What are the important issues in building a sealed enclosure?
First, the enclosure should be airtight. This is mainly because a sealed box uses pressure to limit the excursion. Without this proper pressure, the excursion from the driver can lead to bottoming (exceeding driver maximum excursion). Make sure to apply additional wood glue between every wall joints from enclosure interior to seal off any possible air leak. Second, apply internal bracing if possible (however it is not absolutely necessary). Sealed boxes are in general smaller than vented boxes so the free panel areas are quite small. To further improve on that, the internal bracing needs to be substantial to be effective. Using minimal bracing only gives us a slight improvement. Third, use only a moderate amount of polyfill. In Direct servo subs, the purpose of the polyfill is not to control the damping of bass roll-off, because the damping of bass roll-off is controlled by the servo loop. Rather, it is used to control the tonal balance of the mid bass range.
