All you need is an existing cheap pair of protective headphones, and a scrap set of audio headphones. I used a pair I got from Jet Blue, but any old pair you might have around will do. You can wire the speakers into your headphones and your good to go. The basic conversion is trivial, but I include some tips/photos about making it look a bit more like a professional set.
Let me put a disclaimer in here. You're modifying the protective headphones, so they may be slightly less good at hearing protection after this. There are some small holes for wires passing though the foam/outer shell. So this might not be a good thing to do for headphones used for super high noise levels like shooting ranges or lots of work with jet engines. However they still seem to work very well for table saw/weed whacker level noise.
The second warning is that this adds a dangling cord to your headphones. (Which might get pulled into machinery, etc.) I considered putting an 1/8" jack into the headphones and only using a patch cord to wire them into my iPod so I could have the wire out for normal use, but I discovered:
1) super cheap/free headphone wires are amazingly weak, so the cable is unlikely
to pull me in it'll just break.
2) I can just have different protective phones around for when I do operations with
more danger of being pulled in.
So I decided not to bother with the 1/8" jack since that was going to be more damage to the foam inside the headphones.
Alright. Enough with the disclaimers. Lets get on to making the headphones.
Step 1: Find Some Free Headphones and Tear The Up
Step 2: Grommets Make it Look Fancy (and protect the wires)
To drill the plastic of the headphones first drill a very small pilot hole where you want the full sized hole to go, and then drill a full sized hole. Use a drill bit the size of the waste of the smallest grommets in the radio shack bag. (I think about a #12 drill bit)
Once you've drilled the full size holes, you can make putting the grommet in easier by reaming the hole a little by hand with a great big drill bit (or use a 60 deg deburring bit, etc)
Then you can push the grommet into the hole, and poke at it with a little flat bladed screwdriver until it sitting with one half on the inside and one half on the outside. Then when you're finally ready to wire you can just push the wires though the center hole of the grommet.
You can use a bit of dish soap if they're a tight fit.
Step 3: Running a Wire From One Headphone to the Other
This turned out to be by far the hardest part of the project. But only because I decided to make it hard on myself. If you have headphones like the 3M ones that just have a solid plastic band that goes from one ear to the other I recommend just drilling some 1/8" holes along the band, and snaking the wire up/down though those holes to keep it out of the way and tidy.
The headphones I had were more complicated. They had a two springy steel wires that went between the sides, with a padded plastic band at the top for your head. The wires passed though this thing, and I decided to pass my wire though as well. This turned into about 30 mins of soaped wire and deep tissue massage to get that wire to behave and come out the other side.
It looks nice, but really was not worth it. If you have a small stainless wire or something you could thread that through, and use it to pull the shielded wire though that might work better, but really it wasn't worth it. I probably should have just converted my 3M headphones instead, a few drilled holes and some threading would have been much less frustrating.
Don't be tempted to use gaffers tape or something for this step since your hair is up there and you really don't want something sticky providing depilatory services while you're wearing these things. Also remember to leave enough slack in the wires so they can be adjusted to their biggest setting and also have the ear pieces tilted out to their normal position when in use.
Step 4: Solder the Wires and You're Done
Here you can see the 1/8" shielded wire just before I tinned it and soldered it to the speaker. The normal wires for the headphones are TINY. They each of 1 strand of hair like wire that has the signal, and a small set of bare strands that are the shield.
So the wiring goes something like this:
One headphone has 1 grommeted hole at the top, the other has one at the top and bottom. The two top holes are where the 1/8" wire passes between the two sides. The extra hole at the bottom is where the last dual wire from the original audio phones goes out to the 1/8" stereo jack and on to your audio device.
When you push the 1/8" shielded wire though the upper grommeted hole, poke it through the foam on the inside so it comes out in the "main chamber" of the headphone. You might need to use a nail or a piece of thick wire to do this.
Once you pass the dual wire though that bottom grommeted hole, tie a knot in it to keep it from easily pulling out. When I went to pull the knot tight I broke the wire! Crazy. Those tiny copper wires + little bit of plastic means this dual wire is pretty weak. This provides a measure of safety, but also means they're not going to hold up for ever. So pull the knot tight very gingerly. Then pass it though the foam into the "main chamber" just like the other wires.
Split/strip the dual wire into two sets. One with the tiny red wire and shield wires, and the other with the tiny blue wire and shield.
Solder one of these to the 1/8" shielded wire to travel over to the other speaker. I did this and the electrical taped the two wire together and put a 1/2" of heat shrink tubing over the whole thing to make sure there wouldn't be any strain on those hair like wire.
When soldering to the speakers be aware that there are 2 tiny hair like wires going from those pads to the coil of the speaker itself. Make sure they stay submerged in solder. When I when to test my wiring one of the speakers was dead and I tracked it down to one of those wires having popped out of the pool of solder while I was soldering down the big 1/8" shielded wire.
The other side of the original dual wire can go directly to the speaker on that side. You can tin the single hairlike signal wire to make sure you've burned of the insulation that's on it. Be quick when you solder the shield wires so they don't overhead the insulation on the signal wire and short to that. If those wires pass though the air for a bit the part that's actually touching the signal wire stays cooler, so make sure they aren't soldered up super close to the other wires.
Once you have things wired, but before you stuff it all into the safey headphones it's nice to test things out. If you have a meter you should read something like 30 Ohms between the shield on the 1/8" jack and the tip/first ring (which go to each of the speakers).
If you measure between the tip and first ring it should read double what the other reading was.
If you don't have a meter you can always hook up an audio device and see if you hear something coming from each speaker.
Once you've got the speakers bedded down in the foam enough that they don't press on your ears, you should be ready to enjoy listening to music/audio books in a noise environment.
My headphones are a bit soft. I now have to crank the volume a bit higher then normal to get a normal listening volume. I think that's because they're not pressed right against my ears any more. Still they work well and because of the level of noise isolation the safety phones give me the whole thing works a treat. Enjoy!