EDIT 07/18/17: This post has been getting a lot of hits lately. Since I don’t have comments enabled for my site, if you have questions or would like to comment on anything, please feel free to email me directly. My email can be found on my CV (I won’t post it here so the spam bots don’t eat me!)
I discovered something fascinating while browsing around the other day: headphones that transmit sound directly to your skull. This method of sound transfer has been dubbed bone conduction. All you do is press the little transducers up to your temple, jaw, or skull, and the vibrations in the little electrical device transfer to the waves through the solid bone medium to your inner ear. This way you can listen to things without blocking your ears with big cans or buds. Rather than go out and purchase one of the little premade units, I decided to make my own DIY bone conduction headphones.
I have my own issues with headphones. I am the type who strongly prefers earbuds. Sure, you can get better sound out of headphones, but I find that the large strap and bulky padding smashes my ears and glasses together in an uncomfortable way. Furthermore, the shape of my skull with causes the applied pressure of the headphones to pull my glasses out of alignment. That puts pressure on one side of my nose or the other, and it makes my vision all out of alignment. To make things just that much worse, my very fine hair is easily molded, a quality that makes it easy to get ready in the morning but causes instant hat hair. Headphone bands give me this weird wave in my perfect, voluminous follicle coif. If you look at the bone conductivity headphones available on the market, they all use a strap to keep them in place like a normal pair of headphones. With the extra pressure required to push the transducers up to my jaw, I can only imagine that they would have even more issues.
So I thought to myself: how can I make a set of DIY bone conduction headphones with properties closer to earbuds?
Answer: use the straps that you wear every day, your glasses.
The Build
For this build, I purchased equipment from Adafruit which was very similar to the stuff recommended in the Ruiz Brothers build. I used
I followed the build similar to the Ruiz Brothers instructions linked above with substitutions made for the parts I sourced elsewhere, such as the battery pack.
I glued the completed breadboard to the battery pack and called it a success. My first tests (using the transducers on my apartment’s door to introduce my partying neighbors to Norwegian black metal) proved the build to be a success.
For future modifications to this, I plan on soldering a jumper on the gain pins instead of using the removable jumper. I also would wire the headphones directly to the board rather than using the European headers included with the breakout board.
The Buds
As you can see in the picture of the build, there are these strange brown boxes wired to the device. These are the containers for the bone conduction transducers. These were designed in Fusion360 to fit the transducers quite snugly. The Adafruit page offers a technical drawing of the transducer, but I found that my units were slightly off of this specification. I wanted the transducers to fit in the housings without much room to rattle and lose volume and quality, so I went with my measurements.
The hooks on the end of each housing are designed to fit onto the earpieces of my glasses. The plastic glasses I wear have a slightly tapered profile. This means with a little push of the parts onto the earpieces, they stay in place without sliding off. The inside of each of the hooks on the housing are slanted, e.g. one end is more open than the other. This means that the housings are not interchangeable, and fit on either the left or the right side of the glasses. The cover of the housing fits into the remaining room around the transducer and these can be glued into place. The slit at the bottom provides enough room for my cheap wires to go through or a bit bigger gauge if you are concerned about sound quality.
The housings were printed on a Makerbot 5th Gen in dark brown PLA filament. The print was scaled at 102.4% from the actual design size. This scaling was based on a previous calibration curve of the printer PLA shrinkage and print accuracy. Like I said, I wanted the tightest tolerance the little 3D printed parts could manage.
The little widget below is clickable, and it will let you move the 3D object around to see for yourself how it is designed.
This is the “left” container.
This is the cover which fits over both the “left” and “right” versions of the above model.
Just want to try the parts out for yourself? The three .stl files you will want to download are below.
bonecond1.stl
bonecond2.stl
bonecond3.stl
The Fit
With my little DIY bone conduction headphones complete, it is time to try them out.
Look at that handsome fellow in the picture. The transducer housings fit as expected on my glasses. The sound transferred well to my skull. If you have ever used bone conducting transducers before, think of it like a medium press sound transfer rather than a super hard press. The brown color is still stands out a bit from the color of my glasses and my hair. Finishing paint would be necessary to make it look nicer.
The sound quality was pretty good as far as these little transducers can go. They won’t be replacing my in-ear monitors any time soon, but they are sufficient to listen to spoken word audio.
Bonus trial: my mother is partially deaf due to Meniere’s disease. She tried my DIY bone conduction headphones and was able to hear things on better on her deaf side. Further development of this technology may prove to be useful for sufferers of mild hearing loss who do not desire the full hearing aid.
