Infinite baffle
Infinite baffle enclosures are the simplest from a design concept. The principle of infinite baffle design is the separation of the front output of the woofers from the rear output of the woofers. One of the advantages of this design is the low resonant frequency of the infinite baffle system. This low resonant frequency allows the system to have a very good low frequency extension. Another advantage of an infinite baffle design is the little amount of trunk space available for this application. For this reason, infinite baffle designs are very popular in vehicles where trunks are small but the desire for big bass is large.
One of the main disadvantages of this application is the reduced mechanical power handling of the loudspeaker used in the application. This is due to the fact that the enclosure used is so large (the volume of the trunk) that the air in the trunk does not assist the mechanical suspension system of the woofer. Not all woofers are best suited for this application. To truly maximize this design, the mechanical suspension of the woofer needs to sturdy enough as not to allow the woofer to reach hard suspension limit. (Like the Woofer’s voice coil hitting the back plate of the speaker).
It is a common misconception that infinite baffle enclosures are the easiest to construct of the different type of enclosures. While it is true that variations are of this enclosure are not too difficult, maximizing the performance of this design can be quite time consuming.
In true infinite baffle designs, the front wave output of the woofer is separated from the rear wave output of the woofer with the air mass behind the woofer does not assist the mechanical operation of the woofer. In real car audio applications, infinite baffles are just really large sealed boxes. The large volume of the trunk of the car serves to separate the rear output of the woofer from the front output of the woofer without affecting the mechanical performance of the woofer.
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topDoes Infinite Baffle Really Exist in a Car?
When a sealed enclosure volume gets large enough with respect to the volume of air equal to the speaker’s compliance, Vas, the sealed volume of air does not add to the suspension stiffness of the woofer. An average "rule of thumb" for determining when the volume of air in the trunk equals of exceeds three times of the Vas total of all the woofers. Even with the large size of many trunks, it is still possible to use woofers where the relationship between the total Vas of the woofer and the volume of the trunk leads to a situation where the air in the trunk does affect the mechanical operation of the woofers. In this situation, the trunk acts like a large leaky sealed enclosure.
Maximizing Performance
Maximizing the performance of an infinite baffle application can be addressed in to different criteria: minimize leaks and maximize the sturdiness of the baffle.
Leaks = Loss
Minimizing the leaks can be very difficult. Some cars are more difficult to seal than others. There are a lot of ways that air can travel between the trunk and the passenger compartment. Properly sealing the baffle and the common walls between the two compartments take considerable time and effort for a complete job. If you cannot completely eliminate the leaks between the trunk and the passenger compartment, the infinite baffle design will still work, but the result will simulate a leaky sealed box with reduced low frequency capability. Although all leaks may not be eliminated, it is still possible to get good performance of the leaks between the two compartments are kept to a minimum.
Plugging the Leaks
There are usually two types of leaks associated with infinite baffle applications: near the baffle mounting and in the corners of the vehicle between the two compartments. Leaks near the baffle have a much greater destructive interference than leaks that are located further away from the woofers. Usually, the best method of reducing leaks near the baffle is sealing the baffle to the metal of the vehicle with close cell tape or expandable close foam. Of the two materials, the tape is much cleaner and seals almost as well as the expandable foam. If you have large gaps between baffle and the metal of your vehicle, it is better to fill the gaps with a solid piece of material like MDF used in the baffle construction. Remember that any leaks around baffle only reduce the efficiency of the application.
Leaks at the corners of the vehicle can contribute to a major loss in low frequency output. The easiest but not the neatest way to seal these areas is to direct expandable close cell foam into these areas. There are two much different types of expandable foam: pre-mixed cans and two-part mix. Pre-mixed cans are ready to use and are available at most local hardware stores. To use, shake the can then hold it upside down and squirt the material into the desired location. The foam is relatively easy to direct but takes a long time to cure (24-48 hours).
The other type of expandable is a two-part mix. This type of foam is generally more expensive and difficult to use. The advantages this type of foam has are a much faster curing time (it can usually be cut and sanded in 30 minutes) and its greater expandability when compared to the can type of foam. Depending on the situation, one type will be better suited than the other will.
WARNING !!
A word to the wise about expandable foam: the foam is not easily removed. It does not come off skin, it wears off skin. If you get it in your hair it is easier to cut your hair than remove the foam. You have been warned!
Working with the foam can be a very interesting experience. It is very important to properly prepare an area for using expandable foam. One you experience working with foam, you will know what I mean. Many Installers have already discovered this feature. Tape off areas that you want to stay clean. Do not ever assume the foam cannot possibly go there: it does. The foam is very difficult to remove one it attaches itself to a surface. The best way to remove unwanted foam is to wait until it dries. Also invest in a box of disposable gloves for this process. The price of the gloves is well worth it.
Finally, make sure the baffle you are mounting the woofers to is very strong. Overkill is acceptable as the greater the structural integrity of the baffle, the better the performance of the woofers. A flimsy baffle will reduce the effectiveness and color the woofer’s sound quality with degraded transient performance and unwanted baffle noises and vibrations.
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