Additional resources: using speaker design software for woofer substitution

When substituting woofers in loudspeakers, you'll need to find one that mechanically fits.  In addition, you'll want to consider the bass performance.

Woofers have three parameters that affect bass performance. They’re collectively known as the Thiele/Small (T/S) parameters and are named fs, Qts, and Vas. Even if we knew what the original woofer T/S parameters were, it’s not important to match them. What’s important is that the replacement woofer is compatible with the speaker box. 

Sealed boxes (i.e., no vent or port) are forgiving of a wide range of T/S parameters. The best simple advice for them is to avoid choosing a replacement woofer that has a Qts value larger than 0.7. Woofers with high Qts values have weak magnet structures and run the risk of producing boomy, resonant bass.

Ported speaker boxes are more complex in design and are much more sensitive to T/S parameter changes than sealed boxes. If you want to do a good job substituting a woofer in a ported box, you’ll need to use loudspeaker box design software to check and compare the performance of candidate woofers in your particular box. You’ll need to enter the cabinet and port dimensions of your box as well as the T/S parameters of the woofers you’re considering as substitutes and then evaluate the results. 

Here's a video showing how to do this using the Subwoofer Design Toolbox by MFR Engineering.  The Subwoofer Design Toolbox works for any woofer, not just subwoofers. Using software also benefits sealed-box woofer substitution projects. 

  How to Choose a Replacement Woofer for a Ported Box
Useful when the original woofer is irreparably damaged and an exact replacement isn't available.

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