As a general rule I try to remember that buying products based solely on name is a very bad idea. Examples: "Fun Size" anything, "As Seen On TV" anything, Doritos.
I ignored that rule as I was scrolling through Adafruit's product list a couple weeks ago, however, and as a result two bone transducers and an amp board showed up in my mailbox a week later. The "bone" part of the title is completely unnecessary, it's to drive home the point that you can put the things up against your skull and it will turn your skull into a speaker so you can hear things inside your head.
I mean, it worked, obviously, but it's completely unnecessary.
Transducers are essentially speakers with a slight modification. Instead of moving a magnet back and forth attached to a paper cone, as speakers do, transducers cause the magnet to contract and expand; thus the transducer resonates, and when held up against another surface (including jawbones/skulls) the surface resonates along with it, becoming a larger speaker. You can buy smaller ones, like the bone transducers, or bigger ones, like these suckers, that can deliver a punch. They require slightly more power than conventional speakers, but that isn't too much of an issue, because as mentioned in the Adafruit tutorial on which I based my work, an amp board (I used this one) nicely boosts the sound to normal levels.
So in a few days I had two transducers, the amp board, and a stereo jack sitting on my desk, along with the built-up anticipation that came from my being, at heart, seven years old with regards to electronics gimmicks.
Part 1: Prototyping
In its simplest form, the two transducers can simply be hooked into the stereo jack, just like headphones. Because they're quieter, though, the volume really needs to be cranked. I properly wired up the amp board and used a microUSB breakout board I had previously to power it with a portable charging battery (the 10000mAh battery I got for $20 on Amazon that I'm still conviced uses magic technology). Surprisingly, it worked, and it worked even better once I remembered the gain jumpers that could be soldered on. I consider it sort of the "bonus level" of the amp board. Because I'm immature. Once it was all protoboard-ed together, I had a nice little set up that let me place my head very close to my desk, hit play on whatever the audio source was, and then quickly hold the two transducers to the sides of my head to listen in. Not an optimal setup, but it worked. Time to make it a little more permanent.
Part 2: Assembly (or, How Not to Solder/Hot Glue Things)
There are a few existing designs for transducer headsets. One has the transducers placed in the seam of a hat, so that they're constantly pressed against the upper earbone; another has an over-the-top band, resembling a hairband; a third has a design that runs around the back of the head and rests on the ears. I opted for the last design, mostly because I wanted something difficult to see from the front or top. With my roommate's kind sacrifice of a wire coat hanger, I managed to bend out a shape that fit my head reasonably well and kept pressure on the parts that rested just in front of both ears (where the transducers would come in contact with the skull). I taped the transducers on (in retrospect, they're the one thing I should have used hot glue on but didn't out of desire for readjustments) to test them out, and they worked rather well.!(/content/images/2015/11/proto_frame.jpg)
Next came the assembly of the actual components I had. I'm going to preface this by saying I am not at all proud of what I had to do, and I hope nobody ever has to resort to the same tactics, but I did what I had to out of desperation, and I admit that there's a good chance I'll do the same thing again in the future if I have to. So without hesitation, I give you: the Frankenboard.!(/content/images/2015/11/bad_solder.jpg)
The stereo jack is hot glued upside-down to the top of the amp board, and the microUSB breakout is just hot glued on to the side--the reason it's at an odd angle is because for structural integrity I hooked the top of one of the headers into the screw hole on the amp board. I know. I said I wasn't proud of it. Then I did some of the sketchiest soldering I ever hope to do, mostly wires to pins, which, while certainly not ideal, was a hell of a lot easier than desoldering the headers. I doubt I'll ever be able to desolder something without either burning myself or the component in question. But in some strange way, I am proud of it, because to my intense shock and surprise, when I hooked up some audio and the transducers...it worked. Despite soldering that would make an electrical engineer vomit, I made a relatively compact little unit that took in power and audio and spit out...well, slightly louder audio, but it worked.
For my final trick, I simply wrapped the transducer leads around the frame to make them more neat and hot glued the entire amp assembly to the back of the frame. I was pleasantly surprised by the fact that the header pins aren't stabbing me in the back of the next when I wear the headset, and it's ever so slightly cumbersome to have the power and audio leads wrapped around my body to their respective sources, but they work darn well for a hack job of a project. And I think they look pretty cyberpunky, and I like that.
There are a few tradeoffs with these things. First, because they work based on solid contact, if they aren't tight enough or your head is angled the wrong way they can sound a little buzzy. They're inherently slightly buzzy in a tactile sense--I mean, they generate sound through resonance/vibration--but you can tell when louder audio clips and one of the transducers isn't quite in place. This would primarily be solved by implanting and actually screwing the things into the surrounding bone (see below) but I'm a little too squeamish to cut things open near my ears and drill into my skull. I'm probably not going to be on the front lines of biohacking. Also interesting was how different sound quality was when the location of the transducers changed slightly--there's a very different timbre when they're placed in front of the hear than behind the ear or on the jawbone or the temple. For me, sound seemed clearest in front of the ear; the closer to the ear, the louder the sound. Placing them behind my ears seemed slightly echoey, but others said that was actually better. It's preference, really. These were also transducers taped to a coat hanger, so considering everything, I think the sound quality is quite good. There's very little distortion or clipping on either end of the spectrum. It takes some getting used to the realization that you can listen to audio without your normal sense of hearing being handicapped. It's like having a speaker in the room that nobody else can hear.!(/content/images/2015/11/finished_1.jpg)
Part 3: Future Plans
There are a few improvements I already plan on making at some point. One is getting a battery that doesn't weigh a pound. Adafruit stocks a ton of rechargable LiPo batteries that can more than power the headset for a while, are fairly inexpensive, and are small enough to be mounted onboard without significant pulling on the headset. At some point I'll pick one up and lay to rest the tenuous connection holding the microUSB breakout on the assembly. Another upgrade would be a Bluetooth chip (I think from Sparkfun this time) that would allow me to ditch the stereo jack as well, completing a reasonably lightweight, portable, wireless headset. I'm ultimately excited for the day when these things are regularly implanted subdermally. I believe implanted transducers are already used for hearing loss patients--tack on a biocompatible rechargable battery and some inductive charging loops along with the Bluetooth chip, and you've got a fully functional headphone set that is under the skin, can be heard only by you, and doesn't block normal hearing. The future's so bright I need shades.
I'm also tempted to make and sell these for others, but we'll see. It's a little difficult due to how personal the shape of the frame must be, but it's a possibility. Thanks for reading, and you can look forward to other ridiculous projects that I pick up and run with!