Air and Bone Conduction Pure-Tone Testing & Audiogram Interpretation

Air and Bone Conduction Pure-Tone Testing & Audiogram Interpretation

Behavioral pure-tone audiometry is the gold standard diagnostic battery in audiological practice. It establishes auditory sensitivity across the speech frequency spectrum, differentiates conductive from sensorineural and mixed hearing loss, quantifies impairment severity, guides medical and surgical management, and serves as the baseline for rehabilitative audiologic interventions.

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The Modified Hughson-Westlake Threshold Search Protocol

Clinical pure-tone audiometry utilizes the modified Hughson-Westlake procedure (Carhart & Jerger, 1959; adapted from Hughson & Westlake, 1944), standardized under ANSI/ASA S3.21 and ISO 8253-1. This psychophysical method uses an ascending approach to determine behavioral threshold.

Psychophysical Rationale of the Ascending Technique

In classic psychophysics, thresholds determined via descending runs are consistently lower (more sensitive) than those obtained via ascending runs due to stimulus perseveration, central expectation, and auditory persistence. The modified Hughson-Westlake protocol deliberately employs an ascending search for three critical clinical reasons:

• It prevents auditory fatigue and tone adaptation, especially in patients with retrocochlear or sensory pathology.
• It establishes a well-defined psychoacoustic criterion: the patient responds to a sound that emerges from complete inaudibility, minimizing false-positive guessing.
• It provides high test-retest reliability across clinical sessions (typically within $\pm 5\,\text{dB}$).

The "Down 10 dB, Up 5 dB" Algorithm

1. Familiarization / Initial Presentation:
   • Present a pure tone at $1000\,\text{Hz}$ at an intensity of $30\,\text{dB HL}$.
   • If the patient responds clearly, threshold search begins immediately.
   • If no response occurs, increase the presentation level to $50\,\text{dB HL}$.
   • If still inaudible, increase the intensity in successive $10\,\text{dB}$ steps until a definite behavioral response is obtained (ASHA, 2005).
2. The Descent ("Down 10 dB"):
   • Following every positive response, attenuate the stimulus by $10\,\text{dB}$ and present the tone again.
   • Continue stepping down by $10\,\text{dB}$ until the patient fails to respond.
3. The Ascent ("Up 5 dB"):
   • Following every absent response (failure to respond), increase the stimulus level by $5\,\text{dB}$ and present the tone.
   • Continue stepping up by $5\,\text{dB}$ until the patient responds.
4. Ascending Run Definition:
   • Only responses obtained on an ascending run (moving from inaudible to audible in $5\,\text{dB}$ increments) are counted toward threshold determination. Responses on the initial descent do not count.

Definition of Behavioral Auditory Threshold

The clinical auditory threshold is formally defined as: $\text{Threshold} = \text{The lowest hearing level (in dB HL) at which a response occurs in } \ge 50\% \text{ of ascending presentations}$ Clinically, this criterion requires a minimum of 2 responses out of 3 presentations (or 3 responses out of 5 presentations) at that specific intensity on ascending sweeps.

Stimulus Characteristics & Presentation Timing

• Tone Duration: Stimulus duration should be standardized between $1.0$ and $2.0\,\text{seconds}$ (typically $1.0-1.5\,\text{s}$). Tone presentations shorter than $200\,\text{ms}$ suffer from temporal integration roll-off, artificially elevating thresholds because the auditory system integrates acoustic energy over $\approx 200-300\,\text{ms}$. Presentations longer than $2.5\,\text{s}$ induce auditory adaptation.
• Pulsed Tones vs. Continuous Tones: Pulsed tones (2–3 pulses per second with $50\%\,\text{duty cycle}$ and $20-50\,\text{ms}$ rise/fall times) are strongly preferred over continuous tones. Pulsed tones reduce patient listening fatigue, enhance signal detection in background physiologic noise, and prevent confusion in patients experiencing subjective high-frequency tinnitus.
• Inter-Stimulus Intervals: Clinicians must randomize the interval between presentations ($1.5-4.0\,\text{seconds}$). Regular or rhythmic stimulus delivery leads to false-positive rhythmic conditioning, where the patient presses the response button in anticipation of the cadence.

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Test Frequency Sequence & Inter-Octave Rules

Standard Testing Sequence

Pure-tone air conduction testing must adhere to a standardized sequence to maximize reliability and identify participant inconsistency:

1. Initial Frequency ($1000\,\text{Hz}$): Testing begins at $1000\,\text{Hz}$ because human hearing exhibits superior test-retest reliability, low susceptibility to physiologic background noise, and central placement within the speech-frequency spectrum.
2. High-Frequency Progression: Proceed sequentially in ascending octaves: $2000\,\text{Hz}$, $3000\,\text{Hz}$, $4000\,\text{Hz}$, $6000\,\text{Hz}$, and $8000\,\text{Hz}$.
3. Mandatory 1000 Hz Retest (Internal Reliability Check): Before evaluating low frequencies, retest threshold at $1000\,\text{Hz}$ in the first ear tested.
   • Acceptable Reliability: The retest threshold must agree with the initial threshold within $\pm 5\,\text{dB}$.
   • Recording Rule: When the two thresholds agree within $\pm 5\,\text{dB}$, record the lower (better) one. If they differ by more than $5\,\text{dB}$, ASHA (2005) allows the lower threshold to be accepted but calls for retesting at least one other frequency; also reinstruct the patient and check transducer placement.
4. Low-Frequency Progression: Proceed in descending order: $500\,\text{Hz}$, $250\,\text{Hz}$, and (when indicated) $125\,\text{Hz}$.
5. Contralateral Ear Sequence: Test the second ear starting at $1000\,\text{Hz}$, proceeding to high frequencies, and finishing with low frequencies. The $1000\,\text{Hz}$ retest is not required in the second ear unless responses appear erratic or questionable.

Inter-Octave Testing Rule

Standard octave frequencies include $250, 500, 1000, 2000, 4000,\,\text{and } 8000\,\text{Hz}$. Inter-octave testing ($750, 1500, 3000,\,\text{and } 6000\,\text{Hz}$) is clinically mandatory under the following conditions:

• The $\ge 20\,\text{dB}$ Rule: Whenever a threshold difference of $20\,\text{dB}$ or greater is observed between two adjacent octave frequencies (e.g., $1000\,\text{Hz} = 20\,\text{dB HL}$ and $2000\,\text{Hz} = 45\,\text{dB HL}$), the intermediate inter-octave frequency ($1500\,\text{Hz}$) must be tested.
• Hearing Aid Fittings: Testing $1500\,\text{Hz}$ and $3000\,\text{Hz}$ is standard when prescribing modern multi-channel hearing aids to ensure accurate targets.
• Noise-Induced Hearing Loss & Ototoxicity: Testing $3000\,\text{Hz}$ and $6000\,\text{Hz}$ is routine in occupational noise surveillance (OSHA/NIOSH standards) and ototoxicity monitoring protocols because acoustic trauma notches and drug toxicity frequently emerge between $3000$ and $6000\,\text{Hz}$ before manifesting at standard octaves.

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Transducer Selection & Acoustic Coupling Mechanics

The selection of acoustic transducers dictates calibration standards, ambient noise attenuation, collapsed canal vulnerability, and cross-hearing thresholds.

Insert Earphones (Etymotic ER-3A / ER-5C / 3C)

Insert earphones couple to the patient's external auditory canal via disposable expandable foam or silicon eartips.

• Interaural Attenuation (IA): Provides much greater interaural attenuation than supra-aural earphones (conservatively about $70\,\text{dB}$ at $1000\,\text{Hz}$ and below and about $50\,\text{dB}$ above $1000\,\text{Hz}$ with deep insertion). This greatly reduces how often masking is needed in asymmetrical losses.
• Canal Collapse Prevention: Deep insertion mechanically stents the cartilaginous external meatus, completely preventing collapsed ear canal artifacts.
• Occlusion Effect Reduction: Deep insertion into the bony canal dramatically minimizes the occlusion effect ($0\,\text{dB}$ at $\ge 500\,\text{Hz}$).
• Ambient Noise Attenuation: Superior passive low-frequency ambient noise rejection compared to supra-aural earphones, facilitating valid testing in non-ideal sound suites.
• Contraindications: Obstructing cerumen impaction, active otorrhea/exudate, canal atresia or severe stenosis, acute otitis externa, or unhealed post-surgical incisions.

Supra-Aural Headphones (Telephonics TDH-39 / TDH-49 / TDH-50)

Supra-aural earphones consist of dynamic transducers housed in MX-41/AR cushions resting directly on the pinnae.

• Interaural Attenuation (IA): Conservative standard $\text{IA} = 40\,\text{dB}$ across all frequencies.
• Ear Canal Collapse: Cushion headband tension compresses the pinna against the mastoid, causing the posterior cartilaginous meatal wall to collapse over the canal opening in elderly individuals (loss of cartilage elasticity) and infants. This produces an artificial, pseudo-conductive air-bone gap of $15-35\,\text{dB}$ localized to $1000-4000\,\text{Hz}$.
• Sanitation & Occlusion: Higher occlusion effect ($30\,\text{dB}$ at 250 Hz, $20\,\text{dB}$ at 500 Hz) and lower ambient noise attenuation.

Circumaural Headphones (Sennheiser HDA 200 / HDA 300)

Circumaural earphones feature large earcups with cushions that completely encircle the pinnae, resting against the cranium.

• Application: Standard for extended high-frequency audiometry ($8000-20000\,\text{Hz}$) used in ototoxicity surveillance, cisplatin/aminoglycoside monitoring, and advanced tinnitus evaluations.

Bone Conduction Transducers (Radioear B-71 / B-81)

Bone vibrators convert electrical signals into mechanical vibrations to directly stimulate the temporal bones and cochlear fluids.

• Frequency Limits: Testing is restricted to $250-4000\,\text{Hz}$. Frequencies above $4000\,\text{Hz}$ are excluded due to severe harmonic distortion and physical output limits of magnetic transducers.
• Interaural Attenuation: Bone conduction $\text{IA} = 0\,\text{dB}$ across the entire skull. Contralateral masking is required to isolate ear-specific bone conduction whenever an unmasked air-bone gap exceeds $10\,\text{dB}$.

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Bone Conduction Mechanics: Mastoid vs. Forehead Placement

Bone conduction sound energy reaches the organ of Corti via three physical pathways:

1. Compressional / Distortional Bone Conduction: Skull vibrations alternate between compression and expansion, distorting the bony otic capsule and cyclically compressing the inner ear perilymph. Because the scala vestibuli has greater volume than the scala tympani and the round window is 20 times more compliant than the oval window, fluid displacement creates a traveling wave along the basilar membrane.
2. Inertial Bone Conduction: The ossicular chain is suspended elastically within the middle ear cavity. Cranial vibration moves the temporal bone around the ossicles; due to inertia, the ossicles lag behind, generating relative stapedial footplate displacement at the oval window.
3. Osseotympanic Bone Conduction: Cranial vibration vibrates the cartilaginous and bony walls of the external auditory meatus, radiating acoustic sound pressure into the canal. In an unoccluded ear, this energy escapes into the room; in an occluded ear, it drives the tympanic membrane.

Mastoid Placement vs. Forehead Placement

Clinical Recommendation: Mastoid placement is universally favored in diagnostic practice because its lower RETSPLs provide $10-15\,\text{dB}$ greater testable dynamic range, allowing accurate evaluation of severe sensorineural thresholds before reaching transducer output limits.

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Pure-Tone Averages (PTAs) & Clinical Applications

Pure-tone averages synthesize discrete thresholds into a single numerical index for speech cross-checks, disability ratings, and medical classifications.

1. Standard 3-Frequency Pure-Tone Average (PTA)

The standard PTA is the arithmetic mean of air-conduction thresholds at $500\,\text{Hz}$, $1000\,\text{Hz}$, and $2000\,\text{Hz}$: $\text{PTA}_{3} = \frac{\text{AC}_{500} + \text{AC}_{1000} + \text{AC}_{2000}}{3}$

• Speech Recognition Threshold (SRT) Cross-Check: The standard PTA must agree with the measured SRT within $\pm 6-8\,\text{dB}$. A discrepancy $> 10-12\,\text{dB}$ suggests non-organic hearing loss (pseudohypacusis), auditory neuropathy, or cognitive/linguistic barriers.

2. Two-Frequency Fletcher Average (Best-Two PTA)

The Fletcher average is calculated by taking the arithmetic mean of the two best (lowest threshold) frequencies among $500\,\text{Hz}$, $1000\,\text{Hz}$, and $2000\,\text{Hz}$: $\text{PTA}_{\text{Fletcher}} = \frac{\text{Best Threshold}_1 + \text{Best Threshold}_2}{2}$

• Clinical Indication: Utilized in steeply sloping, precipitous, or ski-slope high-frequency hearing loss, where thresholds at $2000\,\text{Hz}$ drop precipitously by $\ge 20-30\,\text{dB}$ relative to $500$ and $1000\,\text{Hz}$. In such patients, the standard 3-frequency PTA is skewed artificially poor, whereas the Fletcher average aligns accurately with the SRT.

3. High-Frequency Pure-Tone Average (HF-PTA)

The high-frequency PTA averages thresholds at $1000\,\text{Hz}$, $2000\,\text{Hz}$, and $4000\,\text{Hz}$ (or $1000, 2000, 3000\,\text{Hz}$ in industrial audiometry): $\text{HF-PTA} = \frac{\text{AC}_{1000} + \text{AC}_{2000} + \text{AC}_{4000}}{3}$

• Clinical Indication: Predicts speech perception difficulty in competing background noise and monitors noise-induced hearing loss, presbycusis, and ototoxic deterioration.

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Audiometric Interpretation: Degree, Configuration & Type

Degree of Hearing Loss (ANSI / Clark / Goodman Scales)

Hearing loss degree is defined by pure-tone air-conduction thresholds across the speech frequency spectrum.

Pediatric Imperative: The pediatric cutoff for normal hearing is strictly $15\,\text{dB HL}$. Thresholds between $16$ and $25\,\text{dB HL}$ in children represent minimal/slight hearing loss, which compromises phonological awareness, academic performance, and reading comprehension.

Audiogram Configurations

• Flat: Thresholds do not vary by more than $5-10\,\text{dB}$ across octaves.
• Sloping: Thresholds worsen gradually across octave frequencies ($5-10\,\text{dB/octave}$). Characteristic of sensory presbycusis and metabolic hearing loss.
• Precipitous / Steeply Sloping: Thresholds drop sharply by $\ge 20\,\text{dB}$ per octave. Classic for acoustic trauma, ototoxicity, or apical-to-basal lesions.
• Rising: Low-frequency thresholds are poorer than high-frequency thresholds. Classic for early Ménière's disease (endolymphatic hydrops) or middle-ear stiffness pathologies (otosclerosis, Eustachian tube dysfunction).
• Cookie-Bite / Trough-Shaped: Mid-frequency thresholds ($500-2000\,\text{Hz}$) are abnormal, with preserved low and high frequencies. Often hereditary, for example autosomal dominant TECTA-related loss (DFNA8/12).
• Reverse Cookie-Bite / Saucer: Mid-frequency hearing is preserved, while low and high frequencies are impaired.
• Acoustic Notch: Sharp drop in thresholds at $3000\,\text{Hz}$, $4000\,\text{Hz}$, or $6000\,\text{Hz}$, with significant recovery at $8000\,\text{Hz}$. Pathognomonic for noise-induced hearing loss (NIHL), driven by outer ear acoustic resonance ($2.5-3.5\,\text{kHz}$) focusing high sound pressure on the 4 kHz cochlear partition.
• Corner Audiogram: Responses obtained only at the lowest frequencies ($250-500\,\text{Hz}$) at the upper limits of the audiometer ($90-110\,\text{dB HL}$), with no response at all remaining frequencies.

Type of Hearing Loss

The relationship between air-conduction (AC) and bone-conduction (BC) thresholds determines the pathology type:

1. Sensorineural Hearing Loss (SNHL):

   • Air-conduction thresholds: Abnormal ($> 25\,\text{dB HL}$ in adults, $> 15\,\text{dB HL}$ in children).
   • Bone-conduction thresholds: Abnormal ($> 25\,\text{dB HL}$ in adults, $> 15\,\text{dB HL}$ in children).
   • Air-Bone Gap (ABG): Absent or clinically insignificant ($\text{ABG} \le 10\,\text{dB}$).
   • Pathophysiology: Outer and middle ear transmission is intact. Deficit resides in the cochlea (sensory, outer/inner hair cells) or VIII cranial nerve (neural).

2. Conductive Hearing Loss (CHL):

   • Air-conduction thresholds: Abnormal ($> 25\,\text{dB HL}$ in adults, $> 15\,\text{dB HL}$ in children).
   • Bone-conduction thresholds: Completely normal ($\text{BC} \le 25\,\text{dB HL}$ in adults, $\le 15\,\text{dB HL}$ in children).
   • Air-Bone Gap (ABG): Clinically significant ($\text{ABG} \ge 15\,\text{dB}$).
   • Pathophysiology: Cochlear and retrocochlear structures are intact. Obstruction or mechanical lesion in the external canal, tympanic membrane, or ossicular chain (cerumen, otitis media, otosclerosis, ossicular discontinuity).

3. Mixed Hearing Loss (MHL):

   • Air-conduction thresholds: Abnormal ($> 25\,\text{dB HL}$ in adults, $> 15\,\text{dB HL}$ in children).
   • Bone-conduction thresholds: Abnormal ($> 25\,\text{dB HL}$ in adults, $> 15\,\text{dB HL}$ in children).
   • Air-Bone Gap (ABG): Clinically significant ($\text{ABG} \ge 15\,\text{dB}$).
   • Pathophysiology: Concurrent presence of sensorineural damage in the inner ear/nerve and conductive pathology in the outer/middle ear (e.g., presbycusis complicated by otitis media with effusion, or advanced otosclerosis with cochlear involvement).

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Diagnostic Pitfalls & Vibrotactile Artifacts

Low-Frequency Vibrotactile Responses

At high stimulus intensities, bone vibrators (and occasionally supra-aural headphones) generate strong physical mechanical vibrations that activate somatosensory mechanoreceptors (cutaneous Pacinian and Meissner corpuscles) in the periosteum and skin, rather than stimulating the auditory hair cells.

• Vibrotactile Threshold Limits:
  • $250\,\text{Hz}$: Tactile responses occur at $\ge 40-45\,\text{dB HL}$.
  • $500\,\text{Hz}$: Tactile responses occur at $\ge 55-60\,\text{dB HL}$.
  • $1000\,\text{Hz}$: Tactile responses occur at $\ge 70-75\,\text{dB HL}$.

The Severe Diagnostic Trap: A patient with bilateral profound sensorineural hearing loss (true hearing threshold $> 100\,\text{dB HL}$) is tested with unmasked bone conduction. At $250\,\text{Hz}$, the patient feels the vibrator buzzing at $45\,\text{dB HL}$ and presses the response button. If the audiologist plots $45\,\text{dB HL}$ as a true hearing threshold, it creates a false $55\,\text{dB}$ air-bone gap ($100\,\text{dB AC} - 45\,\text{dB BC}$), misdiagnosing profound SNHL as a mixed hearing loss. The clinician must explicitly ask the patient: "Did you hear a tone in your ear, or did you only feel a vibrating or buzzing sensation on your head?" Tactile responses must be documented on the audiogram with a tactile symbol ("V" or "T") or annotated as "no response at maximum output."

Ear Canal Collapse

Canal collapse occurs when pressure from supra-aural headphone cushions pinches the cartilaginous meatus closed. This creates an artificial conductive component of $15-30\,\text{dB}$ across $1000-4000\,\text{Hz}$. Always verify suspicious high-frequency air-bone gaps by switching to insert earphones, which physically stent the canal open.