More than just a box

Posted in FYI on April 11, 2013 by Adam
More than just a box

“I need a box for this sub” is one of the more common statements we hear from people coming in to Santa Fe Auto Sound.  While this sounds like a simple enough request, there is quite a bit more that goes into choosing the correct box, or “enclosure” for a particular subwoofer.

While many people realize they need an enclosure for their woofer, not many realize the differences between enclosure types and their performance characteristics.  Here I will briefly go over enclosure types and how to pick the correct one for your application.

There are three basic enclosure types found in car audio.  Those box types are sealed, ported and bandpass.  Because bandpass boxes are extremely difficult to design, construct and implement correctly in the automotive environment I’m going to focus on the other two. (However, if you are interested in bandpass enclosure design, JL Audio gives a nice primer on the subject here)

Sealed enclosures (or acoustic suspension/air suspension) have been around since the 1940s.  They have a fixed volume of air that acts as an air spring, which interacts with the subwoofer to help control the movement of the speaker.  As the volume of air in the enclosure changes, the effect the “spring” has on the subwoofer changes as well.  Sounds simple, right?  It is. Sealed enclosures are the simplest to design, construct and are quite forgiving if slight errors or miscalculations are made.  Sealed enclosures are usually small, have extremely good transient response (allow the subwoofer to respond quickly), allow for higher power handling, but are relatively inefficient.

Ported, or “vented” enclosures are like sealed enclosures with a hole in them.  However, this is not just any hole.  The hole is actually a port or duct containing specific volume of air coupled to the volume of air in the enclosure.  As the subwoofer interacts with the air in the enclosure, the enclosure in turn interacts with the air in the port.  If designed correctly, the air moving in the port combines with the air being moved by the subwoofer cone itself to re-enforce the low frequency (sub-bass) response.  There is a delicate balance, however, between the characteristics of the subwoofer, the volume of air in the enclosure, and the volume of air in the port.  So no, you can’t just slap a random tube in a sealed enclosure and have the result sound good.  Ported enclosures are larger than their sealed counterparts, have good transient response, have very low distortion around the tuning frequency, and are more efficient than sealed enclosures. 

Below is a cross section view of  JL Audio H.O. 10W7 ported enclosure

JL Audio high output 10W7 enclosure

How do you pick the correct enclosure?  Different speakers behave differently in different types of enclosures. Sounds obvious, right?  One way to see how a woofer will behave is to look at the Thiele/Small Parameters of that woofer.  These are basically measurements of the speaker that predict how it will interact with a particular enclosure.  If this seems really elaborate and full of difficult math, well, it is.  (If you are interested in Thiele/Small Parameters, here is a good place to start.)  Luckily, most subwoofer manufacturers have taken the guesswork out and designed enclosures that work well with their woofers.  Some even build these enclosures and sell them loaded with a woofer that works really well in them.  Convenient, eh?

At Santa Fe Auto Sound we are also able to design and build enclosures for vehicles that may not be accommodating to pre-fabricated enclosures.  Check out our gallery for some examples of different enclosures we have built.

Like with any speaker the best way to find out what you like is to have a listen for yourself.  We have a wide selection of subwoofers loaded in proper enclosures for you to audition.  We encourage you to come by the store and listen to not a “box” but a true “subwoofer system” and hear how much better your car can sound.