabinet and driver can also result in inadequate excursion control, allowing the cone to move excessively and increasing the risk of mechanical damage.

Unless you are confident in your understanding of the original cabinet design, we recommend avoiding drivers with very low Fs values intended for infra-sub applications. Drivers with resonant frequencies below around 32Hz require cabinets specifically designed for them, and are generally a poor choice for drop-in replacements in standard bass reflex enclosures. Read more about Fs (resonant frequency).

Cabinet volume is also a key factor in bass reflex designs. Modern trends favour more compact bass reflex cabinets, which generally work best with 18" drivers having a Qts between approximately 0.31 and 0.4. Drivers with higher Qts values tend to perform better in larger enclosures. Historically, very large subwoofer cabinets were common, and higher Qts drivers worked well in those designs.

This is a guideline rather than a hard rule. A driver with a Qts of 0.41 may still work acceptably in your cabinet, but large deviations from this range increase the risk of sub-optimal performance.

Below is a short list of popular high-power 18" woofers commonly used in small to medium bass reflex cabinets:

B&C 18TBX100

Beyma 18Lex12000Fe

RCL L18P300

B&C 18TBW100

Fane Colossus 18XB

Fane Colossus Prime 18XS

Faital Pro 18XL1600

There are of course many other suitable drivers available. These examples are simply some of the best-known and most widely used models that have proven to work well in bass reflex cabinets.