how to design a sealed enclosure for your subwoofer
this particular how to is designed to teach you the basics of designing an enclosure for your subwoofer. a lot has changed in the last decade and computer simulation has made things much easier to predict how a speaker will perform in a given enclosure. there are a wide variety of programs out there that come highly recommended. for this how-to, i will show you how to use my favourite free program called UniBox.
download that and extract with a program like Winzip or WinRAR. the only other requirement is to have microsoft excel on your computer or a similar spreadsheet program. if you prefer a free standing program, you can download something like WinISD. now on to the how to.
for this particular how-to, i will be modelling the JL Audio 10W6V2-D4. in order to do any modelling, you will need to get a hold of some of the theile/small parameters for you speaker. this can be found often in your owner's manual or on the manufacturer's website. once opening UniBox, you will first need to enter in these parameters. i have highlighted in blue all of the boxes that must be changed in order to produce an accurate simulation of the performance.
under the column entitled "Parameters of a Single Unit", you will notice an area that says "Suggested box type". in this case, it suggests a sealed enclosure. again, i have highlighted this in blue. please note that this is a suggestion only and is not a steadfast requirement.
when scrolling down, you will notice a section entitled "Closed Box" that shows a graph to the right as well. this graph is what will tell you how you can expect your speaker and enclosure to perform. on the graph, there are two axis' to consider. the vertical axis with numbers ranging from 70.0 to 130.0 refers to the spl of the speaker. the horizontal axis with number ranging from 10-1000 refers to frequency. these two axis combine to create what is called the "response curve". please note that the main areas we will want to focus on for a subwoofer are from 30hz-70hz; most bass frequencies common in music fall in this range. anything above 70hz would generally be played by other speakers (ie. your midrange or midbass). you will notice a button that says "graphs not updated". it is important to click this whenever it says that so you will be saying the most up to date simulation.
one of the boxes to fill in here is titled "Physical Vb." this refers to the size of the enclosure and is listed in litres. here i have changed the Physical Vb to 40L (be sure to update those graphs). you will notice that the response curve has changed substantially. now you are starting to see how we figure out what size enclosure we would like. if you don't feel like tinkering, you can just go with the suggested Vb (enclosure size) suggested under the column titled "Standard Design".
there is an additional way in which you can change the response curve. you will notice there is a spot that says "Wanted Qtc" with a button below that which allows you to optimize Vb (enclosure size) for your wanted Qtc. don't get confused here; Qtc is just a fancy way of generalizing how a speaker will perform. it is generally considered that a speaker with a .707 Qtc is considered optimal. the higher the Qtc, the smaller the enclosure. if you're not sure what Qtc your enclosure is, there is an area on the right where the Qtc of your enclosure is listed in red. i have highlighted in blue the area where you can change the Qtc.
one of the most important things to consider is the effect of what is called "cabin gain". generally speaking, around the 50-60hz mark, a subwoofer and enclosure installed in a vehicle will begin to experience a rise in the response curve as it moves down to the lower frequencies because the waves at this point become longer than the cabin of the vehicle. this particular customer wanted the flattest response curve possible (what some would call good "sq"). to achieve this, i had to consider that cabin gain would cause the response curve to begin to increase around the 50-60hz area and therefore, go with an enclosure whose response would counteract this by decreasing around the 50-60hz area. after some tinkering, the final enclosure i decided to go with was the following. you will notice it is approximately 16L large, has a Qtc of 0.703, and is very similar in size to what JL Audio has recommended in their owner's manual.
this can be done with any subwoofer in a fairly short amount of time. i will get into the basics of ported enclosure design another day. if you have any questions, find any inaccuracies, or any questions at all, please feel free to pm me anytime.