The 4000 Hz Notch: Audiogram Guide for Safety Managers

The 4000 Hz notch is the most recognizable pattern in occupational audiology. It appears on audiograms of noise-exposed workers around the world, across industries and exposure levels, and it is the finding that OSHA 1910.95’s STS formula is specifically designed to detect. But reading a 4 kHz notch correctly — understanding what it means clinically, when it triggers regulatory action, and what it says about WC exposure — requires more than knowing that it exists. According to the CDC, approximately 22 million U.S. workers are exposed to hazardous occupational noise annually, and a significant portion of their annual audiograms will show some variant of this pattern. This guide explains what the 4 kHz notch looks like, why it appears there, and how to use it in clinical and legal contexts.

What the 4 kHz Notch Looks Like on an Audiogram

An audiogram displays hearing threshold in dB HL on the vertical axis (with 0 at top representing normal hearing, worsening downward) and frequency in Hz on the horizontal axis from low frequencies at left to high frequencies at right. A 4 kHz notch appears as:

• Thresholds at 500 Hz and 1,000 Hz within or near normal limits (typically 0–25 dB HL)
• Thresholds at 2,000 Hz normal or mildly elevated (typically 10–30 dB HL)
• Threshold sharply elevated at 4,000 Hz — the bottom of the notch (may be 40–70+ dB HL in moderate-severe NIHL)
• Threshold at 8,000 Hz somewhat better than at 4,000 Hz — the “recovery” that creates the notch shape

The notch is defined by this configuration: worse at 4,000 Hz than at both 2,000 Hz and 8,000 Hz. When 4,000 Hz threshold is worse than 8,000 Hz by 10 dB or more, the notch is considered audiometrically significant.

Why 4,000 Hz Is the Characteristic Frequency

The 4 kHz notch location is not arbitrary — it reflects the acoustic physics of the human ear canal combined with the mechanical properties of the cochlea. The ear canal resonates at approximately 3,000 Hz, amplifying incoming noise by 10–15 dB at that frequency before it reaches the cochlea. The cochlear region processing 3,000–6,000 Hz — centered near the 4 kHz position on the basilar membrane — is also mechanically vulnerable because of the fluid dynamics of the basal cochlear turn and the vascular anatomy of the stria vascularis in that region.

The 8,000 Hz recovery occurs because the basilar membrane motion for 8 kHz is mechanically isolated from the peak stress zone; the 2,000 Hz relative preservation occurs because that frequency is processed more apically with better blood supply. The combined effect produces the characteristic notch shape that is essentially a fingerprint of cochlear hair cell damage from sustained noise exposure.

Reading the Notch on an Audiogram

For safety professionals and EHS managers reviewing audiometric results, the practical interpretation of the 4 kHz notch:

• Bilateral symmetric notch: Both ears show similar depth and configuration at 4,000 Hz. This pattern is consistent with occupational noise exposure in a symmetric bilateral noise environment — which describes most industrial workplaces.
• Bilateral asymmetric notch: One ear has a significantly deeper notch than the other. This pattern warrants investigation for non-occupational causes, particularly recreational firearms use (classically worse in the non-dominant ear for right-handed shooters, i.e., the left ear).
• Unilateral notch: Significant 4 kHz notch in one ear only. Not typical of bilateral occupational noise; suggests prior single-ear acoustic trauma, medical pathology, or recreational exposure to directional sound sources.
• Deep vs. early notch: An early notch with threshold at 4,000 Hz of 25–35 dB HL represents early NIHL. A deep notch with 4,000 Hz threshold of 55–70+ dB HL represents advanced NIHL with substantial cochlear hair cell loss.

Audiograms aren’t always easy to read, especially when you’re reviewing a stack of them for a hundred workers at once and none of your training prepared you for what a 4 kHz notch actually looks like versus normal age-related decline. This guide exists because that interpretation gap is real — and it matters more than most EHS professionals realize when a WC claim arrives years later.

When the 4 kHz Notch Triggers an STS Under 1910.95

The presence of a 4 kHz notch does not automatically constitute an STS under OSHA 1910.95. STS is defined as an average threshold shift of 10 dB or more at 2,000, 3,000, and 4,000 Hz in either ear compared to baseline. A notch may be present without triggering STS if:

• The notch was already present in the baseline audiogram at the same depth
• The change in the three-frequency average is less than 10 dB even though the notch is visible
• Age correction reduces the calculated shift below the STS threshold

A notch that deepens or broadens over successive annual audiograms may eventually trigger STS when the cumulative change in the 2k/3k/4k average reaches 10 dB. Serial audiograms that show progressive notch deepening without yet meeting the STS threshold still represent real cochlear damage — and should prompt review of HPD adequacy and noise exposure controls, even if the regulatory STS action threshold has not been reached.

Notch Progression Over Time

The 4 kHz notch in a noise-exposed worker does not appear all at once — it develops gradually over years of exposure. Serial audiograms showing the notch history of a long-tenured worker tell a story that individual cross-sectional audiograms cannot:

• When did the notch first appear?
• At what rate did it deepen and broaden?
• Did the rate of progression change when HPD programs were implemented?
• Has the notch stabilized, or is it continuing to progress?

This longitudinal story is why annual audiometric records must be retained for the full duration of employment — and ideally beyond. The 22-year serial record in a WC proceeding tells a fundamentally different story than a single audiogram taken at the time of claim.

Non-NIHL Causes of High-Frequency Loss at 4,000 Hz

Not all 4 kHz notches are occupational. Other causes of high-frequency hearing loss that may mimic or overlap with NIHL include:

• Age-related hearing loss (presbycusis): Typically produces a downward-sloping configuration across high frequencies without the characteristic notch shape; usually bilateral and symmetric but smoothly declining rather than notched
• Ototoxic medication exposure: Aminoglycoside antibiotics, cisplatin, and loop diuretics can produce high-frequency sensorineural loss; typically bilateral but may have different frequency distribution than NIHL
• Acoustic neuroma: Usually produces unilateral sensorineural loss; asymmetric pattern should prompt referral for otological evaluation
• Meniere’s disease: Classically low-frequency loss early in the disease; may produce broader sensorineural loss in later stages; associated with fluctuating hearing, vertigo, and tinnitus

WC and Legal Implications of the 4 kHz Notch

In workers’ compensation proceedings, the 4 kHz notch serves several roles:

• Establishing occupational etiology: A bilateral symmetric 4 kHz notch in a worker with documented noise exposure is consistent with occupational NIHL — supporting causation arguments
• Challenging occupational attribution: An asymmetric notch, unilateral notch, or audiometric configuration that deviates from the expected NIHL pattern creates openings for non-occupational attribution arguments
• Documenting progression timing: Serial audiograms showing when the notch appeared and how quickly it deepened allow precise chronological attribution of when damage occurred — potentially implicating specific employment periods
• Supporting apportionment: Pre-employment audiograms documenting a pre-existing 4 kHz notch before the current employment began establish that a portion of the total loss predated the employment relationship

Frequently Asked Questions