Bone Conduction Headphones
Social Media these days seems to be full of companies launching earphones that play music 'via the magic of bone conduction' and often pitch them as this meaning you cannot damage your hearing from the music heard via them, usually backed up by some very woolly and often incorrect science.
How you hear noise
There is only one way you hear something and that doesn't change no matter how the sound is presented to you. You hear via vibrations stimulating the tiny sensory cells in the cochlea and these cells trigger nerve impulses which the brain then interprets as a sound. There is no other way to hear - there is no way to by-pass the cochlea and no way to hear anything at all other than via physical vibrations of the cells. You cannot send a vibrated sound 'directly to the brain' or 'to the nerves' as they have no means of receiving those vibrations and turning them into a signal the brain can recognise.
Bone conduction is part of the way we hear every day, albeit a minor part, and is just the vibrations of the sound passing through your skull to the cochlea rather than reaching the cochlea via the vibrations passing through the air and into the ear. The two methods meet at the cochlea.
This is an important principle - with bone conduction headphones, the way the noise gets to the cochlea is different but you are still receiving that noise at the cochlea in exactly the same way as noise coming via air transmission. And as the noise from either way ends up at the same place, the damage caused by too much noise is exactly the same.
Damage to hearing via bone conduction
There is a long and inglorious history of many people damaging their hearing by bone conduction, going back decades, and this is among engineering staff in factories. It was common, and occasionally still is, for engineers to try and diagnose a problem in a motor by listening to it. What they would do is place a screwdriver against the housing of the motor and place their skull against the other side, just above or behind the ear. That would transmit the sounds from inside the motor directly to their ears via bone conduction, often enabling them to work out what was wrong within the motor without stripping it down.
This is not without consequence however and resulted in thousands of people suffering hearing damage as a result of the bone conduction vibrations being passed directly to their cochlea from the motors. In a hearing test with someone exposed to noise you often see a dip in the results around the 6,000Hz frequency range, whereas with engineers who have been using this screwdriver trick you would see the drop in hearing ability to be focused more around the 4,000Hz range. I have done tens of thousands of hearing tests over the years and have seen this effect myself many times and it is still reasonably common to see even today.
Bone conduction can encourage excess noise
One of the small benefits of in-ear headphones is that they can reduce external noises, even if only by a little. Even the cheapest in-ear headphones will stop a small amount of external noise getting into the ear which is better than none at all. Why is this important? Because for most people, the volume they set the music to is determined by how well they can hear the music over other background noises and the louder the background noise, the more they will turn the music up to drown it out. This more often than not, even for very modest background noise levels, means the in-ear volume of the music is well into the danger areas for causing hearing loss.
With bone conduction headphones, there is nothing stopping the background noise entering the ears so the wearer hears that volume exactly as normal. That means they have to crank the bone conduction headphones up louder to hear the music clearly over the uninterrupted background noise, and as bone conduction is just stimulating the cochlea in the same was as normal headphones this means more sound in the form of vibration is still damaging the sensory cells.
This means the attendant risk of hearing damage is still there with bone conduction headphones, or it could be argued is even more likely than with in-ear headphones which are blocking some of the background noise.
Bone conduction headphone summary
There is no doubt that bone conduction headphones work - you can hear perfectly well via bone conduction (but with less clarity than regular headphones), but they are not safer than in-ear or over-ear headphones, do not reduce or eliminate the risk of hearing damage, can still damage hearing in just the same way as regular headphones, and offer no safety benefits to most people. Any company which is claiming bone conduction headphones somehow eliminate the risk of hearing damage is using nonsense-science to sell a myth.
Away from these occasional spurious health claims they are not a bad product at all and have some good use-cases which some of the more reputable suppliers base their sales on:
Some people who hate the pressure of in-hear headphones or the heat of over-hear headphones may find bone conduction ones to be a great solution, but that is about comfort and nothing to do with safety.
They may also be good for motorcyclists and also lycra-clad pedal bikers who would be able to listen to music with nothing blocking their ears so enabling them to be more aware of other traffic around them.
People with outer or middle-ear problems such as otosclerosis may benefit as the bone conduction headphones by-pass the outer and middle ear areas completely and send the sound straight to the cochlea.
Unfortunately these good use-cases are often buried in the nonsense that they cannot damage hearing or somehow present the sound in a way which is safer than regular headphones. Sound is sound and how you hear it is exactly the same no matter what the transmission path is.