Clinical Masking Principles, Interaural Attenuation & Plateau Method

Clinical Masking Principles, Interaural Attenuation & Plateau Method

In diagnostic pure-tone audiometry, the primary goal is obtaining ear-specific thresholds. When auditory sensitivity differs significantly between ears, acoustic energy delivered to the poorer ear can cross the skull and stimulate the better, contralateral cochlea. Clinical masking is the psychoacoustic process of introducing a calibrated noise into the non-test ear (NTE) to isolate the test ear (TE) and obtain valid, ear-specific hearing thresholds.

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Cross-Hearing & The Psychoacoustics of Shadow Curves

The Cross-Hearing Mechanism

Cross-hearing occurs when sound presented to the test ear crosses the head via bone conduction (cranial bone vibration) or air-leakage radiation and stimulates the contralateral cochlea. The stimulus always stimulates the cochlea of the non-test ear via bone conduction, regardless of whether the transducer delivering the sound to the test ear is an air-conduction earphone or a bone vibrator.

The "Shadow Curve" Phenomenon

When cross-hearing occurs during unmasked testing of an ear with severe or profound unilateral hearing loss, the resulting audiometric thresholds do not reflect the test ear. Instead, they produce a shadow curve that closely mirrors the threshold configuration of the normal or better contralateral ear, shifted downward by the interaural attenuation of the transducer.

The Severe Clinical Trap: In a patient with unilateral total sensorineural deafness (dead right ear, normal left ear):

• Unmasked air-conduction testing of the right ear with supra-aural headphones ($\text{IA} = 40\,\text{dB}$) produces false thresholds at $40-50\,\text{dB HL}$ across all frequencies.
• Unmasked bone-conduction testing with the vibrator on the right mastoid yields thresholds of $0-10\,\text{dB HL}$ (identical to the normal left cochlea, because bone $\text{IA} = 0\,\text{dB}$).
• If unmasked thresholds are accepted without contralateral masking, the patient appears to have a moderate-to-severe conductive hearing loss ($40-50\,\text{dB AC}$, $0-10\,\text{dB BC}$, $40\,\text{dB ABG}$) in the right ear. This error can lead to inappropriate middle-ear surgical exploration on a dead ear.

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Interaural Attenuation (IA) Parameters Across Transducers

Interaural attenuation (IA) is defined as the loss of acoustic energy (in decibels) as sound travels from the test ear across the skull to stimulate the contralateral non-test cochlea.

$IA = \text{Signal Level at TE} - \text{Signal Level Reaching Contralateral Cochlea}$

Transducer-Specific Interaural Attenuation Values

Why Insert Earphones Provide Higher IA

Insert earphones achieve high interaural attenuation (about $50-70\,\text{dB}$, depending on frequency and insertion depth) through two physical mechanisms:

1. Reduced Contact Area: The foam eartip contacts only a tiny surface area of the cartilaginous meatus ($\approx 1.5\,\text{cm}^2$), compared to supra-aural cushions which couple to a massive surface area of the cranium and pinna ($\approx 35-45\,\text{cm}^2$). Smaller contact area reduces mechanical vibration transfer to the skull.
2. Acoustic Decoupling: Deep insertion isolates the sound pressure within the deep canal, preventing radiation of sound waves to the temporal bone.

Clinical Impact: Insert earphones greatly reduce how often air-conduction masking is needed and lower the risk of overmasking, saving test time and patient fatigue.

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Mathematical Rules for Initiating Clinical Masking

Audiologists must memorize and apply exact mathematical rules to determine when masking is required.

1. Air-Conduction Masking Rule

Masking of the non-test ear during air-conduction testing is mandatory whenever the air-conduction presentation level to the test ear exceeds the bone-conduction threshold of the non-test ear by an amount equal to or greater than the interaural attenuation of the transducer:

$AC_{\text{TE}} - IA \ge BC_{\text{NTE}}$

• Conservative Clinical Rule (when $BC_{\text{NTE}}$ is unknown): If bone conduction thresholds for the non-test ear have not yet been established, the air-conduction threshold of the NTE ($AC_{\text{NTE}}$) is substituted conservatively: $AC_{\text{TE}} - IA \ge AC_{\text{NTE}}$

Application Example:

• Patient tested at $1000\,\text{Hz}$ with TDH-39 headphones ($\text{IA} = 40\,\text{dB}$).
• Right Ear (TE) unmasked $AC = 65\,\text{dB HL}$.
• Left Ear (NTE) $BC = 15\,\text{dB HL}$.
• Calculation: $AC_{\text{TE}} - IA = 65 - 40 = 25\,\text{dB}$.
• Comparison: $25\,\text{dB} \ge 15\,\text{dB}$ ($BC_{\text{NTE}}$) $\implies$ Masking of the Left Ear is mandatory.
• If tested with ER-3A insert earphones ($\text{IA} \approx 70\,\text{dB}$ at $1000\,\text{Hz}$): $65 - 70 = -5\,\text{dB} < 15\,\text{dB}$ $\implies$ No masking required.

2. Bone-Conduction Masking Rule

Because bone conduction interaural attenuation is $0\,\text{dB}$, an unmasked bone vibrator placed on either mastoid stimulates both cochleae with equal mechanical energy. Therefore, an unmasked bone conduction threshold represents the sensitivity of the better cochlea, regardless of vibrator placement.

Masking of the non-test ear during bone-conduction testing is mandatory whenever there is an air-bone gap in the test ear exceeding $10\,\text{dB}$:

$\text{ABG}_{\text{TE}} = AC_{\text{TE}} - BC_{\text{TE}} > 10\,\text{dB} \quad (\text{i.e., } \text{ABG} \ge 15\,\text{dB})$

Clinical Imperative: Even if the bone conduction vibrator is physically mounted on the right mastoid, if $AC_{\text{Right}} = 50\,\text{dB HL}$ and unmasked $BC = 10\,\text{dB HL}$, the air-bone gap is $40\,\text{dB}$. That $10\,\text{dB}$ bone threshold may belong entirely to the left cochlea. Contralateral masking of the left ear is mandatory to establish whether the right ear has a conductive, mixed, or sensorineural loss.

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Masking Stimuli: Narrow-Band Noise & Critical Band Theory

Fletcher's Critical Band Concept

Harvey Fletcher (1940) established that the human basilar membrane acts as a bank of overlapping band-pass filters (auditory critical bands). When masking a pure tone, only the acoustic energy contained within the critical band centered at the test frequency contributes to threshold shift.

• Noise energy outside the critical band does not increase masking effectiveness.
• Unfiltered broad-band white noise delivers excessive acoustic energy across the entire spectrum, inducing unnecessary loudness, patient discomfort, and premature central masking without providing additional threshold shift.

Narrow-Band Noise (NBN)

Narrow-band noise is geometrically centered at the test frequency with a bandwidth slightly wider than the auditory critical band (typically $\frac{1}{3}$ to $\frac{1}{2}$ octave bandwidth) and steep filter roll-off slopes ($\ge 24-48\,\text{dB/octave}$).

• NBN provides the highest masking efficiency with minimum overall sound pressure level.
• Calibration: Audiometers calibrate NBN in dB Effective Masking (dB EM). An audiometer setting of $50\,\text{dB EM}$ of narrow-band noise is calibrated to shift the pure-tone threshold in that ear to exactly $50\,\text{dB HL}$.

Speech Noise

For masking during speech audiometry (SRT and Word Recognition Score), narrow-band noise is unsuitable because speech is broadband. Clinical speech masking uses Speech-Spectrum Noise, which has a flat spectrum from $100$ to $1000\,\text{Hz}$ and rolls off at $12\,\text{dB/octave}$ above $1000\,\text{Hz}$ to simulate the long-term average speech spectrum (LTASS).

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The Occlusion Effect (OE) & Middle Ear Mechanics

Biophysical Basis of the Occlusion Effect

When a bone vibrator stimulates the skull, the cartilaginous walls of the external auditory meatus vibrate, radiating acoustic energy into the ear canal lumen. In an open (unoccluded) ear, this sound escapes out the concha into the free field. When the non-test ear is covered by a headphone or insert earphone to deliver masking noise, the ear canal is occluded.

• The acoustic energy cannot escape and is reflected back toward the tympanic membrane.
• This sound pressure drives the ossicular chain and cochlea, artificially boosting low-frequency bone conduction sensitivity by up to $30\,\text{dB}$.
• This phenomenon is the occlusion effect (OE).

Standard Clinical Occlusion Effect Values

The Preexisting Middle Ear Pathology Exception

Critical Diagnostic Rule: The occlusion effect occurs only when the middle ear of the non-test ear is normal. If the NTE has a preexisting conductive pathology (e.g., otitis media with effusion, perforated tympanic membrane, otosclerosis, ossicular discontinuity), the middle ear transmission system is already mechanically impaired or disconnected. In such ears: $\text{OE} = 0\,\text{dB}$ If an audiologist adds $20\,\text{dB}$ of OE compensation when masking an NTE that has a conductive loss, the masking level will be set $20\,\text{dB}$ too high, inducing immediate overmasking of the test ear.

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Hood's Plateau Method: Execution & Step-by-Step Logic

Developed by J.D. Hood (1960), the plateau method (threshold search method) is the gold standard for clinical masking.

The Three Stages of Masking

As masking noise in the NTE is increased in $5\,\text{dB}$ steps, the test ear threshold responds across three distinct stages:

1. Undermasking: Masking noise in the NTE is insufficient to prevent the NTE cochlea from hearing the test tone. As masking noise is increased by $5\,\text{dB}$, the apparent TE threshold shifts upward by $5\,\text{dB}$ in direct 1:1 proportion to the noise.
2. Effective Masking (The Plateau): The masking noise in the NTE is sufficient to eliminate cross-hearing, but not intense enough to cross the skull back to the test ear. As masking noise is raised across a range of $15-20\,\text{dB}$ (at least three consecutive $5\,\text{dB}$ steps), the test ear threshold remains completely stable ($\pm 0\,\text{dB}$). This stable level represents the true threshold of the test ear.
3. Overmasking: The masking noise in the NTE becomes so intense that it crosses the skull via bone conduction and begins masking the test ear cochlea ($BC_{\text{TE}}$). The test ear threshold begins shifting upward again in 1:1 proportion with further noise increases.

Mathematical Formulas for Initial Masking Levels ($EM_{\text{initial}}$)

• For Air Conduction Masking: $EM_{\text{initial}} = AC_{\text{NTE}} + 10\,\text{dB safety margin}$
• For Bone Conduction Masking: $EM_{\text{initial}} = AC_{\text{NTE}} + \text{OE} + 10\,\text{dB safety margin}$ (Note: Omit OE if the NTE exhibits an air-bone gap).

Clinical Step-by-Step Execution Protocol

1. Introduce $EM_{\text{initial}}$ into the NTE earphone.
2. Re-present the pure-tone to the test ear at the unmasked threshold level.
3. If the patient responds: Increase the masking noise in the NTE by $5\,\text{dB}$. (Do not change tone level).
4. If the patient fails to respond: Increase the pure-tone in the TE in $5\,\text{dB}$ steps until the patient responds again. Once they respond, increase noise in the NTE by $5\,\text{dB}$.
5. Plateau Confirmation: Continue until the pure-tone threshold remains completely unchanged across at least three consecutive $5\,\text{dB}$ masking noise increments (a minimum $15\,\text{dB}$ plateau width, e.g., noise at $+5\,\text{dB}$, $+10\,\text{dB}$, $+15\,\text{dB}$ above initial level without threshold shift). Record this value as the masked threshold.

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Overmasking Calculations, Central Masking & The Masking Dilemma

Overmasking Mechanics

Overmasking occurs whenever the effective masking level delivered to the NTE exceeds the interaural attenuation and reaches the bone-conduction threshold of the test ear:

$EM_{\text{NTE}} - IA \ge BC_{\text{TE}}$

The maximum permissible masking level ($EM_{\text{max}}$) before overmasking occurs is: $EM_{\text{max}} = BC_{\text{TE}} + IA - 5\,\text{dB}$

Central Masking

When masking noise is introduced into the contralateral ear, the test ear threshold frequently shifts upward by a fixed $5\,\text{dB}$ (rarely $10\,\text{dB}$), even when noise levels are well below cross-over intensity. This is central masking, a central auditory nervous system phenomenon mediated by inhibitory neural pathways in the superior olivary complex. Central masking is physiologically normal and is not an error; clinicians should anticipate a $5\,\text{dB}$ elevation when applying contralateral noise.

The Masking Dilemma

A masking dilemma arises when the minimum initial masking level required to prevent cross-hearing immediately exceeds the maximum permissible masking level, producing overmasking on the very first step. In this scenario, no plateau exists.

• Clinical Etiology: Bilateral conductive or mixed hearing loss with substantial bilateral air-bone gaps (e.g., bilateral chronic suppurative otitis media, bilateral advanced otosclerosis, bilateral canal atresia).

Mathematical Demonstration:

• Bilateral otosclerosis: $AC_{\text{Right}} = 60\,\text{dB HL}$, $BC_{\text{Right}} = 10\,\text{dB HL}$; $AC_{\text{Left}} = 60\,\text{dB HL}$, $BC_{\text{Left}} = 10\,\text{dB HL}$.
• Transducer: Supra-aural TDH-39 ($\text{IA} = 40\,\text{dB}$; $\text{OE} = 20\,\text{dB}$ at $500\,\text{Hz}$).
• Minimum masking required in NTE: $EM_{\text{initial}} = AC_{\text{NTE}} + 10 = 60 + 10 = 70\,\text{dB EM}$
• Level at which overmasking of the TE begins: $EM_{\text{over}} = BC_{\text{TE}} + IA = 10 + 40 = 50\,\text{dB EM}$
• Analysis: Minimum required noise ($70\,\text{dB EM}$) is $20\,\text{dB}$ higher than the level that causes overmasking ($50\,\text{dB EM}$). Applying $70\,\text{dB EM}$ immediately crosses the skull and masks the test ear, falsely elevating thresholds.

Clinical Management of the Masking Dilemma

1. Switch to Insert Earphones: Raising IA from $40\,\text{dB}$ to roughly $70\,\text{dB}$ at $500\,\text{Hz}$ moves the overmasking point up to about $10 + 70 = 80\,\text{dB EM}$, and deep inserts make the occlusion effect negligible, so a usable plateau often appears.
2. Tuning Fork Cross-Checks: Perform the Weber test (lateralizes to the ear with greater conductive loss) and Bing test.
3. Objective Diagnostic Cross-Checks: Perform 1000-Hz/multi-frequency tympanometry, acoustic reflex testing, and electrophysiologic threshold evaluation (e.g., bone-conduction ABR or ASSR with insert masking).
4. Clinical Documentation: If unresolvable, document thresholds as "untestable due to masking dilemma" and annotate unmasked bone conduction as representing the better cochlea.