4.3 Pure-Tone Bone Conduction Audiometry & The Occlusion Effect

4.3 Pure-Tone Bone Conduction Audiometry & The Occlusion Effect

While air conduction audiometry establishes the overall degree and configuration of hearing impairment, it cannot localize the anatomical site of the lesion. A patient presenting with an air conduction threshold of $50\text{ dB HL}$ could have impacted cerumen, otitis media, sensory hair cell death, or acoustic neuroma. Pure-tone bone conduction audiometry bypasses the outer and middle ear transmission systems, transmitting vibrational energy directly through the cranium to stimulate the cochlear fluids. Comparing air conduction and bone conduction thresholds reveals the sensorineural reserve and isolates the precise conductive contribution to the hearing loss.

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Physiological Mechanisms of Bone Conduction

Bone conduction perception is not a single physiological event; rather, it is the vector summation of three distinct biomechanical mechanisms acting concurrently across different frequency bands (as formulated by Georg von Békésy and Juergen Tonndorf):

+-------------------------------------------------------------+
|            Three Modes of Bone Conduction Hearing           |
+-------------------------------------------------------------+
|  1. Distortional / Compressional Mode (High Frequencies)    |
|     - Skull vibrates -> Otic capsule compresses/expands     |
|     - Asymmetric window impedance -> Basilar membrane wave  |
|                                                             |
|  2. Inertial Ossicular Mode (Mid Frequencies: 800-1500 Hz)   |
|     - Skull moves -> Suspended ossicles lag behind          |
|     - Stapes footplate pumps into oval window               |
|                                                             |
|  3. Osseotympanic Mode (Low Frequencies: <1000 Hz)          |
|     - Canal walls vibrate -> Sound radiated into canal lumen|
|     - In open ear: escapes; In occluded ear: drives TM      |
+-------------------------------------------------------------+

1. The Distortional / Compressional Mode (High Frequencies: $>1500\text{ Hz}$)

• Mechanism: When the bone oscillator vibrates against the skull at high frequencies, the cranial bones do not move as a rigid unit; instead, microscopic vibrational traveling waves compress and expand the dense otic capsule encasing the cochlea.
• Fluid Displacement: The fluid inside the cochlea is incompressible. Because the round window membrane is approximately $20\text{ times}$ more compliant than the rigid stapes footplate in the oval window, and because the volume of the scala vestibuli exceeds that of the scala tympani, alternating compression of the otic capsule forces fluid toward the compliant round window. This creates a pressure differential across the basilar membrane, initiating a traveling wave that shears outer hair cell stereocilia identical to normal air conduction.

2. The Inertial Ossicular Mode (Mid Frequencies: $800\text{ to }1500\text{ Hz}$)

• Mechanism: The ossicular chain (malleus, incus, stapes) is suspended within the middle ear cavity by delicate ligaments and tendons. When the skull vibrates, the temporal bone moves back and forth.
• Inertial Lag: Because of their mass inertia, the loosely suspended malleus and incus lag behind the moving skull. This relative displacement causes the stapes footplate to pivot and pump inward and outward relative to the oval window margin. This inertial movement drives cochlear fluids directly through middle ear mechanical linkage.
• Pathological Disruption: If the stapes footplate is locked by bony ankylosis (as in otosclerosis), the inertial ossicular mode is eliminated, producing an artificial depression in bone conduction thresholds known as Carhart's notch.

3. The Osseotympanic Mode (Low Frequencies: $<1000\text{ Hz}$)

• Mechanism: Cranial vibrations cause the bony and cartilaginous walls of the external auditory canal to vibrate. These vibrating walls act as miniature acoustic transducers, compressing the air in the canal lumen and radiating airborne sound waves into the canal space.
• Open vs. Occluded Pathway: In an open (unoccluded) ear, the acoustic impedance of the outside atmosphere is low; consequently, this radiated low-frequency acoustic sound simply escapes out of the ear canal into room air (acting as an acoustic high-pass filter). However, when the ear canal is occluded by an earphone, earmold, or finger, the trapped acoustic energy cannot escape; it is reflected inward against the tympanic membrane, vibrating the ossicles and reinforcing low-frequency cochlear stimulation. This phenomenon is the foundation of the Occlusion Effect.

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Bone Oscillator Instrumentation & Headband Calibration

Bone conduction audiometry requires specialized electromechanical transducers and standardized coupling pressure.

+-------------------------------------------------------------+
|            Transducer & Headband Standards (ANSI S3.6)      |
+-------------------------------------------------------------+
|  - Transducer: Radioear B-71 (Standard) or B-81 (High-power)|
|  - Headband: P-3333 spring steel band                       |
|  - Static Coupling Tension: 5.4 +/- 0.5 Newtons (~500 grams)|
|  - Contact Area: Circular plastic flat face (1.75 cm^2)     |
+-------------------------------------------------------------+

1. Transducer Typology: Radioear B-71 vs. B-81

• Radioear B-71: The historical worldwide standard planar bone vibrator. It provides linear output up to approximately $60\text{–}70\text{ dB HL}$ in mid frequencies, but exhibits significant harmonic distortion at $250\text{ and }500\text{ Hz}$ at elevated outputs.
• Radioear B-81: A modern balanced-armature bone transducer designed to achieve higher maximum output levels ($+5\text{ to }10\text{ dB}$) with drastically reduced low-frequency harmonic distortion, providing superior diagnostic separation of true bone conduction from vibrotactile artifacts.

2. Headband Static Force Calibration

• ANSI S3.6 mandates that the bone oscillator must be coupled to the skull using a calibrated P-3333 spring steel headband exerting a static coupling force of $5.4\pm 0.5\text{ Newtons}$ (equivalent to approximately $500\text{ grams}$ of static weight).
• Under-Tension Danger: If the headband is stretched or loose ($<4.9\text{ N}$), acoustic transmission through the skin and periosteum is inefficient, resulting in artificially elevated (worse) bone conduction thresholds and false-positive air-bone gaps.
• Over-Tension Danger: Excessive headband force ($>5.9\text{ N}$) causes severe patient discomfort, localized ischemia, and altered skull bone resonance.

3. Skin-Transducer Coupling Protocols

• Clear Hair Completely: The clinician must part and push aside all hair over the mastoid process. Hair acts as an acoustic shock absorber, dampening vibrational transmission by $5\text{ to }15\text{ dB}$.
• Avoid Pinna Contact: The casing of the bone oscillator must never touch the auricle (pinna). If the oscillator touches the pinna, sound vibrations radiate directly from the plastic shell into the concha as airborne sound, producing a false air conduction signal rather than true bone conduction.

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Mastoid vs. Forehead Oscillator Placement

Clinical guidelines permit bone oscillator placement at either the mastoid process or the midline of the forehead.

Why Mastoid Placement Dominates Hearing Instrument Sciences:
Audiometer bone conduction output is physically limited by transducer saturation.
Testing via the mastoid preserves 10 dB of vital dynamic headroom, allowing clinicians
to measure severe sensorineural losses up to 70 dB HL that cannot be reached on the forehead.

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Standard Test Frequencies & The Vibrotactile Danger Zone

Bone conduction audiometry is conducted across octave frequencies from $500\text{ to }4000\text{ Hz}$ (specifically $500, 1000, 2000, \text{and }4000\text{ Hz}$):

• Why 8000 Hz is Not Tested: The mass of the bone oscillator transducer cannot physically oscillate at 8000 Hz without generating massive harmonic distortion, acoustic radiated leakage, and severe skin radiation. Audiometers are not calibrated for routine bone conduction at 8000 Hz.
• Why 250 Hz is Frequently Omitted: Bone conduction testing at 250 Hz is notoriously fraught with severe vibrotactile artifacts and large occlusion effect variances.

Vibrotactile Threshold Boundaries

At high intensity levels in the low frequencies, patients no longer hear the test stimulus acoustically; instead, they feel the physical mechanical shaking of the skin, skull, and periosteal mechanoreceptors (Pacinian corpuscles). The patient presses the response button because they feel a "buzzing" sensation, not because their cochlea perceived sound.

+-------------------------------------------------------------+
|            Vibrotactile Threshold Limits (Mastoid)          |
+-------------------------------------------------------------+
|  - 250 Hz:   30 to 35 dB HL  (Extreme Vibrotactile Risk)    |
|  - 500 Hz:   50 to 55 dB HL  (High Vibrotactile Risk)       |
|  - 1000 Hz:  75 to 80 dB HL  (Moderate Vibrotactile Risk)   |
+-------------------------------------------------------------+

Critical Clinical Trap: Consider a patient with profound, dead-ear sensorineural loss in the right ear ($AC = 95\text{ dB HL}$). When bone conduction is presented at $250\text{ Hz}$ at $35\text{ dB HL}$ or at $500\text{ Hz}$ at $55\text{ dB HL}$, the patient feels the vibration and pushes the button. If the novice clinician records this as an acoustic threshold, the audiogram will display an artificial $40\text{ to }60\text{ dB}$ air-bone gap, incorrectly diagnosing a severe conductive or mixed pathology in a cochlea that is biologically non-functional! Whenever unmasked bone thresholds match these vibrotactile limits, the clinician must question the patient ("Did you hear a sound like a tone, or did you just feel a buzzing vibration on your skin?") and mark the threshold with a vibrotactile notation (VT).

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The Occlusion Effect (OE) and Masking Calculations

The Occlusion Effect (OE) is defined as the artificial enhancement (lowering of numerical threshold, or subjective increase in loudness) of low-frequency bone conduction thresholds that occurs when the external auditory meatus is covered, plugged, or occluded by an earphone, earmold, or probe.

                     THE OCCLUSION EFFECT MECHANISM
                     
       OPEN EAR CANAL                      OCCLUDED EAR CANAL
    [ Bone Oscillator ]                   [ Bone Oscillator ]
             |                                     |
             v                                     v
     ( Skull Vibrates )                    ( Skull Vibrates ) 
             |                                     |
             v                                     v
    Canal Walls Radiate                   Canal Walls Radiate
       Low-Freq Sound                        Low-Freq Sound
             |                                     |
             v                                     v
     /---------------\                     /---------------\ 
    |  Sound ESCAPES  |                   | [ EARPHONE ]   |
    |  out of open    |                   | Traps sound!   |
    |  meatus         |                   | Sound forced   |
    |  (High-pass)    |                   | through TM     |
     \---------------/                     \---------------/ 
             |                                     |
             v                                     v
    NORMAL BC THRESHOLD                   ARTIFICIALLY ENHANCED BC
                                          (Threshold drops 10-30 dB!)

Quantitative Occlusion Effect Values

The magnitude of the Occlusion Effect depends directly on the volume of trapped air inside the canal and whether the transducer contacts the cartilaginous canal:

Why Insert Earphones Minimize the Occlusion Effect

• Supra-aural cushions enclose a massive air volume ($~50\text{ cm}^3$) outside the canal and compress the cartilaginous auricle, maximally driving osseotympanic vibrations into the canal.
• Insert earphones seat an expandable foam tip inside the canal. When inserted to standard depth, the residual canal volume is reduced to $<1.0\text{ cm}^3$, diminishing the OE to only $10\text{ dB}$ at 250 Hz and $5\text{ dB}$ at 500 Hz.
• If the insert tip is pushed deeply past the cartilaginous-osseous junction into the bony canal, the Occlusion Effect drops to $0\text{ dB}$ across all frequencies, because the rigid bony canal walls do not flex to generate osseotympanic sound pressure.

Clinical Calculation: Incorporating OE into Masking Equations

When bone conduction is tested with masking, the non-test ear (NTE) must be covered by an earphone to deliver narrowband masking noise. Covering the NTE instantly induces the Occlusion Effect in that ear, artificially enhancing its bone conduction sensitivity for low-frequency tones crossing the skull.

To prevent undermasking, the clinician must add the standardized OE value into the initial masking formula:

$M_{initial} = AC_{NTE} + OE + 10\text{ dB (Safety Factor)}$

Clinical Masking Scenario:
- Testing Bone Conduction at 500 Hz
- Non-Test Ear (NTE) AC Threshold = 20 dB HL

Case A: Using Supra-Aural Earphones (TDH-39)
  OE at 500 Hz = 15 dB
  M_initial = 20 dB (AC) + 15 dB (OE) + 10 dB (Safety) = 45 dB Effective Masking (EM)

Case B: Using Insert Earphones (ER-3A)
  OE at 500 Hz = 5 dB
  M_initial = 20 dB (AC) + 5 dB (OE) + 10 dB (Safety) = 35 dB Effective Masking (EM)

Exam Alert: Notice that forgetting the Occlusion Effect in Case A would lead the clinician to introduce only $30\text{ dB EM}$ ($20 + 10$), which is $15\text{ dB}$ undermasked! The non-test ear will hear the tone via the enhanced occlusion pathway, producing a false air-bone gap in the test ear.

Pathological Absence of the Occlusion Effect

If a patient has a pre-existing conductive hearing loss in the non-test ear (e.g., middle ear effusion, otosclerosis, tympanic membrane perforation), the middle ear transmission system is already mechanically impaired. In these ears, the Occlusion Effect is absent ($0\text{ dB}$), because trapped osseotympanic sound waves cannot cross the disrupted middle ear to reach the cochlea. This physiological principle forms the basis of the classic Bing tuning fork test.