About speaker boxes (enclosures) Part 1
Here we will examine what a speaker enclosure (box) does, different types, the differences, and general issues around buying or planning boxes.
Part one will briefly describe the function of a box and common types of boxes used in mobile audio (and a couple others that I just like). This will be a general overview to be followed by more in depth examinations and discussions.
First of all, why do you need a box at all? If you have ever hooked up a woofer just laying on the floor with no box, you’ll notice instantly that the cone moves wildly about and it really doesn’t produce a lot of low bass. If you’ve done a bit of experimenting or been around a while, you’ve probably also seen a woofer in a well built box producing an amazing amount of bass with very little cone movement. What’s the story here you may ask?
A woofer will produce two sound waves, a front wave and a rear wave. In order for them to not just cancel each other out, they need to be separated. The “ideal” box for perfectly linear sound (the box neither adds nor takes away anything from the woofer) is the “infinite baffle” or free air. Which is in theory a wall with sides infinitely high and wide with a woofer in the center. In practice this is often done in the home in a ceiling or a wall with an entire room or attic becoming a box. Of course with a large enough room for practical purposes it will perform the same as a true infinite wall. In cars this is often attempted by completely sealing off a trunk and mounting the woofer on the rear deck or in a wall where rear seats would be or behind rear seats.
The other important thing a box will do is controll the woofer. A sealed box or proper vented design will provide a cushion of air that will keep the cone of the woofer from moving too freely. This will help prevent unwanted cone movement and generally improve power handling. Woofers that are designed for infinite baffle applications will have either a very stiff motor structure or very high excursion or both to compensate.
Types of boxes include but are not limited to sealed, ported (also called vented), bandpass, horn loaded, and transmission line (which is just an advanced form of ported).
A sealed box is simply that. A rigid airtight box the volume of which is calculated to match the woofer based on various mechanical and electrical properties of the woofer.
A ported or vented box adds a tuned port which is carefully calculated to resonate with the woofer to provide extra output and controll cone motion (down to tuning frequency).
A bandpass box is a hibrid of ported and sealed. It uses one sealed chamber in conjunction with a ported chamber and all sound originates from the port.
Horn loaded boxes are a large version of the smaller horns most folks have seen in PA or DJ applications. The actual science of horns to this day has an element of mystery and guess and test. The science behind this is rather complex, and numerous resources exist and many books have been published on the subject for advanced readers. Though fantastic results can be had, horn loaded low frequency boxes would need to be larger than most cars to function properly so we will not discuss this further.
Transmission line boxes are a fantasic variation of the ported box. In this application the soundwave from the rear of the speaker travels through a very long port until it is inverted 180 degrees, in phase with the front wave, (normally where the woofer starts to drop in output naturally) thus re-inforcing the woofer at it’s weak spots. When done properly the sound wave at the system’s peaks will come around out of phase with the front wave to cancel out unwanted peaks in volume as well. (This is the science used in most Bose speaker systems). If done properly, this application is capable of amazing sound, but for practical purposes boxes are large (coffin sized or bigger), complex, heavy, and expensive to build and are almost never used in mobil applications.
In the upcoming parts of this series, we will examine each type in detail, discuss strengths and weaknesses, and hopefully at the end of it all, bring it all together so people can make more informed decisions when planning, shopping and building boxes for any application.
Stay tuned!
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Ive always liked bandpass enclosures. They give the deepest bass and look really nice as well.