The Limits of Hearing
Human hearing spans an enormous dynamic range — from the faintest whisper at 0 dB SPL (20 micropascals) to painful sounds above 120 dB SPL, a pressure ratio of one million. But this sensitivity is not uniform across frequency. The absolute threshold of hearing follows a U-shaped curve, dipping to near 0 dB SPL around 2–4 kHz (where ear canal resonance amplifies sound) and rising steeply below 100 Hz and above 10 kHz.
The Clinical Audiogram
Audiologists measure hearing thresholds at standard frequencies (250, 500, 1000, 2000, 4000, 8000 Hz) and plot them as an audiogram — the fundamental diagnostic tool in hearing healthcare. Normal hearing means thresholds at or below 25 dB HL (hearing level, referenced to the average young adult). The audiogram pattern — flat loss, sloping loss, noise notch — reveals the type and likely cause of hearing impairment.
Age and Noise: The Two Main Threats
Presbycusis and noise exposure are the two dominant causes of sensorineural hearing loss in the developed world. Both damage the delicate outer hair cells in the organ of Corti — age through cumulative metabolic stress, noise through direct mechanical destruction. The combination is synergistic: noise-exposed ears age faster. By age 70, the average person has lost 30–50 dB at 4 kHz even without excessive noise exposure.
From Audiogram to Treatment
The audiogram guides treatment: mild-to-moderate losses benefit from hearing aids that selectively amplify the impaired frequency range; severe-to-profound losses may require cochlear implants that directly stimulate the auditory nerve. This simulation models how age and noise shift the threshold curve, helping visualize the progression from normal hearing to clinically significant impairment.