Well, a 2 X 4 is certainly an unexpected place for sound, as necessity is the mother of invention this passive speaker, for me, is just that. While preparing for my day I generally listen to Pandora on my iPhone. The problem with that is, if you don't put it in the sink, you can't hear the music. After searching for some blue tooth speakers I came across a "passive" speaker amplifier. it requires no power, is small, light, and compact. There are several similar devices and designs found on the web, and with that decided it would be easy enough to make and could also make some pretty cool gifts at the same time.
Step 1: The Prototype
The prototype started without a plan, and nothing but an iPhone. Initially a 2" thick piece of hardwood was used. Using a Drill press and a router I was able to create the proto type. This speaker can be built using nothing but a hand held drill but there will be some cleanup afterwards. The rough dimensions are shown in a few of the photos attached however, I have uploaded the Sketchup file for your convenience.
Start with your dimensionally cut piece of 2X4, Initially route the top slot for the iPhone to sit in. Doing this first gives you more "meat" to hang on to and reduces the amount of tearout. It is strongly recommended that you set stops for the beginning and end of your cut with the router as you will not be able to accurately judge the distance of the cut. You can see in the photos that I had that issue several times before installing fence stops on the router.
Next, the larger of the two "speaker" holes and then the smaller diameter hole cut to within 1/4" of the back of the speaker. I used large (cheap) forstner bits from Harbor Freight that worked well.
Finally bore a 1/2" hole from the iPhone slot to the smaller diameter hole in the center. You may have to angle your drill slightly to line up the holes but its a pretty easy process.
Step 2: Let the Production Begin
Now with a working prototype and some surprising volume from the unit I proceeded to make several from a 2X4 I had lying around. I did make a few adjustments in a couple of the dimensions to make the device more stable but all in all it still follows the initial design.
As always I would caution safety, when working with large tools and small pieces of wood there can be catastrophic effects.