Most novices tend to focus on power handling when choosing a replacement speaker, unfortunately this is often the least relevant specification to try to match. The mechanical properties of a speaker will often have more effect on the final result, in terms of tonality and efficiency, than power handling. These mechanical properties govern how the speaker interacts with the cabinet, and give useful insight into expected performance.

This part can get quite complicated, and is where cabinet designers can spend days trying different combinations of cabinet shapes, sizes, and drivers to fine-tune a design.

If you check the specifications of most drivers on the market, they will include the Thiele-Small, or T/S parameters (read more here). These provide numerical values that help quantify the mechanical properties of a speaker, such as cone mass, compliance, resonant frequency, and damping.

When selecting a replacement, its best to choose a driver with broadly similar mechanical properties to the original. Unfortunately this information is rarely available for the original driver, so an educated guess is often required. Which parameters matter most depends on the cabinet type and the intended application.

For bass speakers, pay particular attention to Fs (resonant frequency), Qts, and Vas. Ideally these should be reasonably close to the values of the driver you are replacing.

For mid-bass speakers, such as a 12" or 15" driver in a full-range cabinet, several factors matter. Avoid very high power woofers (900W or more) with extremely large voice coils, as these tend to be heavy and sluggish, with poor mid-range response. Look for a sensible Fs of around 60Hz or lower for usable bass output, high sensitivity (typically 98dB or higher), and a smooth frequency response extending to 2kHz or above.

Click here for guidelines on selecting a woofer for an 18" bass reflex cabinet.