Interaural attenuation (IA) is a foundational concept in audiology that describes the reduction in sound energy as an acoustic signal travels from one side of the head to the other. During a hearing test, sound is deliberately presented to one ear, known as the test ear, but the sound waves do not always remain confined to that ear. The head acts as a natural barrier that absorbs or reflects some of the sound energy.
Understanding this natural sound loss is necessary for accurate hearing measurement. If an audiologist presents a sound that is loud enough, the signal can bypass the intended ear and be perceived by the opposite ear, leading to inaccurate test results. The specific value of interaural attenuation informs the clinician when the test results might be compromised.
Defining Interaural Attenuation
Interaural attenuation is defined as the difference, measured in decibels (dB), between the sound pressure level delivered to the test ear and the sound pressure level that ultimately reaches the cochlea of the non-test ear. This physical phenomenon occurs because the head’s tissues, including the skin, cartilage, and bone, absorb and reflect the acoustic energy.
When an air-conducted sound is presented at a high intensity, the vibration sets the skull bones into motion. The sound energy then travels via bone conduction pathways to the inner ear of the opposite side. The IA value quantifies how much the signal is weakened during this journey across the head.
For example, if a 100 dB tone is presented to the right ear, and the interaural attenuation is 40 dB, the sound arriving at the left ear’s cochlea is 60 dB. This difference highlights the sound loss across the head. The cochlea of the non-test ear is stimulated directly, bypassing that side’s outer and middle ear.
IA Values Based on Hearing Test Equipment
The amount of sound energy lost across the head is not a single fixed number; it varies significantly depending on the type of equipment, or transducer, used to deliver the sound. Different transducers provide different degrees of acoustic isolation, which alters the IA value. Audiologists rely on established minimum IA values for each device to determine when the sound might be heard by the non-test ear.
Supra-aural Headphones
The IA value for traditional supra-aural headphones, which rest over the ear, is conservatively assumed to be approximately 40 dB across most frequencies. This means that any sound presented to the test ear at an intensity 40 dB or more above the hearing threshold of the non-test ear risks being heard by the opposite side.
Insert Earphones
A higher degree of sound isolation is achieved with insert earphones, which are placed directly into the ear canal. The tight seal increases the resistance to sound transmission across the head. Consequently, the minimum IA value for insert earphones is substantially higher, typically ranging from 60 dB to 70 dB for pure tone testing.
Bone Conduction Vibrators
Bone conduction vibrators bypass the outer and middle ear entirely by vibrating the skull directly. Because the entire skull vibrates almost as a single unit, there is virtually no loss of sound energy as the signal travels to the opposite inner ear. For clinical purposes, the interaural attenuation for a bone conduction signal is assumed to be 0 dB.
Cross Hearing and the Necessity of Masking
The clinical consequence of insufficient interaural attenuation is known as “cross hearing,” or “crossover.” This occurs when the sound presented to the test ear is loud enough to overcome the head’s natural attenuation and stimulate the cochlea of the non-test ear. If the sound arriving at the non-test ear is above that ear’s hearing threshold, the patient will respond, making it impossible for the audiologist to know which ear actually heard the sound.
When cross hearing is suspected, the resulting measurement is invalid. This uncertainty is problematic when there is a significant difference in hearing sensitivity between the two ears, such as asymmetrical hearing loss. A measured threshold that is falsely elevated by the better ear is referred to as a “shadow curve.”
To obtain accurate ear-specific thresholds, the clinical technique of “masking” is employed. Masking involves presenting a controlled noise, typically a narrow-band noise, to the non-test ear simultaneously with the test tone. This noise raises the non-test ear’s threshold, effectively neutralizing it. By introducing masking, the audiologist ensures the non-test ear is temporarily unable to perceive the crossed-over signal, forcing the patient to respond only if the signal is truly audible to the test ear.