Sound is a form of mechanical energy that travels through a medium, such as air, in waves. The process of hearing relies on capturing these sound waves and converting them into electrical signals the brain can interpret. Humans perceive the vast majority of sounds through air conduction. This mechanism involves the entire auditory system, from the visible outer ear to the fluid-filled structures deep within the head.
The Physical Pathway of Air Conduction
The journey of sound begins in the outer ear, where the pinna, the visible part of the ear, collects incoming sound waves and channels them down the ear canal (external auditory meatus) toward the middle ear. The canal amplifies sound frequencies in the 2,000 to 5,000 Hertz range, which includes many frequencies important for human speech.
The sound waves strike the tympanic membrane (eardrum), causing it to vibrate. This vibration transfers acoustical energy from the air-filled outer ear to the fluid-filled inner ear via a chain of three tiny bones (ossicles) in the middle ear: the malleus (hammer), the incus (anvil), and the stapes (stirrup). These form a mechanical lever system.
The middle ear acts as an impedance matching device because sound waves travel less efficiently in liquid than in air. The ossicular chain overcomes this mismatch by concentrating the force of the eardrum onto the much smaller surface of the stapes’ footplate. This mechanical advantage amplifies the sound pressure roughly twenty-fold, ensuring the energy is strong enough to create a wave in the inner ear fluid.
The stapes presses against the oval window, an opening into the fluid-filled cochlea of the inner ear. This action creates a pressure wave that travels through the cochlear fluid, where specialized hair cells convert the mechanical motion into electrical nerve impulses. These impulses are then sent to the brain via the auditory nerve.
Air Conduction Versus Bone Conduction
Air conduction and bone conduction are the two ways sound energy can reach the inner ear. Air conduction utilizes the external and middle ear structures as its main route, relying on the eardrum and ossicles to transmit and amplify the sound.
Bone conduction bypasses the outer and middle ear, relying instead on the skull bones to vibrate. When sound waves cause the skull to vibrate, these mechanical movements are transmitted directly to the fluid within the inner ear.
Audiologists use air conduction testing to assess the function of the entire hearing system, from the ear canal to the inner ear. Bone conduction testing specifically measures the ability of the inner ear to process sound.
By comparing the results of air conduction and bone conduction tests, a professional can determine if a hearing loss is caused by a problem in the outer/middle ear or the inner ear. If air conduction thresholds are significantly worse than bone conduction thresholds, it suggests an issue within the sound-conducting structures of the outer or middle ear.
Air Conduction and Conductive Hearing Loss
Any obstruction or damage to the outer or middle ear can impair hearing. Conductive hearing loss occurs when sound is prevented from efficiently traveling through the external auditory canal or the middle ear to reach the cochlea.
A common cause is a blockage within the external ear, such as an excessive buildup of cerumen (earwax). Middle ear infections (otitis media) can also cause fluid to accumulate behind the eardrum. This fluid prevents the eardrum and the ossicles from vibrating freely.
Damage to the middle ear can also lead to conductive hearing loss. A perforated eardrum, for example, cannot vibrate properly, interrupting the sound transfer process. Problems with the ossicular chain, such as damage from trauma or abnormal bone growth like otosclerosis, directly impede the amplification and transmission of sound waves to the inner ear.
Because the inner ear remains healthy in cases of conductive hearing loss, the impairment is often treatable. Addressing the underlying issue, such as removing a blockage or surgically repairing a damaged eardrum, can often restore the air conduction pathway and improve hearing.