Altering the Qtc is not only dependent on the size of the speaker enclosure. Another way to change this is by adding "enclosure fill." Using fill can be an invaluable tool to alter box response in order to achieve certain parameters and Qtc value that may be otherwise difficult to achieve due to size constraints within the vehicle. Dacron, polyfill, fiberglass and acoustic foam are among the most commonly used enclosure fills. Adding these materials within an enclosure can effectively reduce the Qtc, reduce standing waves and lower the resonance of the system. As well, proper selection of the amount, type of material and the location of the material within the enclosure can also increase efficiency by up to 15 percent.
Although not every woofer works well in a sealed box, there are many competent speakers designed for this type of application. Most will operate in a variety of enclosure volumes that can produce good high-fidelity sound, and most consist of a simple design and easy construction. Perfect for the do-it-yourselfer.
Ported EnclosuresThe reputation of ported enclosures has not been one of quality sound. Rather, they are often perceived as being sloppy and generally not very accurate. This common misconception is likely a product of a poorly designed or an improperly ported enclosure. It is not uncommon for ported (vented) enclosures to have better transient response than underdamped sealed enclosures with a high Qtc.
As odd as it may sound, ported enclosures have been around longer than the simpler sealed box. The patent on this design was first 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 and vent. In 1961, A.N. Theile published a landmark paper that was later (1973) refined by R.H. Small who defined the behavior of a speaker, thus allowing for ported enclosures to be perfected. Their contribution is well known as Theile-Small parameters and most commonly related to ported speaker systems.
Ported enclosures are often referred to by many names such as bass reflex, tuned or, more commonly, vented. What distinguishes a ported enclosure from that of a sealed is the port or duct that allows the back wave to interact with the front wave. The coupling of a port to the air inside the enclosure reinforces the low-frequency response of the subwoofer system. By changing either the length or surface area of the port, the resistance to motion of the column of air within the port changes its resonant characteristics, thus causing the tuning frequency of the enclosure to change.
In a ported enclosure, there are three main factors that have to be considered: the parameters of the speaker being used; the volume of air in the enclosure; and the size (diameter and length) of the port. Proper integration of these three factors will have a positive effect on the output of a ported system. The primary goal of integrating these three factors is to delay the rear output wave of the speaker so as it comes out of the port it is relatively in phase with the wave produced by the front of the speaker. The result is a gain in efficiency of about 3dB compared to a sealed system using the same woofer.