Bone Conduction Headphones

The following tutorial will teach you how to make bone conduction headphones which transmit sound via the bones of the skull.

-2 bone conduction transducers (dayton audio bone conduction transducers) https://www.amazon.co.uk/gp/product/B00HFG6AZG/ref...

-1 male to male 3.5 audiojack https://www.amazon.co.uk/gp/product/B00C76RGPC/ref...

-1 PAM8403 dual audio amplifier https://www.amazon.co.uk/gp/product/B08MVKTSVN/ref...

-stripboard

-4 wire cable

-2 wire cable

-single solid core wires (for circuit connections)

-Polycaprolactone (low melting point polymer) https://www.amazon.co.uk/mouldable-colouring-polym...

-power source (3-5V battery/powerbank - NOT adapter connected to mains)

-1 3 ring audio socket (female) https://www.amazon.com/Tegg-PJ-307-Stereo-Headphon...

-2 2 ring (dual) audio socket (female) https://www.amazon.com/CESS-Socket-Adapter-Cables-...

-2 2 ring audio jacks (male - mono audio) https://www.amazon.co.uk/Ancable-10-Pack-3-5mm-Sol...

Tools:

standard soldering supplies

Sound is a wave that can travel through everything except a vacuum. Normally, we hear sound via air conduction as it hits the eardrum, vibrates the 3 bones in the middle ear and stimulates the hair cells in the inner ear (cochlea).

However, in certain situations, such as underwater, we hear sound via bone conduction in this case the sound travels to the inner ear directly through the bones. You can also experience this by closing you ears, placing your chin on a table and have someone tap the table.

The device consits of the amplifer board and the bone transducer circuit.

Amplifier Board:

-note the amplifier circuit layout; the bottom is for power and input signals, and the top is for the outputed amplified signals.

-The stereo audio socket (3 connections) should be connected to the bottom left minding the R,L and GND signs. This is where the male to male audio cable connects to (with the other end being plugged into your computer).

-Each individual mono audio socket should be connected to the top of the amplifier circuit. The orientation is not important although the -ve output terminal should connect to the larger/outer contact plate of the audio socket.

-Connect a suitable (socket for) power source to the bottom right of the amplifier circuit minding the polarity. The power source should produce around 3-5V. This can be a lithium ion battery, 2AA batteries or a 5V power bank. Do NOT use a power adapter connected to main to ensure total circuit isolation. Beware that any noise in the power source could be amplified and be heard when using the headphones.

Bone transducer circuit:

-solder 2 solid core wires (those used in breadboards) into the bone conduction transducers.

-Cut 1 meter of 4-wire cable (thin cable with 4 wires inside it) and asign 2 wires to each bone conduction transducers.

-at the other end, solder the 4 wires in pairs to 2 sets of 15cm 2-wire cables. Bear in mind which pair belongs to which transducer. Then solder each of the 2-wire cables to a mono audio jack (2 connections).

-Reinforce the soldered joints with PCL or a hot glue gun.

The entire setup will require one mounting device (headband) and one case (box for amplifier and battery).

The mounting device should be hand molded from PCL (see supplies) and the box can be handmolded, 3D printed or created in other ways.

Not all frequencies are evenly transmitted via bone conduction; lower frequencies will be conducted better than higher frequencies. This means that the sound will contain more bass that treble.

As such you can download an equaliser on you computer to compensate for this difference. You can also use the equalizer to adjust for any hearing loss you may have. I recommend getting equalizer APO from: https://sourceforge.net/projects/equalizerapo/.

Try out the above settings in order to equalize the audio.

Bone conduction headphones will affect different types of hearing loss differently:

For conductive hearing loss (healthy hair cells, dysfunctional sound conduction), bone conduction headphones will be very effective and give much clearer sound than regular headphones as they completely bypass the conductive components of the ear.

For sensory neural hearing loss (damaged hair cells, functional sound concdution - most common hearing loss) bone conduction headphones will sound no different compared to regular air conduction headphones as the sensory mechanism of the inner ear will not recieve the sound properly regardless of sound conduction method.

An Equalizer can also be used to compensate for certain types of hearing loss, especially with reference to an audiogram.

Extra Details:

Although bone conduction headphones do not strain the ear drum and excessive sound can be dissapated at the point of contact, hearing could still be damaged when listening at high volumes.

Bone condcution headphones do not cut off the user from the environment however using them with earplugs will acoustically isolate the user even more than regular in/over ear heaphones.