4.4 Audiometric Synthesis: Degree, Configuration & Type of Loss
Key Takeaways
- Standard ANSI S3.21 symbology strictly governs clinical charting: red circles and blue X's indicate unmasked air conduction (right and left), red triangles and blue squares designate masked air conduction, while brackets ([ and ]) and carats (< and >) designate masked and unmasked bone conduction.
- Hearing loss severity in adults is classified under the Goodman/ASHA scale into Normal (-10 to 25 dB HL), Mild (26-40 dB HL), Moderate (41-55 dB HL), Moderately-Severe (56-70 dB HL), Severe (71-90 dB HL), and Profound (91+ dB HL).
- The Air-Bone Gap (ABG = AC - BC) defines the diagnostic type of hearing loss: Conductive (normal BC <= 25 dB, abnormal AC, ABG > 10 dB), Sensorineural (abnormal AC and BC within 10 dB of each other), and Mixed (abnormal BC > 25 dB, worse AC, ABG > 10 dB).
- The standard Pure-Tone Average (PTA) is the arithmetic mean of AC thresholds at 500, 1000, and 2000 Hz and must agree with the Speech Recognition Threshold (SRT) within ±6 dB; the Fletcher two-frequency PTA averages the best two frequencies to resolve discrepancies in steeply sloping configurations.
- Specific audiogram contours correlate with underlying pathology: a 4000 Hz acoustic notch indicates noise-induced damage, an upward-sloping low-frequency loss indicates early Meniere's disease, and Carhart's notch at 2000 Hz in bone conduction reflects stapedial otosclerosis.
4.4 Audiometric Synthesis: Degree, Configuration & Type of Loss
Diagnostic pure-tone audiometry culminates in the synthesis of air conduction and bone conduction thresholds into three fundamental diagnostic dimensions: degree of hearing loss, audiometric configuration, and type of hearing impairment. Correctly interpreting these dimensions enables the Hearing Instrument Specialist to identify otologic pathology requiring medical or surgical referral, counsel patients with acoustic precision, and establish evidence-based prescriptive amplification parameters.
The Audiogram: Graphic Architecture and ANSI S3.21 Symbology
The audiogram is a standardized Cartesian coordinate grid displaying hearing threshold levels as a function of acoustic frequency. According to ANSI S3.21, the graphic display adheres to a standardized aspect ratio: one octave along the horizontal frequency axis (abscissa) must correspond exactly to a $20\text{ dB}$ interval along the vertical hearing level axis (ordinate). Maintaining this fixed $1\text{ octave : }20\text{ dB}$ aspect ratio ensures that audiometric slope contours (e.g., sloping, precipitous) appear visually consistent across all diagnostic software and clinical printouts.
THE AUDIOGRAM GRID
Frequency in Hertz (Hz) -> Logarithmic Scale (Octaves)
125 250 500 1000 2000 4000 8000 Hz
-10 +------+------+------+------+------+------+------+ Normal
0 | | | | | | | | Range
10 | | | | | | | | (-10 to 25 dB)
20 | | | | | | | |
30 +------+------+------+------+------+------+------+ Mild (26-40)
40 | | | | | | | |
50 +------+------+------+------+------+------+------+ Moderate (41-55)
60 | | | | | | | |
70 +------+------+------+------+------+------+------+ Mod-Severe (56-70)
80 | | | | | | | |
90 +------+------+------+------+------+------+------+ Severe (71-90)
100 | | | | | | | |
110 +------+------+------+------+------+------+------+ Profound (91+)
120 | | | | | | | |
ANSI S3.21 Standard Symbology
To eliminate diagnostic confusion, standardized symbols, colors, and placement conventions must be followed. The universal color rule is: RED = RIGHT EAR and BLUE = LEFT EAR.
| Transducer & Condition | Right Ear (Red) | Left Ear (Blue) | Unspecified / Soundfield |
|---|---|---|---|
| Air Conduction (Unmasked) | O (Circle) | X (Cross) | — |
| Air Conduction (Masked) | $\Delta$ (Triangle) | $\square$ (Square) | — |
| Bone Conduction (Unmasked, Mastoid) | $<$ (Open to right) | $>$ (Open to left) | — |
| Bone Conduction (Masked, Mastoid) | $[$ (Open to right) | $]$ (Open to left) | — |
| Bone Conduction (Unmasked, Forehead) | — | — | $\wedge$ (Inverted V) |
| Bone Conduction (Masked, Forehead) | — | — | $\lceil$ or $\rceil$ (Bracket) |
| Soundfield (Unmasked) | — | — | S |
| Soundfield (Aided with Hearing Instruments) | — | — | A |
| Soundfield (Cochlear Implant) | — | — | CI |
| No Response (At maximum equipment limit) | $\searrow$ (Arrow attached at $45^\circ$) | $\swarrow$ (Arrow attached at $45^\circ$) | Attached to symbol |
Symbol Placement Rules:
- Right bone symbols (< or [) are drawn immediately to the LEFT of the frequency line.
- Left bone symbols (> or ]) are drawn immediately to the RIGHT of the frequency line.
- When air and bone thresholds overlap at the same intensity, the bone symbol is drawn
adjacent to the air circle or X without obscuring it.
- If no response is obtained at the audiometer's maximum output, an arrow pointing
downward at a 45-degree angle is appended to the bottom-corner of the symbol.
Classification of Hearing Loss by Degree (ASHA / Goodman Scale)
The clinical severity of hearing loss is categorized using the established Goodman (1965) / ASHA classification system. Degree of loss quantifies the patient's functional auditory deficit and loss of conversational speech audibility:
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| Hearing Loss Severity Categories |
+-------------------------------------------------------------+
| -10 to 15 dB HL: Normal Hearing (Pediatric Cutoff) |
| 16 to 25 dB HL: Slight / Minimal Loss (Adult Normal) |
| 26 to 40 dB HL: Mild Hearing Loss |
| 41 to 55 dB HL: Moderate Hearing Loss |
| 56 to 70 dB HL: Moderately-Severe Hearing Loss |
| 71 to 90 dB HL: Severe Hearing Loss |
| 91+ dB HL: Profound Hearing Loss (Deafness) |
+-------------------------------------------------------------+
| Severity Level | Threshold Range (dB HL) | Percentage of Speech Missed | Functional & Communicative Impact |
|---|---|---|---|
| Normal (Adult) | $-10\text{ to }25\text{ dB}$ | $0%$ | Effortless communication in quiet; subtle noise challenges |
| Slight (Child) | $16\text{ to }25\text{ dB}$ | $10\text{–}15%$ | Misses soft consonants; academic difficulty in school classrooms |
| Mild | $26\text{ to }40\text{ dB}$ | $25\text{–}40%$ | Misses soft speech, whispering, and distant voices; noise deficits |
| Moderate | $41\text{ to }55\text{ dB}$ | $50\text{–}75%$ | Misses normal conversational speech; requires visual cues and amplification |
| Moderately-Severe | $56\text{ to }70\text{ dB}$ | $80\text{–}100%$ | Cannot hear normal conversation; understands only loud or shouted speech |
| Severe | $71\text{ to }90\text{ dB}$ | $100%$ | Cannot hear shouted speech; perceives only loud environmental noises |
| Profound | $91+\text{ dB}$ | $100%$ | Deafness; relies on speechreading, tactile cues, or cochlear implants |
Pediatric vs. Adult Normal Boundary: In adults, thresholds up to $25\text{ dB HL}$ are classified as clinically normal because adults possess fully developed language, vocabulary, and top-down cognitive closure. In children, however, the normal cutoff is strictly set at $15\text{ dB HL}$. Thresholds between $16\text{ and }25\text{ dB HL}$ (classified as slight or minimal hearing loss) cause pediatric listeners to miss up to $15%$ of teacher instruction in reverberant classrooms, leading to documented academic deficits and delayed language acquisition.
Diagnostic Types of Hearing Loss: The Air-Bone Gap Criterion
The fundamental diagnostic classification of hearing impairment depends on the mathematical relationship between air conduction (AC) and bone conduction (BC) thresholds—specifically the Air-Bone Gap (ABG):
- An ABG of $0\text{ to }10\text{ dB}$ is considered within normal calibration tolerance and test-retest variability; it represents an insignificant gap.
- An ABG of $>10\text{ dB}$ (i.e., $\ge 15\text{ dB}$) is clinically significant and indicates the presence of a mechanical conductive pathology in the outer or middle ear.
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| The Three Audiometric Types of Loss |
+-------------------------------------------------------------+
| 1. Conductive Hearing Loss (CHL): |
| - Bone Conduction is NORMAL (BC <= 25 dB HL) |
| - Air Conduction is ABNORMAL (AC > 25 dB HL) |
| - Air-Bone Gap is SIGNIFICANT (ABG > 10 dB) |
| |
| 2. Sensorineural Hearing Loss (SNHL): |
| - Bone Conduction is ABNORMAL (BC > 25 dB HL) |
| - Air Conduction is ABNORMAL (AC > 25 dB HL) |
| - Air-Bone Gap is ABSENT (ABG <= 10 dB; AC == BC) |
| |
| 3. Mixed Hearing Loss (MHL): |
| - Bone Conduction is ABNORMAL (BC > 25 dB HL) |
| - Air Conduction is WORSE (AC > BC) |
| - Air-Bone Gap is SIGNIFICANT (ABG > 10 dB) |
+-------------------------------------------------------------+
1. Conductive Hearing Loss (CHL)
- Diagnostic Profile: Bone conduction is normal ($\le 25\text{ dB HL}$); air conduction is depressed ($>25\text{ dB HL}$); $ABG \ge 15\text{ dB}$.
- Site of Lesion: Outer ear (cerumen impaction, atresia, foreign body, canal collapse) or middle ear (otitis media with effusion, perforated TM, otosclerosis, ossicular discontinuity, cholesteatoma).
- Audiologic Characteristics: The cochlea and auditory nerve are pristine. Word recognition scores (WRS) typically reach $95\text{ to }100%$ once speech is amplified sufficiently to overcome the mechanical conductive barrier. The patient experiences simple sound attenuation without significant distortion, recruitment, or phonemic regression. Often treatable medically or surgically.
2. Sensorineural Hearing Loss (SNHL)
- Diagnostic Profile: Both air conduction and bone conduction are abnormal ($>25\text{ dB HL}$) and agree within $10\text{ dB}$ of each other ($ABG \le 10\text{ dB}$). Air and bone thresholds track together.
- Site of Lesion: Cochlea (outer/inner sensory hair cells) or retrocochlear structures (cranial nerve VIII, cerebellopontine angle, central auditory nervous system).
- Audiologic Characteristics: Caused by presbycusis, acoustic trauma, ototoxicity, genetic conditions, or acoustic neuroma. Permanent impairment. Accompanied by abnormal loudness growth (recruitment), reduced speech clarity, and degraded word recognition in background noise. The primary candidate for hearing aid amplification.
3. Mixed Hearing Loss (MHL)
- Diagnostic Profile: Bone conduction is abnormal ($>25\text{ dB HL}$), air conduction is abnormal and significantly worse than bone conduction, and a significant air-bone gap exists ($ABG \ge 15\text{ dB}$).
- Site of Lesion: Concurrent presence of both a sensorineural pathology in the cochlea and a conductive pathology in the outer or middle ear.
- Clinical Examples: A patient with long-standing age-related sensory presbycusis who develops acute otitis media with effusion; or advanced otosclerosis where stapedial ankylosis has invaded the cochlear otic capsule (cochlear otosclerosis).
Audiometric Configurations and Diagnostic Contours
The geometrical "shape" or contour of the air conduction thresholds across the frequency spectrum provides powerful diagnostic clues regarding the underlying otologic etiology:
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| Common Audiometric Configurations |
+-------------------------------------------------------------+
| 1. Flat: Thresholds across all octaves vary by <=20 dB |
| 2. Sloping: Thresholds worsen by 5-10 dB per octave |
| 3. Steeply Sloping (Ski-slope): Thresholds drop >=20 dB/oct|
| 4. Rising (Reverse Slope): Lows worse than highs |
| 5. Trough (Cookie-Bite): Mid-frequency dip (1-2 kHz) |
| 6. Acoustic Noise Notch: Sharp drop at 4 kHz, 8 kHz recovery|
| 7. Carhart's Notch: BC dip at 2 kHz in stapedial fixation |
+-------------------------------------------------------------+
| Configuration | Definition & Audiometric Profile | Typical Underlying Etiologies |
|---|---|---|
| Flat | Thresholds across 250–8000 Hz differ by no more than $20\text{ dB}$ | Metabolic (strial) presbycusis; otitis media with effusion; genetic loss |
| Sloping | Thresholds worsen gradually toward high frequencies ($5\text{–}10\text{ dB/octave}$) | Classic age-related sensory presbycusis; gradual progressive SNHL |
| Steeply Sloping (Ski-Slope) | Thresholds worsen precipitously by $\ge 20\text{ dB}$ per octave | Severe sensory presbycusis; extensive basal outer hair cell loss; dead regions |
| Precipitous | Normal low/mid frequencies followed by an abrupt, cliff-like drop | Ototoxicity (aminoglycosides); severe acoustic trauma |
| Rising (Reverse Slope) | Low-frequency thresholds are poorer than high-frequency thresholds | Early Meniere's disease (endolymphatic hydrops); middle ear stiffness |
| Trough-Shaped ("Cookie-Bite") | Mid-frequency depression ($1000\text{–}2000\text{ Hz}$) with better lows and highs | Congenital, non-syndromic genetically inherited sensorineural hearing loss |
| Tented (Inverted Cookie-Bite) | Thresholds best at $1000\text{–}2000\text{ Hz}$, poorer at lows and highs | Rare congenital or vascular genetic etiologies |
| Acoustic Noise Notch | Sharp threshold drop at $3000\text{–}6000\text{ Hz}$ (peaking at $4000\text{ Hz}$) with recovery at $8000\text{ Hz}$ | Noise-Induced Hearing Loss (NIHL); occupational noise or gunfire |
| Carhart's Notch | Artificial dip in bone conduction centered at $2000\text{ Hz}$ ($15\text{ dB}$) | Stapedial Otosclerosis (Mechanical disruption of inertial ossicular mode) |
Pathophysiological Focus: The 4 kHz Noise Notch vs. Carhart's Notch
- The 4 kHz Acoustic Notch: Chronic industrial noise or acoustic blast trauma damages outer hair cells centered at $4000\text{ Hz}$ with partial or complete recovery at $8000\text{ Hz}$. This specific localization occurs because the external auditory canal exhibits a natural acoustic resonance peaking between $2500\text{ and }3500\text{ Hz}$. Energy entering the ear is amplified by $15\text{ to }20\text{ dB}$ at this resonant peak; basilar membrane fluid dynamics displace the point of maximum mechanical shearing stress approximately one-half octave above the resonant frequency, directly targeting the 4000 Hz cochlear region.
- Carhart's Notch: In stapedial otosclerosis, bone conduction thresholds display an apparent sensorineural loss centered at $2000\text{ Hz}$ (typically $5\text{ dB}$ at $500\text{ Hz}$, $10\text{ dB}$ at $1000\text{ Hz}$, $15\text{ dB}$ at $2000\text{ Hz}$, and $5\text{ dB}$ at $4000\text{ Hz}$). This is an artifactual mechanical depression caused by stapes footplate fixation, which abolishes the inertial ossicular mode of bone conduction (the ossicular chain resonates at ~1500–2000 Hz). Following successful surgical stapedectomy, Carhart's notch resolves, restoring bone conduction thresholds to normal baseline.
Pure-Tone Average (PTA) & The Fletcher Two-Frequency Exception
The Pure-Tone Average (PTA) is a single metric summarizing air conduction hearing sensitivity across the primary speech frequencies. It serves as an indispensable clinical cross-check against speech audiometry.
1. Standard 3-Frequency Pure-Tone Average ($PTA_3$)
The standard Pure-Tone Average is calculated as the arithmetic mean of air conduction thresholds at $500\text{ Hz}, 1000\text{ Hz}, \text{and }2000\text{ Hz}$:
2. The Speech Cross-Check Rule
In diagnostic audiometry, the PTA must agree with the Speech Recognition Threshold (SRT) within $\pm 6\text{ to }7\text{ dB}$ (frequently stated as within $\pm 5\text{ to }8\text{ dB}$; discrepancies $>10\text{ dB}$ are abnormal).
- Good Agreement ($PTA \approx SRT$): Confirms high patient test reliability, authentic biological thresholds, and accurate equipment calibration.
- Discrepancy ($PTA - SRT > 10\text{ dB}$): If the SRT is substantially better than the PTA (e.g., $PTA = 50\text{ dB HL}$, but $SRT = 20\text{ dB HL}$), the clinician must suspect pseudohypacusis (malingering, non-organic hearing loss), equipment miscalibration, or an asymmetrical sloping configuration.
3. The Fletcher Two-Frequency PTA (Best-Two PTA)
When an audiogram slopes steeply—defined as a threshold difference of $\ge 20\text{ dB}$ between adjacent octave frequencies—the standard 3-frequency PTA becomes artificially skewed by the high-frequency drop, creating an apparent, false discrepancy with the SRT.
Clinical Dilemma Example:
- 500 Hz = 20 dB HL
- 1000 Hz = 30 dB HL
- 2000 Hz = 70 dB HL (Steep 40 dB ski-slope drop between 1 and 2 kHz)
- Patient's Measured SRT = 25 dB HL
Standard 3-Frequency PTA Calculation:
PTA_3 = (20 + 30 + 70) / 3 = 120 / 3 = 40 dB HL
Discrepancy: PTA (40 dB) - SRT (25 dB) = 15 dB! (Appears to fail reliability check!)
Fletcher Two-Frequency PTA Resolution:
Harvey Fletcher established that speech understanding in steep slopes is mediated
almost entirely by the two best-hearing frequencies.
Best two frequencies = 500 Hz (20 dB) and 1000 Hz (30 dB)
PTA_Fletcher = (20 + 30) / 2 = 50 / 2 = 25 dB HL!
Agreement: PTA_Fletcher (25 dB) == SRT (25 dB) -> PERFECT CLINICAL AGREEMENT!
Exam Alert: On board certification exams, candidates are routinely presented with a steeply sloping audiogram and asked to calculate the Pure-Tone Average that correctly cross-checks with the patient's SRT. Candidates who blindly average 500, 1000, and 2000 Hz fail the question; candidates must recognize the $\ge 20\text{ dB}$ shift and apply Fletcher's two-frequency rule (averaging the lowest two threshold values among 500, 1000, and 2000 Hz).
Audiometric Synthesis: Case Formulations and Diagnostic Cross-Check
To synthesize audiometric data into a definitive clinical profile, the specialist systematically integrates all four dimensions:
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| Systematic Audiometric Diagnostic Statement |
+-------------------------------------------------------------+
| "[Ear] presents with a [Degree] [Configuration] |
| [Type] hearing impairment, characterized by an |
| Air-Bone Gap of [X] dB, with a 3-Frequency PTA of [Y] dB |
| agreeing within [Z] dB of the Speech Recognition Threshold."|
+-------------------------------------------------------------+
Representative Diagnostic Synthesis Matrix
| Patient Presentation | Air Conduction (500, 1k, 2k, 4k) | Bone Conduction (500, 1k, 2k, 4k) | Diagnostic Classification | Clinical Action |
|---|---|---|---|---|
| Case 1: Industrial Worker | $20, 25, 35, 65\text{ dB}$ | $15, 20, 30, 60\text{ dB}$ | Mild-to-severe 4 kHz notched SNHL | Hearing aid candidate; hearing conservation counsel |
| Case 2: 7-Year-Old Child | $35, 40, 40, 35\text{ dB}$ | $10, 5, 10, 5\text{ dB}$ | Mild flat Conductive Loss ($ABG \approx 30\text{ dB}$) | Medical referral to ENT; suspect otitis media with effusion |
| Case 3: Senior Citizen | $25, 35, 55, 75\text{ dB}$ | $20, 30, 50, 70\text{ dB}$ | Mild-to-severe sloping SNHL ($ABG \le 5\text{ dB}$) | Presbycusis; prescribe binaural RIC amplification |
| Case 4: 42-Year-Old Female | $45, 50, 50, 45\text{ dB}$ | $15, 15, 35, 15\text{ dB}$ | Moderate Conductive Loss with Carhart's Notch | Medical referral for otosclerosis / stapedectomy |
| Case 5: Advanced Smoker | $60, 65, 75, 80\text{ dB}$ | $35, 40, 50, 55\text{ dB}$ | Moderately-severe Mixed Loss ($ABG \approx 25\text{ dB}$) | Medical workup for conductive component; fit amplification |
An otoscopic inspection reveals normal tympanic membranes bilaterally. Pure-tone audiometry reveals bilateral air conduction thresholds of 45 dB HL across 500 to 2000 Hz, while bone conduction thresholds are 15 dB HL at 500 Hz, 15 dB HL at 1000 Hz, 30 dB HL at 2000 Hz, and 15 dB HL at 4000 Hz. What specific audiological phenomenon is observed at 2000 Hz, and what underlying pathology does it classically indicate?
An adult patient's diagnostic audiogram reveals the following pure-tone thresholds in the right ear: Air Conduction = 55 dB HL at 500 Hz, 60 dB HL at 1000 Hz, 65 dB HL at 2000 Hz; Bone Conduction = 35 dB HL at 500 Hz, 40 dB HL at 1000 Hz, 45 dB HL at 2000 Hz. What is the correct classification of the type and degree of hearing loss in this ear?
A patient presents with a steeply sloping sensorineural hearing loss with the following air conduction thresholds: 250 Hz = 15 dB HL, 500 Hz = 20 dB HL, 1000 Hz = 30 dB HL, 2000 Hz = 70 dB HL, 4000 Hz = 80 dB HL. The patient's Speech Recognition Threshold (SRT) is established at 25 dB HL. The standard 3-frequency Pure-Tone Average (PTA at 500, 1000, 2000 Hz) is 40 dB HL, creating a 15 dB discrepancy with the SRT. How should the Hearing Instrument Specialist resolve this clinical discrepancy?
According to standard ANSI S3.21 audiometric symbology conventions, which pair of symbols correctly depicts masked air conduction and masked mastoid bone conduction thresholds for the left ear?