Pure-tone testing, audiogram interpretation, and prescription formulas
SEER SONUS Research·
Figure · Reading the Audiogram
The audiogram plots frequency (250–8000 Hz) against intensity (-10 to 120 dB HL). Typical presbycusis slopes downward — lows preserved while 4000–8000 Hz drops sharply, so sounds are "heard but not understood."
Hearing Assessment Methods
A complete audiological evaluation is the foundation of hearing-aid fitting. The standard protocol includes the following tests:
Pure Tone Audiometry (PTA) — in a sound booth, thresholds are measured at 250–8000 Hz using air-conduction headphones and a bone-conduction oscillator. This is the most fundamental and important hearing test.
Speech Recognition Testing — covers the Speech Reception Threshold (SRT) and Word Recognition Score (WRS). WRS directly predicts hearing-aid outcomes: WRS > 80% usually signals good hearing-aid benefit.
Tympanometry — assesses eardrum compliance and middle-ear pressure, ruling out conductive pathology such as middle-ear effusion and otosclerosis.
Otoacoustic Emissions (OAE) — tests outer hair cell function, commonly used for newborn hearing screening and differential diagnosis.
Reading the Audiogram
X-axis (frequency): 250–8000 Hz, low pitch left to high pitch right. Most speech energy lies in 500–4000 Hz (the speech frequency range)
Y-axis (intensity): -10 to 120 dB HL, soft at top and loud at bottom. The lower the marks, the more severe the loss
[ ] △: bone-conduction thresholds (right/left) — the gap from air-conduction thresholds is the "air-bone gap"
Sample audiogram: bilateral sloping sensorineural hearing loss (a typical presbycusis pattern)
The classic high-frequency sloping audiogram above is the hallmark of presbycusis — low frequencies are relatively preserved while high frequencies (4000–8000 Hz) drop sharply. Clinically, patients can "hear sound but not understand it," because consonant recognition (especially high-frequency consonants like /s/, /f/, /θ/) is severely impaired.
Fitting Prescription Formulas
Prescription formulas are the core algorithms of hearing-aid fitting — they compute the required gain, compression ratio, and maximum output from the threshold at each frequency:
Formula
Developed by
Core philosophy
Primary population
Characteristics
NAL-NL2
Australian National Acoustic Laboratories
Maximize speech intelligibility
Mainly adults
Accounts for binaural effects and empirical data
DSL v5
University of Western Ontario, Canada
Ensure speech audibility
Mainly children
Higher gain, wide dynamic range
Real Ear Measurement (REM) is the gold standard for verifying that prescribed gain is achieved in the actual ear canal. Studies show REM-verified fittings improve speech understanding by 20–30% on average over fittings based on first-fit estimates. Yet only about 30% of fittings worldwide include REM.
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