Our ears capture and interpret sound vibrations, allowing us to perceive the world. This process begins when sound waves enter the ear and are transformed into signals our brain can understand. Various parts of the ear work in harmony, from the outer structures to the delicate internal mechanisms. Understanding how sound travels through these pathways is fundamental to grasping the concept of an air-bone gap.
Understanding Sound Conduction
Sound reaches the inner ear through two pathways: air conduction and bone conduction. Air conduction occurs when sound waves travel through the air into the outer ear canal, causing the eardrum (tympanic membrane) to vibrate. These vibrations transfer through three tiny middle ear bones—the malleus, incus, and stapes (ossicles)—to the fluid-filled cochlea in the inner ear. The cochlea’s hair cells convert these mechanical vibrations into electrical signals, which are then sent to the brain via the auditory nerve.
Bone conduction provides an alternative route for sound to reach the inner ear, bypassing the outer and middle ear structures. When a vibrating object is placed on the skull, it causes the skull bones to vibrate. These vibrations are directly transmitted to the cochlea, stimulating the hair cells and generating electrical signals for the brain. Both air and bone conduction are normal mechanisms by which we perceive sound.
Defining an Air-Bone Gap
An air-bone gap indicates a difference in hearing sensitivity between air-conducted and bone-conducted sound. This gap is determined during a hearing test by comparing thresholds for sounds presented through headphones (air conduction) versus sounds presented directly to the skull via a bone vibrator (bone conduction). The air conduction threshold represents the softest sound heard through the entire ear pathway, while the bone conduction threshold reflects the inner ear’s ability to hear, bypassing the outer and middle ear.
A significant air-bone gap exists when the air conduction threshold is noticeably poorer (requires a louder sound) than the bone conduction threshold, typically by 10 decibels (dB) or more at a specific frequency. This difference is calculated by subtracting the bone conduction threshold from the air conduction threshold for a given frequency. For instance, if a person can hear a sound at 20 dB via bone conduction but requires 40 dB via air conduction, there is a 20 dB air-bone gap. This measurement provides valuable information about where a potential hearing issue might be located within the auditory system.
What an Air-Bone Gap Indicates
An air-bone gap suggests a problem within the outer or middle ear that impedes sound transmission through the air conduction pathway. This hearing issue is known as conductive hearing loss, where sound cannot effectively reach the inner ear at its full intensity. A substantial air-bone gap can also be a component of mixed hearing loss, involving both a conductive problem in the outer or middle ear and a sensorineural problem in the inner ear.
Various conditions can lead to an air-bone gap. Common examples include a buildup of earwax (cerumen) blocking the ear canal, fluid accumulation in the middle ear (otitis media with effusion), or a perforation (hole) in the eardrum. Problems with the ossicles, such as their malformation, discontinuity (a break), or fixation (fusion), can also create an air-bone gap. In rare cases, certain inner ear disorders, like an enlarged vestibular aqueduct or semicircular canal dehiscence, can also present with an air-bone gap. Identifying an air-bone gap helps audiologists and medical professionals diagnose the specific type and potential cause of hearing loss.