When purchasing IB speakers, keep in mind the following parameters to assure that the speaker is right for the application. The first is the Qts (the total resistance of the driver at its resonance). Speakers with a Qts greater than 0.6 typically work well in an IB environment. The second is the Vas (the volume of air having the same compliance as the driver's suspension). The Vas should be smaller than the total volume of the environment the speaker(s) are being placed in.
Sealed EnclosuresSealed enclosures have stood the test of time and have grown in popularity throughout the years. This design was first patented by Harry Olson in 1949 and then exploited by Acoustic Research in the early 1950's. Today, sealed enclosures (aka: acoustic suspension) are the most commonly used by car audio enthusiasts around the world.
Industry aficionados often associate sealed enclosures with "sound quality". It is generally accepted that the transient response exhibited by these enclosures is superior to other box designs when they are built to the optimum size required by a particular speaker. Much of this can be contributed to the "acoustic-spring" that adds to the restoring force of the speaker. The acoustic-spring is a reaction from the air trapped within the enclosure that compresses or rarefies as the cone moves back and forth. This is greatly affected by the distance that the cone of the subwoofer moves. The longer the stroke of the woofer, the greater the force exerted by the acoustic-spring in the opposite direction.
Power handling of a sealed system is a trait directly related to the acoustic-spring. It is the increasing load on the cone of the woofer that maintains control as the excursion increases, thus allowing for more power to be applied to the speaker. But power handling can be greatly affected by the size, or tuning if you will, of the enclosure. Larger enclosures have less of a limiting factor on the speaker cone, allowing the woofer to play lower with a more gradual role off while compromising power handling and some efficiency. On the other hand, decreasing the size of the enclosure, the acoustic-spring exerts more control over the cone motion with a trade-off in low frequency response. While this may increase power handling, the system will not play as low and often produce a rise, or bump, in the frequency response.
The size of the enclosure also determines the shape of the response. This is stated as the "Qtc." To be specific, Qtc is the value of damping provided of a speaker system. To give an example, a sealed system of optimum performance has a Qtc of .707. What this means is that the frequency response of the subwoofer will roll off at a -10.5dB/octave slope in the lower frequencies. Figures 1 and 2 give examples of different Qtc values and the shape of their response.
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 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%.
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.