Audiometry determines the quietest sound a person can hear, known as the hearing threshold. Testing includes air conduction (AC), which assesses the entire auditory pathway, and bone conduction (BC), which evaluates the inner ear (cochlea) by bypassing outer and middle ear structures. The goal of BC testing is to identify whether hearing loss is sensorineural, conductive, or mixed. Masking is employed to ensure accuracy. Masking introduces controlled noise into the non-test ear (NTE) to prevent it from hearing the tone intended for the test ear (TE), guaranteeing the measured threshold belongs only to the TE.
Understanding Cross-Hearing in Bone Conduction
Bone conduction testing utilizes a small vibrator placed against the mastoid bone or the forehead. This vibrator introduces sound directly to the skull, causing vibrations that stimulate the cochlea. Because skull bones are highly efficient at transmitting vibrations, the stimulus delivered to one side causes the entire skull to vibrate.
This vibration stimulates both cochleas simultaneously, regardless of where the bone vibrator is physically placed. The phenomenon of sound traveling across the head is known as cross-hearing. For bone conduction testing, the sound energy loss as it crosses the skull, termed Inter-Aural Attenuation (IAA), is considered to be approximately 0 dB.
The assumption of 0 dB IAA means the sound reaches both inner ears at almost the same intensity. If one ear has significantly better hearing sensitivity than the other, the better ear’s cochlea will respond first, leading to an inaccurate threshold measurement. Masking temporarily occupies the better-hearing non-test ear so the true threshold of the poorer-hearing test ear can be measured without interference.
The Clinical Criteria for Initiating Masking
The decision of when to mask for bone conduction is based on a direct comparison between the two ears’ hearing abilities. Masking is required if the unmasked bone conduction threshold of the test ear (TE BC) is equal to or better than the air conduction threshold of the non-test ear (NTE AC). This comparison is often expressed clinically as: Masking is needed if the difference between the TE BC and the NTE AC is 0 dB or greater.
This rule is used because the sound delivered by the bone vibrator crosses the skull and reaches the non-test ear’s cochlea with virtually no loss. If the signal is loud enough to be heard by the non-test ear’s air conduction pathway, the patient’s response is actually from the NTE, not the TE.
A secondary clinical guideline focuses on identifying a conductive hearing component in the test ear. Masking is needed when there is an Air-Bone Gap (ABG) in the test ear of 15 dB or more between the unmasked AC and BC thresholds. An air-bone gap suggests that the test ear’s outer or middle ear is hindering the air-conducted sound. This means the inner ear is functioning better, as reflected by the bone conduction threshold.
In this scenario, the clinician must confirm that the better bone conduction threshold belongs to the test ear, not the non-test ear’s better cochlea. If the unmasked bone conduction threshold is better than the air conduction threshold in the same ear by 15 dB or more, masking the opposite ear is necessary. This process ensures the measured bone conduction threshold truly reflects the sensitivity of the test ear’s inner ear, allowing for accurate diagnosis of the type and degree of hearing loss.
Determining Effective Masking Levels
Once the criteria for masking are met, the clinician must determine the appropriate level of noise to introduce into the non-test ear. The type of noise used for pure-tone audiometry is narrowband noise (NB), which is centered around the frequency of the pure tone being tested. This noise only masks the small range of frequencies around the test tone, preventing the non-test ear from perceiving the signal without being excessively loud.
Determining the correct amount of noise involves finding the minimum effective masking level and avoiding the maximum effective masking level. The minimum level is the quietest noise that successfully prevents the non-test ear from hearing the crossed-over sound. This starting point is calculated using the non-test ear’s air conduction threshold, a safety factor, and a correction for the occlusion effect.
The standard technique for finding the true masked threshold is called the Plateau Method. This involves gradually increasing the masking noise in the non-test ear while keeping the pure tone stimulus constant in the test ear. The true threshold is found when the patient’s response remains stable across at least three consecutive 5 dB increases in the masking noise. This stable range indicates that the non-test ear is effectively masked and the test ear is accurately responding.
It is important to avoid both undermasking and overmasking during this procedure. Undermasking occurs when the noise is too quiet, allowing the non-test ear to continue responding to the test tone and yielding a false threshold. Conversely, overmasking happens if the noise is too loud, causing the masking noise itself to cross over to the test ear and artificially elevate the threshold.