Audiometry·NIHL·13 min read·Updated March 2026
If you run a hearing conservation program, you will see the 4000 Hz notch. It will appear on annual audiograms. Workers will ask what it means. Audiologists will reference it in their reports. And OSHA’s entire standard threshold shift calculation — the compliance mechanism that drives notifications, refitting, recordability, and workers’ compensation — is built around detecting it early. The 4 kHz notch is not an obscure clinical finding. It is the earliest measurable sign that occupational noise is damaging a worker’s cochlea. Understanding it precisely is one of the most practical things a safety manager can do.
Soundtrace audiometric surveillance detects the 4 kHz notch at its earliest appearance, flags STS automatically, and generates the documentation record your program needs for OSHA compliance and WC defense.
The One Sentence That MattersThe 4000 Hz notch is the first thing noise destroys and the last thing it lets go — it appears before any functional hearing loss, it deepens and widens with continued exposure, and it is still identifiable in the audiogram even in workers with advanced NIHL decades later.
What Is the 4000 Hz Notch?
On a standard pure-tone audiogram, hearing thresholds are plotted across six test frequencies: 500, 1000, 2000, 3000, 4000, and 6000 or 8000 Hz. The vertical axis shows hearing level in decibels (dB HL), with 0 at the top (best hearing) and higher numbers downward (worse hearing). In a person with normal hearing, thresholds at all frequencies fall near 0–25 dB HL.
The 4000 Hz notch is a characteristic pattern where the threshold at 4000 Hz is significantly worse than the thresholds at neighboring frequencies — particularly better thresholds at both 2000 Hz (below) and 8000 Hz (above). The threshold “dips” at 4000 Hz and then recovers at higher frequencies. This notch-and-recovery shape is the pathognomonic audiometric signature of noise-induced hearing loss.
Figure 1 — The 4 kHz Notch: Anatomy of the Pattern
Reading down = worse hearing. The notch dips at 4 kHz then recovers at 8 kHz. This recovery is the defining NIHL signature.
dB HL ↓ worse
500 Hz
1 kHz
2 kHz
4 kHz
6 kHz
8 kHz
Audiogram
500
1k
2k
4k ▼
6k
8k
Early notch
10
10
15
30 ▼
20
15 ▲
Classic notch
10
10
20
50 ▼
35
20 ▲ recovery
Wide notch
10
15
30
60 ▼
55
35 ▲ partial
Advanced
15
20
45
70 ▼
65
50 ▲ faint
Notch depth (4 kHz)
Recovery at 8 kHz (NIHL signature)
Partial recovery (widening notch)
The recovery at 8000 Hz is the diagnostic key. When thresholds at 8000 Hz are better than thresholds at 4000 Hz, the pattern is consistent with NIHL. This single feature distinguishes NIHL from age-related hearing loss (presbycusis), where 8000 Hz is typically the worst frequency tested.
Why 4000 Hz and Not Other Frequencies?
The consistency of the 4 kHz notch across industries, noise types, and populations is remarkable. A welder and a textile worker and a military veteran may have very different noise exposure histories, but their audiograms often show the same characteristic dip at 4000 Hz. This is not coincidence — it reflects three converging anatomical and physiological forces that make the 4 kHz cochlear region the most mechanically and metabolically vulnerable point in the human auditory system.
The Three Converging Forces at 4000 Hz
Figure 2 — Why 4 kHz Is the Vulnerable Zone
Three independent factors converge to make 4000 Hz the first frequency noise destroys.
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Peak Sensitivity
Human hearing is most sensitive between 1–5 kHz. Hair cells here are doing the most work and generate the most metabolic stress under loud exposure.
Max amplification = max fatigue
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Acoustic Reflex Gap
The stapedius muscle reflex attenuates loud sounds below ~2 kHz. The 3–6 kHz range is left largely unprotected by this biological defense.
Body protects speech; noise hits 4 kHz
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Cochlear Geometry
The basilar membrane’s 4 kHz region sits ~¼ from the basal end — a mechanical stress concentration zone for most broadband industrial noise spectra.
Anatomy concentrates damage here
All three factors point to the same cochlear address: the outer hair cells at 4000 Hz.
These three forces don’t operate independently — they compound each other. The hair cells at 4 kHz are working hardest (peak sensitivity zone), have no protective reflex, and sit in the most mechanically stressed region of the cochlea. Under sustained industrial noise, they are the first to exhaust their metabolic reserves and the first to die.
What the Notch Looks Like on an Audiogram
Pure-tone audiometry tests hearing at discrete frequencies. The audiogram is plotted with frequency (Hz) on the horizontal axis and hearing level (dB HL) on the vertical axis, with 0 dB at the top. Thresholds falling at 0–25 dB HL are within normal limits. Thresholds above 25 dB HL represent hearing impairment at that frequency.
A classic NIHL 4 kHz notch has these characteristics:
- Thresholds at 500–2000 Hz are within normal limits (0–25 dB HL)
- Threshold at 3000 Hz may be slightly elevated
- Threshold at 4000 Hz is the worst point — often 30–60 dB HL in moderate NIHL
- Threshold at 6000 Hz is worse than 2000 Hz but better than 4000 Hz
- Threshold at 8000 Hz is better than at 4000 Hz — the recovery
Figure 3 — 4 kHz Notch Depth: Severity Scale
How deep the notch at 4 kHz translates to clinical and regulatory significance. Bar height = threshold in dB HL (taller = worse hearing).
No trigger
STS possible
STS likely
300 Log eval
Recordable
NIHL at 4 kHz rarely exceeds 75–80 dB HL even in workers with decades of noise exposure. This self-limiting ceiling is itself a diagnostic feature of NIHL.
How the Notch Deepens and Widens Over Time
The 4 kHz notch is not static. With continued noise exposure, it evolves in two ways: it deepens (the threshold at 4000 Hz worsens) and it widens (adjacent frequencies at 3000 Hz and 6000 Hz are drawn in). Understanding this progression is essential for reading audiometric trends in your workers over time.
Figure 4 — Notch Progression Over Years of Noise Exposure
How threshold values change at each frequency as occupational NIHL advances. Each row = one worker’s audiometric history over ~15 years.
Year
500
1k
2k
3k
4k ▼
6k
8k
STS?
Baseline
10
10
10
10
10
10
10
—
Year 3
10
10
10
15
25
15
10
Borderline
Year 6
10
10
15
25
40
25
15
✓ STS
Year 10
10
10
25
40
55
50
30
✓✓ STS
Year 15
15
15
35
55
65
60
50
300 Log
Note the notch widening: by Year 10, both 3 kHz and 6 kHz are drawn into the damage zone. The 8 kHz recovery persists but narrows. Low frequencies (500–1k Hz) remain protected until very late stages.
The progression also slows over time. Research shows that NIHL at 3000, 4000, and 6000 Hz typically reaches its maximum level after 10–15 years of stable noise exposure. This is not because the cochlea is recovering — it is because the hair cells in the vulnerable zone have already been destroyed, and there are fewer remaining to damage. The threshold reaches a ceiling and stays there as long as noise exposure continues. When noise exposure stops, progression halts entirely.
Notch vs. Slope: How to Distinguish NIHL from Presbycusis
The most important differential diagnosis for the 4 kHz notch is age-related hearing loss (presbycusis), which also produces high-frequency hearing loss but with a completely different audiometric shape.
Figure 5 — Notch vs. Slope: The Diagnostic Comparison
The single most important question: is 8 kHz better than 4 kHz? If yes → NIHL notch. If no → presbycusis slope.
▲ NIHL — Notch Pattern
8 kHz (20) is BETTER than 4 kHz (50) → NIHL notch confirmed
▼ Presbycusis — Slope Pattern
8 kHz (65) is WORSE than 4 kHz (45) → Presbycusis slope, no recovery
When both NIHL and presbycusis are present (common in older workers), the 8 kHz recovery may be partial or absent. The audiometric history — not just one snapshot — provides the most diagnostic information in mixed cases.
▶ The rule: if 8 kHz is better than 4 kHz, the pattern is NIHL. If 8 kHz is equal to or worse than 4 kHz, the pattern is presbycusis or mixed. For a detailed comparison see NIHL vs. Age-Related Hearing Loss: Audiogram Guide.
How the 4 kHz Notch Drives the OSHA STS Calculation
OSHA’s standard threshold shift (STS) is defined as a 10 dB or greater average change in hearing threshold at 2000, 3000, and 4000 Hz in either ear, compared to the baseline audiogram. This specific frequency selection is not arbitrary — it is designed to bracket the 4 kHz notch zone and catch NIHL at the earliest audiometric opportunity.
Figure 6 — OSHA STS Calculation: Worked Example
How a developing 4 kHz notch produces an STS trigger. The average is calculated at 2k, 3k, and 4k Hz only.
Audiogram
500 Hz
1k Hz
2k Hz
3k Hz
4k Hz
8k Hz
Avg
Baseline
10
10
10
10
10
10
10.0
Annual
10
10
15
25
40
15
26.7
Shift
0
0
+5
+15
+30
+5
+16.7
▲ STS TRIGGERED — Average shift = 16.7 dB (threshold: 10 dB)
Required: Notify employee within 21 days • Refit HPDs • Retest within 30 days • Audiologist referral if shift persists
The STS was driven almost entirely by the 4 kHz value (+30 dB). Without the 4 kHz component, the average of 2k (+5) and 3k (+15) alone would be +10 dB — exactly on the threshold. The notch depth at 4 kHz is the decisive factor in most STS triggers.
This is why the 4 kHz notch matters operationally, not just clinically: it is the primary driver of STS flags, notifications, refitting requirements, 30-day retests, and OSHA 300 Log recordability evaluations. A safety manager who understands the 4 kHz notch understands why each of these program elements exists.
What OSHA Requires When You See the Notch
Figure 7 — OSHA Response: What the Notch Depth Triggers
From notch first appearance through recordability. Each threshold is cumulative — later steps include all prior obligations.
Finding
OSHA Trigger?
Required Actions
Notch present, no STS yet
No direct trigger
Program review recommended; HPD compliance check; document in audiometric record
10 dB avg STS at 2/3/4k
Yes — STS
Notify employee within 21 days; refit and retrain on HPDs; retest within 30 days
STS persists on 30-day retest
Yes — confirmed PTS
Physician or audiologist referral; evaluate 300 Log recordability; consider revised baseline
25 dB avg at 2/3/4k above audiometric zero (work-related)
Yes — 300 Log
Record on OSHA 300 Log (no age correction permitted); medical referral; WC exposure review
Progressive STS across multiple annuals
Yes — Program failure
Systematic program review; engineering controls assessment; noise re-survey; WC audit
When a 4 kHz Notch Is Not NIHL
Although the bilateral 4 kHz notch in a noise-exposed worker is NIHL until proven otherwise, other causes of audiometric notches exist and should be recognized:
- Ototoxic medications: Aminoglycoside antibiotics (gentamicin, tobramycin), loop diuretics (furosemide), and platinum-based chemotherapy agents (cisplatin) can produce high-frequency notch patterns. Cisplatin typically causes broader high-frequency loss beginning at 8000 Hz and progressing inward. A worker with recent cancer treatment or IV antibiotics warrants flagging for medical review before attributing the notch to occupational noise.
- Viral cochleitis: Some viral infections (including measles, mumps, cytomegalovirus) can produce unilateral or bilateral high-frequency notches. Unlike NIHL, these may be asymmetric, may appear suddenly rather than progressively, and may be associated with a history of acute illness.
- Hereditary hearing loss: Some genetic hearing loss patterns preferentially affect 4000–6000 Hz, particularly in younger workers. A 4 kHz notch in a worker with minimal noise exposure history, or asymmetric presentation, warrants medical evaluation.
- Auditory neuropathy: Rare, but can produce variable frequency-specific deficits. Audiological evaluation with speech recognition testing can help distinguish from NIHL.
Practical RuleFor workers with documented significant occupational noise exposure and a bilateral 4 kHz notch, OSHA treats the STS obligations as triggered. The question of whether the notch is “truly” NIHL vs. another cause is a medical determination made by a physician or audiologist during the referral process — not a determination that delays the employer’s notification and refitting obligations.
Frequently asked questions
What is the 4000 Hz notch on an audiogram?
The 4000 Hz notch is a dip in hearing sensitivity at 4000 Hz with relatively better hearing at both lower frequencies (500–2000 Hz) and higher frequencies (8000 Hz). It is the characteristic early audiometric signature of noise-induced hearing loss and results from the particular mechanical and metabolic vulnerability of the 4 kHz cochlear region.
Why does noise-induced hearing loss start at 4000 Hz?
Three factors converge: (1) human hearing sensitivity peaks at 1–5 kHz, maximally stressing hair cells in that range; (2) the acoustic reflex protects frequencies below 2 kHz but leaves 3–6 kHz unprotected; (3) the 4 kHz region of the cochlea sits at a point of peak mechanical stress from most industrial noise spectra. These three forces make 4 kHz the first and most vulnerable frequency.
What does OSHA require when an audiogram shows a 4 kHz notch?
A notch alone does not trigger OSHA obligations. OSHA’s STS trigger requires a 10 dB average increase at 2000, 3000, and 4000 Hz compared to baseline. When that average is met, the employer must notify the employee within 21 days, refit and retrain on HPDs, and retest within 30 days. If the shift persists, physician or audiologist referral is required.
Can the 4000 Hz notch be reversed?
No. The permanent 4 kHz notch cannot be reversed. The cochlear hair cells responsible for that frequency have been permanently destroyed. Progression halts when noise exposure stops, but the existing notch remains. Only temporary threshold shift (TTS) — which may produce a transient notch — resolves after adequate quiet.
Is a 4000 Hz notch always caused by noise?
Not always, but it is the defining NIHL pattern. A bilateral 4 kHz notch with recovery (8000 Hz better than 4000 Hz) in a noise-exposed worker is strong evidence of NIHL. Other causes include ototoxic medications, viral cochleitis, and some hereditary conditions. The occupational context, symmetry, and audiometric history all inform the differential diagnosis.
Catch the 4 kHz Notch Before It Becomes an STS
Soundtrace flags threshold changes at 4 kHz on every annual audiogram — before the notch deepens enough to trigger OSHA obligations, while intervention can still slow progression.
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