Your Audiology Tutorial: Bone Conduction

The conclusion (usually) of a comprehensive audiometric battery is the bone conduction test.  It is another way to measure hearing sensitivity.  Rather than present pure tones via air conduction through foam inserts or over the ear headphones, the tester will transmit them via an oscillator over the mastoid bone behind the pinna (cartilaginous outer ear).  An efficient method.  Any student of acoustics can tell you that solid matter is a better way for sound to travel.  Air is subject to dampening due to temperature, humidity, etc.  

The oscillator may also be placed on the forehead, though this is a less common. 

Air conduction testing requires the tone to travel through the outer ear, eardrum, and middle ear space on its way to the cochlea.  If a patient has earwax occlusion, fluid in the ear, or numerous other ear abnormalities, the signal is compromised.  Bone conduction testing is thus very useful to determine if a hearing loss is conductive (outer/middle ear) or sensorineural (inner ear).  

Audio test results for someone who has a fluid build up behind the ear drum (confirmed with tympanometry) will reveal better bone thresholds than air.  This is because during BC the tones bypass the problem area - the fluid filled middle ear.   Such results may also be seen in patients with otoschlerosis, a condition where the three middle ear bones attached to the eardrum become fused. 

There are bone conduction headphones marketed for everyday use.  You can listen to music, news, podcasts, whatever through them via a Bluetooth connection to your phone or pad..  Some folks like them because it keeps the outer ear open, especially useful when you're outdoors, surrounded by potential hazards.