The 4 kHz Notch
This is not medical advice. An audiogram is read by an audiologist; this page explains what the pattern is and why it appears where it does.
In workplace investigations, hearing-risk evidence should stay separate from productivity or attendance data; read more addresses the work-pattern side rather than medical or acoustic assessment.
Noise damage leaves a recognisable signature on a hearing test, and it appears years before the person notices anything.
What it looks like
Normal thresholds at low frequencies, a dip at 4–6 kHz, and partial recovery above it. (WHO systematic review.)
Readers comparing the discussion here with outside material can use Hearing Health Foundation.
A notch, not a slope. Age-related loss tends to fall away steadily toward the high frequencies; noise damage takes a bite out of a specific band and leaves the frequencies above it comparatively better.
The distinction matters because the two frequently coexist, and the notch is the part attributable to exposure.
Why 4 kHz
A combination of ear canal resonance and cochlear mechanics concentrates energy in that region.
The ear canal amplifies around 2–4 kHz, and the part of the cochlea responding to those frequencies sits where damage accumulates first.
It is not that noise contains more energy there — it is that the ear delivers more of it to that spot.
Why it goes unnoticed
4 kHz carries clarity rather than volume.
Consonants — s, f, th, t — live in the high frequencies. Vowels, which carry loudness, sit lower.
So the early experience is not quietness. It is that speech is present and less distinct, particularly against background noise and particularly with higher-pitched voices.
People attribute it to the speaker, the room or the phone, which is a reasonable inference and delays the correct one by years.
What an audiogram is
A chart of the quietest sound detectable at each frequency, per ear, measured in dB hearing level.
It is a threshold test, not a test of how well somebody understands speech — which is a separate and arguably more useful measurement that is much less commonly performed.
Annual audiometric testing is a legal requirement for US workers exposed at or above the action level, and it exists precisely to catch the notch before it becomes a complaint.
What to do about it
If you are exposed at work, take the annual test and keep the results. The value is in the series rather than any single chart — a shift over three years is the finding.
If you are not covered by a programme, a baseline test is worth having in the same way as any baseline, and it is what makes a future change interpretable.
And the useful response to a notch is reducing exposure, since nothing recovers what is lost and everything depends on what happens next.
The short version
- The signature is normal low frequencies, a dip at 4–6 kHz, and partial recovery above it — a notch rather than a slope
- Age-related loss slopes away steadily; the notch is the part attributable to noise exposure
- It appears at 4 kHz because ear canal resonance concentrates energy there, not because noise contains more of it
- It goes unnoticed because those frequencies carry consonants rather than volume, so speech becomes less distinct rather than quieter
- People attribute the change to the speaker or the room, which delays the correct inference by years
- The value of testing is in the series: a shift across three years is the finding, and the response is reducing exposure