Sound and the decibel: the acoustics behind an audiogram
- Topic
Sound is a pressure wave with a pitch (frequency, in hertz) and a loudness (intensity, in decibels); the audiogram plots both.
What it is
Every hearing test rests on two physical quantities. Frequency, measured in hertz (Hz), is heard as pitch; the young human ear hears roughly 20 to 20,000 Hz and speech carries most of its information between about 500 and 4000 Hz. Intensity is heard as loudness and is expressed in decibels (dB), a logarithmic scale that compares a sound with a reference level. Because the scale is logarithmic, small changes in decibels are large changes in energy, which is why a few extra decibels of noise at work shorten the safe exposure time so much. There are several decibel scales (SPL, HL, SL and A-weighted), and mixing them up is a classic exam trap. This page also explains how the middle ear matches the impedance of air to that of the inner-ear fluids, the reason a broken ossicular chain causes a large conductive loss.
Key facts
- Frequency (Hz) = cycles per second = pitch. One octave is a doubling of frequency; audiograms test octaves from 250 to 8000 Hz.
- Intensity (dB) = loudness. Threshold of pain is around 120 to 140 dB; impulse noise above about 140 dB can damage the ear in a single exposure (acute acoustic trauma).
- Pure tone: a single frequency (tuning fork, audiometer). Complex sound: many frequencies (speech, music). Noise: random frequencies; white noise and narrow-band noise are used for masking.
- Typical levels (NIDCD): conversation 60 to 70 dBA; motorcycles 80 to 110 dBA; headphones at maximum volume and concerts 94 to 110 dBA; sirens 110 to 129 dBA; fireworks 140 to 160 dBA.
- Interaural attenuation: a loud air-conducted tone crosses to the other cochlea through the skull (about 40 dB with supra-aural headphones, almost nothing for bone conduction), so the better ear must be masked.
- Tuning forks: 512 Hz is the standard for Rinne and Weber because it decays slowly and is not felt as vibration like 128 Hz.
Function
- The pinna and ear canal collect sound and boost it a little around 2000 to 4000 Hz by resonance.
- The eardrum is much larger than the stapes footplate; this area ratio, plus the lever of the malleus and incus, concentrates pressure on the oval window.
- Without this impedance matching most sound energy would reflect off the inner-ear fluids, as it does at the surface of water.
- Phase difference: an intact eardrum shields the round window so the two windows do not receive sound at the same time.
- In the cochlea the basilar membrane sorts frequencies by place: high pitches at the base, low pitches at the apex; this is why noise and age damage the high tones first.
The decibel scales you will meet
| Scale | Reference (what 0 means) | Where it is used |
|---|---|---|
| dB SPL (sound pressure level) | A physical pressure, the faintest sound a young ear can detect at 1000 Hz | Physics, calibrating equipment, acoustic trauma thresholds |
| dB HL (hearing level) | Average threshold of young normal ears at each frequency (audiometric zero) | The pure-tone audiogram; WHO hearing loss grades |
| dB SL (sensation level) | This patient's own threshold | Speech audiometry and tone decay tests (for example, 40 dB above threshold) |
| dBA (A-weighted) | SPL filtered to mimic the ear's lower sensitivity to low and very high pitches | Workplace and environmental noise, sound level meters and dosimeters |
In clinical practice
- A patient with a 40 dB HL loss hears soft speech poorly but understands face-to-face conversation; the same number in dB SPL means something different.
- Noise damage is first seen as a notch at 3000 to 6000 Hz (deepest near 4000 Hz) because of cochlear mechanics, not because noise is only high-pitched.
- Masking is needed when the test tone in the poorer ear could be heard by the better ear; unmasked results can show a false «shadow curve».
- Explosions and gunshots can rupture the eardrum or disrupt the ossicles as well as damage hair cells.
Next step / Initial management
- Read every audiogram as dB HL against frequency; check that masking was used when the ears differ by 40 dB or more in air conduction.
- Noise-exposed workers: measure exposure in dBA; in Colombia the GATI-HNIR sets 85 dBA as the permissible limit and asks for baseline and periodic audiometry with hearing protection.
- Acute acoustic trauma (blast, gunshot) with hearing loss or vertigo: otoscopy and audiometry, and same-day referral to ENT; corticosteroids are considered as in sudden hearing loss.
- A failed screening or a new asymmetry: routine referral to audiology for a full audiogram with masking.
Test yourself
Think of the answer, then tap the question to see it.
Noise A is 85 dBA and noise B is 95 dBA. How much more intense is B?
Ten times more intense (10 dB = tenfold intensity), and it is heard as roughly twice as loud.
Under a 3 dB exchange rate, a worker can safely spend 8 hours at 85 dBA. How long at 88 dBA?
About 4 hours: each 3 dB increase doubles the energy, so the allowed time is halved.
What does 0 dB HL mean on an audiogram?
The average threshold of young people with normal hearing at that frequency (audiometric zero); it is not silence.
A man has traumatic ossicular discontinuity with an intact eardrum. What size of conductive loss is expected?
The maximum conductive loss, around 50 to 60 dB, because the middle-ear transformer is lost and the intact drum blocks sound reaching the windows directly.
Elsewhere in DurgiPedia
Connections
Educational content; it does not replace a medical assessment and is not a clinical instruction. For a real case, follow current guidelines and specialist advice.
Sources: Physiology, Ear (StatPearls Publishing (NCBI Bookshelf), 2026); Pure-Tone Audiometry (StatPearls Publishing (NCBI Bookshelf), 2023); Noise Exposure and Hearing Loss (StatPearls Publishing (NCBI Bookshelf), 2023); Acute Acoustic Trauma (StatPearls Publishing (NCBI Bookshelf), 2024); Noise-Induced Hearing Loss (National Institute on Deafness and Other Communication Disorders (NIDCD), 2025); Understand Noise Exposure (CDC, National Institute for Occupational Safety and Health, 2026); Guía de Atención Integral Basada en la Evidencia para Hipoacusia Neurosensorial Inducida por Ruido en el Lugar de Trabajo (GATI-HNIR) (Ministerio de la Protección Social, República de Colombia, 2006).
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