Hearing Health Guide
Definition
Both dB HL (decibels hearing level) and dB SPL (decibels sound pressure level) measure sound intensity, but on different scales for different purposes. dB SPL is a physical, acoustic measurement: it reflects the actual pressure a sound wave exerts in the air, the same at any frequency. dB HL is an audiological measurement: it's calibrated frequency by frequency so that 0 dB HL always represents the average threshold of normal-hearing young adults at that specific frequency, even though the equivalent dB SPL value differs from one frequency to the next.
dB SPL is a straightforward physical measurement of acoustic pressure, referenced to a fixed baseline (20 micropascals, roughly the quietest sound a young, healthy ear can detect at 1,000 Hz). A device like a sound level meter reads dB SPL, typically with A-weighting applied to approximate how the human ear perceives loudness across frequencies.
Because it's a physical unit, 60 dB SPL represents the same acoustic pressure whether the tone is 250 Hz or 8,000 Hz.
The human ear isn't equally sensitive at every frequency. It naturally hears mid-range frequencies (roughly 1,000-4,000 Hz) more easily than very low or very high ones, even at the same physical sound pressure. dB HL corrects for this by calibrating 0 dB at each frequency to match the average normal-hearing threshold at that frequency.
This means 0 dB HL at 250 Hz corresponds to a higher dB SPL value than 0 dB HL at 1,000 Hz. The dB HL scale flattens out that natural variation so a hearing test result is easy to interpret: 0-25 dB HL reads as normal at any frequency on the chart.
If audiograms were plotted in raw dB SPL, a person with completely normal hearing would show a jagged, uneven line across frequencies, simply reflecting the ear's natural sensitivity curve rather than any hearing problem. Converting to dB HL makes a normal-hearing result plot as a flat line near zero, so any dip clearly represents an actual loss rather than an artifact of the measurement scale.
Sound level meters, including LSTN's, report dB SPL (A-weighted) because they're measuring environmental noise, not an individual's hearing thresholds. Audiograms report dB HL because they're measuring a person's hearing relative to a clinical norm.
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