AfterShokz and TEAC Filltune HP-F100 Bone Conduction Headphones

Bone Conduction Headphones
My inbox was pummeled with press releases for the new Aftershokz Bone Conduction Headphone for weeks before CES, and these cans got a lot of attention there. Sure got my curiosity up, so I stopped by their booth for a listen, and walked away with a pair for review. But I don't know diddly about the technology, so I thought I should do a little homework before a serious evaluation. Turns out, there are quite a few very good applications for this type of transducer.

How it Works
It's pretty simple really: The small bones of your middle ear are anchored to the bones of your skull. When you put acoustic vibrations into your skull, they end up vibrating the little bones in your middle ear, and that gets transfered to the opening of your cochlea. Voila! You hear sound. Pretty straight forward.

Bone conduction headphones come in two flavors: mechanical and magnetostrictive transducers. Both are typically worn so that the transducer is pressing against the cheekbone about an inch in front of your ears. Mechanical transducers are much like dynamic speakers, and use a driver coil to magnetically move the part that touches your cheekbone to impart the vibration into the bones of your skull.

Magnitostriction is the impartation of kinetic energy into a non-ferrous material through magnetic force. Normally, we think of magnets only being able to move ferrous materials like steel, but there is another way. Many molecules have an electrical polarity. Water, for example, is an oxygen atom with two hydrogen atoms attached on either side, but because they aren't directly opposite each other, a water molecule has an electrical polarity. Because of this electrical polarity, when put in a magnetic field they will tend to align. In magnetostrictive headphones, the magnetic field is modulated with the audio signal, which torques all the molecules around, and makes a vibration in your skull that you hear.

(This is basically how MRI (Magnetis Resonance Imaging) works. Big magnets vibrate the soft tissue of the body, and because of the differing electrical polarity strength of the various soft tissue of the organs, an image contrast can be sensed. That's a brutally short explanation, but fundamentally accurate. Here's the Wiki.)

Medical Use
aftershokz_photo_bone implantFolks who have outer and middle ear problems can benefit strongly from bone conduction transducers as it bypasses the ear canal, ear drum, and to some extent, the normal workings of the middle ear. People with hearing loss associated with these parts of the ear can oft times regain some hearing with a bone conduction transducer. Here's a Wiki page on bone conduction implants for people with these types of problems.

I like that.

Under Water
aftershokz_photo_swimmp3I've not felt the need for music while swimming, but having been on the high school swim team I know how boring laps can be. Here's a an interesting product: SwimMP3 stores tracks in flash memory, and straps to your goggles to press on your cheek bones to deliver bone conducted tunes under water.

aftershokz_photo_swimbouyAnother very cool underwater application from Amphicom is a system of bouys, short range underwater FM transmitters, and bone conduction headphones to deliver narrated tours of reefs and underwater features.

Ha! That would be fun.

Tactical Communications
aftershokz_photo_tacticalThe most deadly serious application for bone conduction is in communication systems for military and law enforcement tactical teams. Here, team members have headsets that use bone conduction earpieces, and sometimes use vibration transducers pressed against the Adam's Apple. (Not shown in picture.)

The bone conduction earpieces (magnetostriction types) are pressed against the cheek bone, allowing the ear canal to be open to outside noise so team members can remain clearly aware of sounds in their environment. The magnetostrictive transducers make virtually no residual noise, and the neck microphones are able to pick up the merest whisper. Team members can communicate with each other in virtual silence allowing them to sneak around and communicate without offering any clues to their whereabouts.

Mount the amazing night vision capabilities of today (I got to experience a state-of-the-art night vision scope not long ago and was stunned at how well it worked), and equip them with silenced weapons and ... well, I wouldn't want to confront these guys in the dark of night. 'Course, you'd never know they were there til it was too late.

All very cool stuff, but how do they sound?

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Roy G Biv's picture

I dub thee, Bonephones!

ultrabike's picture

Ok, lets forget that I turned out a bit sensitive to IEM due to canal irritation.

I get my Audeos which may lack a little bass, but its there and its really nice (I actually passed some tones and could clearly hear < 30 Hz from them)... Best audio I have heard so far. Micro-phonics? No problem. Get some ear guides and the cable is actually OK. And great portability... The filters to change the sound signature... I almost pulled the trigger to get the green filter for the extra bass...

I take these mosquito-fart resolving cute-tips with me for jogging... Man I'm pump ... Out of my way ... Start jogging PHUM PHUM PHUM ... Stop ... What the ... PHUM PHUM PHUM ... Every single jogging step ... Occlusion effect.

Some say that one gets used to it, but it really ruined my cute-tips bass for jogging applications. Therefore I ended up getting a not as good, but good enough for running Koss KSC75.

Obviously not completely satisfied though. I'm still waiting for the Senns to come down in price or for hell to freeze.

briskly's picture

I like my IEM (Westone 4) when commuting, minor bass boost helps you hear the music and all. But every footfall means into occlusion when you walk around. Sealed IEMs just suck for exercise use.

No isolation means no occlusion either. Something about frequencies resonating inside your ear canal. Never ran with an ety , but I imagine it must be dreadful.

The amperior? I didn't like running with my 25-1 II very much, but that's me.

ultrabike's picture

Yes dude. I learned the hard way that IEM are not for jogging. Maybe weights, but not jogging for sure. Actually Tyll in his article has a very good suggestion IMO: Koss PortaPro. I still got the KSC75 though. Like earbuds on steroids (40mm driver w 2mm titanium coating and a Grado like frequency response - according to HeadRoom). Very good for such applications! given they are open = zero occlusion misery.

I don't mind the clips that much... but if you do then PortaPro uses headband (though not the best one)... anyhow there is a ton of KSC75 mods out there for some good reasons: Great sound for $15!


13mh13's picture

Hertsy, boy, you did it again.

ultrabike's picture

Its an ... Inner-earphone, Amphi-phone, Jar-headphone, ... and an audiophile's Bonehead-phone.

Anyway. I did find this article plenty interesting given the fact that Tyll found them very good @ certain frequency ranges and Jar-headphone applications. Teac has a white paper mentioning 25Hz - 25 kHz:

Tyll found them severely lacking bellow 600Hz though. They were also not cheap:

And it seems one can still buy them:

If however they deliver significant performance at certain frequency ranges, maybe a hybrid headphone could come out of them (crossing them to a different technology bellow 600 Hz and above X kHz...) Who knows, ... it got me thinking :)

shstrang98's picture

Funny you mention "bonephone." When reading this article for a minute there I thought I was reading a Sharper Image ad in popular science in the late 70's.
Bad flashback.

Now I must run over myself with my lawn mower.

Toronto John's picture

This article comes as close to my situation as any I've seen. Having got a pair of HiFiMan HE500's it surprised me to find the supplied cables and parts of the headphones are sensitive to vibrations from rubbing or tapping. The sound is the same as a tonearm feeding back.I've use Grado R2's and this doesn't happen. The slightest movement or outside contact is audible inside my head- it becomes a microphone. Sitting absolutely still is my only option. I love the phones but not the noise sensitivity thing. Is the noise a function of tightly sealed headphones or the supplied cables? Or does the noise travel from the phones into the Cable? Would a more shielded cable stop this? Like DiMarzio or CablePro Earcandy? I solved a lot of feedback/turntable problems over the years but this one stumps me.