A free air subwoofer is a special type of subwoofer meant for car audio which uses your trunk as the resonating chamber. Most car audio subwoofers are meant to go in a box of some type. There are various types including ported boxes, bandpass boxes and sealed boxes. These can sound great and be very power efficient, but they can also take up a lot of room in your trunk. A sub system uses free air subwoofers can use the enclosed trunk space itself as a resonating chamber and give you great bass without taking up a lot of room. Instead of being mounted in a box, these subs are mounted on a board or deck on top or in front of the trunk.
For maximum effectiveness, it’s important that your trunk is sealed virtually air tight. For this reason, they won’t work well with hatchback or SUV vehicles where the the trunk is not sealed. Also, they need more power for the same volume so you may want to buy a higher powered car amplifier than you might have used with a subwoofer box in the same situation. These subwoofers can be slightly less clear and defined than a sealed box system, but then again you also retain the ability to use your trunk for groceries and bags! If you are looking for punchier or more accurate bass, a subwoofer enclosures system is probably your best choice.
Another advantage of a free air subwoofer system are that they are often cheaper and easier to install than comparable systems with a sub enclosure. It’s sometimes also known as an infinite baffle subwoofer. Be aware that not all subs are designed for use in free air systems – be sure to buy one that is meant for this specific car audio purpose.