Enclosures featuring a (high) Qtc - in the vicinity of 0.9 or greater - are considered under-damped. While smaller in size, they begin to exhibit a ripple in their response, giving them a characteristic sound that ranges from warm to robust to outright boomy. A sharper roll-off and a higher f3 point are typical of high Q systems and, therefore, low-frequency extension is somewhat compromised. On the flipside, systems with a Qtc nearing 0.5 are considered over-damped. A good indication of this type of enclosure is an often dry, somewhat lifeless sound. Low Q systems have a more gradual roll-off which improves the frequency extension down low, but their large size degrades power handling by providing less restoring force (acoustic spring). Figures 1 and 2 give examples of different Qtc values and the shape of their response.
Altering the Qtc isn't only dependent on the size of the speaker enclosure. Another way to change this is by adding "fill" to the enclosure. This material can be an invaluable tool to alter box response, helping achieve desired parameters and a Qtc value that may be difficult to accomplish in certain applications. Dacron, poly-fill, fiberglass and acoustic foam are among the most commonly used products. Adding them within an enclosure can effectively reduce the Qtc of an under-damped enclosure, minimize standing waves and lower the resonance of the bass system. As well, proper selection of the amount, type and location of the material within the enclosure can effectively increase efficiency.
Although not every woofer works well in a sealed box, many competent speakers are designed for this type of application. Generally speaking, a driver with a low-resonance frequency (Fs) and Qts of 0.32 or greater indicates that it's suitable for a sealed enclosure (see sidebar by Vance Dickason for EBP measures). Woofers following these traits tend to operate in a wide range of enclosure volumes and are capable of producing good high-fidelity sound. Above all else, sealed enclosures are very forgiving, relatively simple to design, easy to construct and a good step above IB systems in overall performance.
VentedThe idea of vent-assisted enclosures has been around for a long time. The first patent on this design classification was issued to A.C. Thuras of Bell Labs back in 1932. His work contributed to basic theory of the design, describing in detail the interaction between the driver, enclosure and vent. It was not until 1961 that A.N. Thiele published a milestone paper, further developed by R.H. Small, which defined the behavior of a speaker. Their contribution is well known as the Theile/Small Parameters, which are broadly used in the design loudspeaker enclosures, particularly vented speaker systems.