From the Guidelines
Air conduction is typically louder than bone conduction in normal hearing, as evidenced by the positive Rinne test, where a tuning fork sounds louder when placed near the ear than when pressed against the mastoid bone 1. This difference occurs because sound waves traveling through air reach the cochlea more efficiently than vibrations transmitted through the skull bones. During hearing tests, this normal pattern is crucial in diagnosing hearing disorders. In conductive hearing loss, which affects the outer or middle ear, bone conduction may actually be perceived as louder than air conduction (negative Rinne test) because the bone pathway bypasses the blockage. In sensorineural hearing loss, which affects the inner ear or auditory nerve, both air and bone conduction are reduced, but air conduction remains louder. This difference exists because the ossicular chain in the middle ear provides mechanical advantage, amplifying airborne sound vibrations approximately 22 times before they reach the inner ear, making air-conducted sounds naturally more intense than bone-conducted sounds in healthy ears. Key factors to consider in evaluating hearing loss include the measurement of pure tone thresholds, speech audiometry, tympanometry, and acoustic reflex testing, as outlined in the clinical practice guideline for age-related hearing loss 1. The guideline emphasizes the importance of screening patients aged 50 years and older for hearing loss and evaluating and treating or referring patients with significant asymmetric hearing loss, conductive or mixed hearing loss, or poor word recognition on diagnostic testing. Overall, understanding the difference between air and bone conduction is essential for accurate diagnosis and effective management of hearing disorders, ultimately improving patient outcomes and quality of life. Some of the key recommendations from the guideline include screening patients aged 50 years and older for hearing loss, examining the ear canal and tympanic membrane with otoscopy, and evaluating and treating or referring patients with significant hearing loss 1. By following these recommendations and considering the differences between air and bone conduction, clinicians can provide high-quality care for patients with hearing loss and improve their overall health and well-being.
From the Research
Air Conduction vs Bone Conduction
- Air conduction (AC) and bone conduction (BC) are two different methods of sound transmission to the inner ear 2, 3.
- Studies have shown that AC thresholds are generally lower than BC thresholds, indicating that air conduction is louder than bone conduction 2, 3.
- The difference between AC and BC loudness functions may originate from changes with level of the AC sound path, such as contraction of the stapedius muscle, as well as distortion from the bone transducer and tactile stimulation 3.
Factors Affecting Air-Bone Gap
- The air-bone gap (ABG) is the difference between AC and BC thresholds, and it can be affected by various factors, including middle ear mechanics and threshold measurements 2, 4.
- ABGs can occur in listeners with normal hearing and sensorineural hearing loss, and they can be influenced by the frequency of the sound stimulus 2.
- False ABGs can increase with hearing-loss magnitude, and they can place patients at risk for unnecessary medical and surgical intervention 2.
Audiological Evaluation
- Audiological evaluation is crucial for the diagnosis and management of hearing loss, including otosclerosis 4, 5.
- Pure-tone audiometry and impedance audiometry are commonly used tests to evaluate hearing loss, but they have limitations and uncertainties 4, 5.
- Additional tests, such as speech audiometric tests and computed tomography imaging, may be necessary to confirm the diagnosis and plan treatment 4, 5, 6.