Sound is energy conveyed through vibrations. These vibrations travel through a medium, reaching our ears and transforming into the sensations we recognize as sounds. This process allows us to perceive and interpret auditory information. Understanding how these vibrations are processed by our auditory system reveals the mechanisms that underpin our perception of sound.
How Sound Reaches Our Ears
Sound reaches the inner ear, where it is processed into neural signals, through two distinct pathways: air conduction and bone conduction. Air conduction is the more common and efficient route for sound perception. In this process, sound waves are collected by the outer ear and channeled through the ear canal to the eardrum, a membrane that vibrates. These vibrations are then transferred to three tiny bones in the middle ear: the malleus (hammer), incus (anvil), and stapes (stirrup). This chain of ossicles transmits the vibrations to the cochlea, a snail-shaped, fluid-filled structure in the inner ear.
An alternative pathway is bone conduction, where vibrations travel directly through the skull bones to the cochlea, bypassing the outer and middle ear structures. This is how a person hears their own voice or the vibrations felt when chewing food. Bone conduction allows sound to reach the inner ear, but it is less sensitive than air conduction. Both pathways ultimately cause fluid in the cochlea to ripple, stimulating tiny hair cells that convert mechanical vibrations into electrical signals sent to the brain for interpretation.
The Superiority of Air Conduction
Air conduction is superior to bone conduction due to the specialized middle ear structures that efficiently transfer sound energy. A primary reason for this efficiency is impedance matching, where the middle ear overcomes the significant difference in resistance between airborne sound and the fluid-filled inner ear. When sound travels from a low-impedance medium like air to a high-impedance medium like the cochlear fluid, most sound energy would be reflected, resulting in a substantial loss of intensity. The middle ear acts as an impedance transformer, boosting sound wave pressure to ensure effective transmission.
This pressure boost, which can range from 20 to 30 dB, is achieved through two main mechanisms. First, the eardrum has a much larger surface area than the oval window, where the stapes connects to the cochlea. This difference concentrates the force onto a smaller area, increasing pressure on the inner ear fluid by about 17 to 20 times. Second, the middle ear ossicles act as a lever system, providing a mechanical advantage. This combined amplification ensures air-conducted sounds are more sensitive and clearer than bone-conducted sounds.
What the Difference Reveals About Hearing
The distinction between air and bone conduction is important in diagnosing different types of hearing loss. Audiologists use tests like the Rinne test to compare a person’s ability to hear sound through air versus bone conduction. In a person with normal hearing, sound heard via air conduction is louder and perceived for a longer duration than sound heard via bone conduction. This expected finding is termed a “positive Rinne” result.
When a “negative Rinne” result occurs, meaning bone conduction is heard louder or longer than air conduction, it indicates a conductive hearing loss. Conductive hearing loss arises from problems in the outer or middle ear that obstruct the sound’s pathway, such as earwax blockage, fluid in the middle ear, or damage to the eardrum or ossicles. In such cases, the bone conduction pathway bypasses the affected outer or middle ear, allowing sound to reach the inner ear more effectively than through air. Conversely, if both air and bone conduction are equally reduced, but air conduction remains louder than bone conduction, it suggests a sensorineural hearing loss, which involves damage to the inner ear or the auditory nerve. A mixed hearing loss involves components of both conductive and sensorineural issues.