Lots of flexibility here. I wanted a three way design, with a woofer, mid, and tweeter. You could obviously simplify things and make a one way earpiece, and at $25-$50 per driver, that would save you quite a bit of money. However, if you're going to the trouble of making yourself custom IEMs
you might as well go the distance. The drivers I used were based on a few things: drivers known to be used in commercial products, frequency response curves of the various KA drivers, and availability. I went with CI (~$27) as my woofer (this is a common application), DTEC (~$48) as my mid, and WBFK (~$48) as my tweeter. TWFK (~$50) is commonly used as both a mid and a tweeter, and has the added benefit of being a dual driver, but it was unavailable at the time. I chose DTEC because of its frequency response and because it's also a dual driver. WBFK is basically the single driver version of TWFK, and has a nearly identical frequency response. The capacitor acts as high pass filter (blocking low frequencies) for the tweeter, and the resistor a low pass filter (blocking high frequencies) for the woofer (there's more about this back on page 1). Additional shaping of the output is achieved via the acoustic dampers.
Through digging around on the Knowles site, it appears that TWFK is not simply two WBFK drivers sandwiched together. It consists of one WBFK driver and another FK series driver, and they claim that it's internally crossed-over, which means that's it's a mid AND tweeter in one package. And since TWFK is the same price as WBFK, that makes it a pretty good deal. It also means you can make a two driver earpiece with TWFK and CI and still have a three way; this is the setup I would recommend.
First, using some glue (I used an epoxy) attach (and fully seal) a length of acoustic tubing to the driver (about two inches; be sure that your glue doesn't actually get in the tubing, above the output of the driver). Ultimately, you want to woofer to have the longest length of tubing, and the tweeter to have to shortest, but don't worry about this right now. Just give yourself extra to play with later. You can pop in the acoustic dampers if you want (green in the woofer, yellows in the mid and tweeter) or wait till later. Next, solder leads onto all of your drivers. In some of my photos you'll see that at first I went to the trouble of insulating all the connections to each driver, only to surreptitiously discover that, for some reason unbeknownst to me, it doesn't matter. So what I later did was use the capacitor as a lead for the tweeter, a resistor as a lead for the woofer, and for all other leads just a single strand of copper wire (you can kind of see this in the first photo). I think you want the caps and resistors to be used as anodes, which is what I did, but I'm not an electrical engineer, so if someone knows more about this please correct me. Next, join all of your anodes together and solder into one of the female connectors, and then do the same for the cathodes. You have the option here of bypassing the connectors and just soldering the connecting wire directly to the leads (I don't know why, by IEMs always have connectors, so I followed suit). After the leads are soldered to the connectors you have to "seal" the connectors with shrink tubing (female connectors are hollow, and when you pour your casting resin over the internals they will fill up if they aren't sealed). Again, ignoring that my connectors are insulated in some of the photos, you should end up with something that looks like picture 3. If you've made your own connecting wire, you should be all set. If you've cannibalized one, or bought a Westone cable, you'll have to make the male connectors that corresponde to the female connectors in the IEMs, so solder the male connecters to the ends of the wires and fashion some kind of termination. Next, test 'em out. Plug the connectors into your connecting wire and crank up your iPod. Test that each driver is working by blocking the tubes from the other two. If you want to actually test them out in your ears you can take some foam earplugs, fillet them, insert the tubes, and then carefully insert the earplug into your ear. Keep in mind the bass might not be that impressive if you don't get a good seal. Once you're happy everything is working you'll need to fit the internals into the molds. Before you can do this, you'll have to make a hole in the very bottom of each mold, at the tip of the "ear canal" for your tubes to come through, a la photo 5. The two easiest ways to do this are 1) go back in time and position your impression in the shot glass with the end of the ear canal right next the to edge of the glass so it's easily located, or 2) take a bread knife and cut off the bottom of the mold such that you just barely expose the end of the canal (I chose option 1, minus the time travel). Once you have your hole you can feed the tubing through and then situate your internals, keeping in mind that you want the acoustic tubing from the woofer to be the longest (about two inches from where the opening will be at the tip of protrusion into the ear canal to where the driver sits in the earpiece), the followed by the midrange (about an inch), with the tubing to the tweeter to be the shortest (about half an inch). Now, flip your molds upside-down (like photo 5) and seal around
the tubing (the spaces between the tubing and the mold) with some epoxy. IT MUST BE SEALED or it will leak when you pour your resin in the molds.
A note on the SmoothOn Crystal Clear:
I wanted a clear earpiece so I could admire my work when I was done. However, there aren't a ton of products out there that are optically clear. What the pros use is an acrylic compound that is actually cured by exposure to UV light. Acrylic is completely, 100% inert to the human body; it's used to make medical device implants precisely for this reason. The SmoothOn Crystal Clear is not acrylic, and is not inert to the human body. It's a urethane plastic, and urethanes contain chemicals called aliphatic isocyanates, which are potent antigens. I learned this the hard way when, after a week of using these for the first time, ended up in my doctor's office because my eyes and ears were swollen shut. What I ended up doing was coating them with nail polish, which has worked fine, no more itchy ears. But if you don't want your IEMs to be clear, find something else to use so that you don't have to deal with this. If you decide to work with the Crystal Clear be extremely careful: use chemical resistant gloves and a respirator when dealing with the liquids, when handling them after they've cured (ESPECIALLY if you need to sand/smooth/shape them), and until you've coated them with something inert (like nail polish).
I've found a source of crystal clear liquid acrylic casting resin: Electron Microscopy Sciences.
Now then, time to take the plunge. With your internals in the molds, and the tubing sealed at the bottom, connect your male connectors from your connecting wire into your female connectors like in picture 6, and using the helping hands, position the connectors where you'd like them to be (the are typically located where I positioned them). Once you have everything the way you want it, pour in the material you're using to make the body. In the last photo you'll see what the result should look like.
I've included a recommended wiring diagram for a two way design from Sonion.