Trade-offs come in many forms when working with IB enclosures. Sure, using IB enclosures conserves a lot of space, especially in the trunks of smaller cars; however, there are some losses to be noted. The most prominent is efficiency. Without a solid structure built around the woofer, it is difficult to reinforce the sound. Trunk interiors are filled with flimsy panels, thin sheet metal and usually a measly spare tire-none too pleasing for sound reproduction. This same issue holds value with the speaker's power handling. As much as 50 percent of the woofer's nominal power handling can be lost when placed in an infinite baffle. Another problem that plagues IB enclosures is the tendency to lose control of cone movement when played below the Fs (resonance frequency of the driver), much like ported systems when played below the tuning frequency. For this reason, it may be desirable to utilize a subsonic filter below the Fs of the speaker.
When purchasing IB speakers, keep in mind the following parameters to ensure 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 1950s. 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 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 attributed to the "acoustic-spring," which 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 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 roll-off, while compromising power handling and some efficiency. On the other hand, when 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 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.