Sound typically travels through the air to our ears. While this pathway is well-known, sound also reaches our inner ear through the bones of the skull. This article explores this alternative route of sound transmission.
Sound’s Usual Journey and Beyond
Sound usually reaches our inner ear through air conduction. Sound waves travel through the air into the outer ear, funneled through the ear canal to the eardrum. The eardrum, a thin membrane, vibrates in response to these incoming sound waves. These vibrations are then transferred to three tiny bones in the middle ear: the malleus (hammer), incus (anvil), and stapes (stirrup). These ossicles amplify and transmit the vibrations to the oval window, a membrane that separates the middle ear from the fluid-filled inner ear.
Bone conduction offers a distinct pathway for sound transmission. In bone conduction, sound vibrations bypass the outer and middle ear structures entirely. Instead, these vibrations are transmitted directly through the bones of the skull to the inner ear. This alternative mechanism allows sound to be perceived even if the ear canal is blocked or if there are issues with the middle ear.
The Skull’s Role in Hearing
The skull conducts sound vibrations directly to the inner ear. When sound waves encounter the skull, they cause the bones to vibrate. These vibrations then travel through the temporal bone, located at the sides and base of the skull. This direct transmission bypasses the eardrum and ossicles, which are involved in air conduction.
Once the vibrations reach the inner ear, they are transmitted to the fluid-filled cochlea. The movement of this fluid, known as perilymph, creates pressure waves that travel through the cochlea. These waves cause the basilar membrane, a structure within the cochlea, to vibrate. The basilar membrane provides a base for the organ of Corti, which contains sensory hair cells.
The vibrations of the basilar membrane cause these hair cells to bend. This mechanical stimulation triggers the hair cells to convert the mechanical energy into electrical signals. These electrical signals are then transmitted along the auditory nerve to the brain, where they are interpreted as sound. This process is similar to how the ossicles stimulate the cochlea in air conduction, but the initial pathway to the cochlea is through the skull bones rather rather than the outer and middle ear.
Everyday Examples of Bone Conduction
Bone conduction is a phenomenon we experience regularly. One common example is hearing your own voice. When you speak, you hear your voice through a combination of air conduction and bone conduction. Others primarily hear your voice through air conduction, which explains why your recorded voice may sound different to you compared to how you perceive it when speaking.
Another instance of bone conduction occurs when eating crunchy foods. The sounds of chewing are transmitted through your jawbone directly to your inner ear. The vibrations from your teeth and jawbone travel through the skull to the cochlea.
Specialized devices also leverage the skull’s conductive properties. Bone-conduction headphones, for example, sit on the cheekbones or mastoid bone behind the ear, sending vibrations directly to the inner ear. These headphones are often used by individuals who want to remain aware of their surroundings or by those with certain types of hearing loss. Similarly, bone-anchored hearing aids are surgically implanted devices that provide a hearing solution for people with conductive or mixed hearing loss, or single-sided deafness.