Hearing thresholds: why the dB HL to dB SPL offset changes with frequency

I was trying to understand my own audiogram results and assumed the dB HL scale had one fixed relationship to dB SPL, until an audiologist explained the offset actually shifts depending on which frequency you're looking at, since human ears aren't equally sensitive across the spectrum.

The factor below is the one that ends the argument — write it once, then stop improvising mid-job.

The factor that settles arguments

This is the relationship our converters use. Screenshot it if you have to — just stop re-deriving it from a half-remembered blog post.

The dB HL to dB SPL offset varies by frequency (based on the average normal-hearing threshold at that pitch), not a single fixed number

Write that line once at the top of your notes. Every later arithmetic step should point back to it instead of inventing a friendlier number mid-stream.

The mistake that keeps showing up

Applying a single dB HL-to-SPL offset across all frequencies on an audiogram, when the actual offset is calibrated separately for each frequency based on population average hearing sensitivity.

I still write the unit next to every intermediate value. It looks pedantic until it catches the one swap that would have shipped wrong.

Worked example

An audiogram shows a 0 dB HL threshold at both 250 Hz and 1,000 Hz, and you want the actual sound pressure level at each.

At 1,000 Hz, 0 dB HL corresponds to roughly 7 dB SPL, while at 250 Hz, 0 dB HL corresponds to roughly 26 dB SPL — the offset itself depends on frequency.

There's no single dB HL-to-SPL number — always check the frequency-specific reference offset from the audiometer's calibration standard.

If you want a second check, invert the conversion and see whether you recover the original quantity within normal rounding.

Quick reference table

Use the table for speed once you trust the factor. Do not use it as a substitute for understanding which definition of the unit you are on.

Why the exact number matters

Audiometric testing and hearing aid calibration depend on frequency-specific reference levels — using one blanket offset would misrepresent hearing sensitivity at the frequencies where human ears are naturally less sensitive.

Precision is not about showing off decimals. It is about making sure the shopping list, the machine setting, and the acceptance check are all describing the same physical quantity.

Pocket recap

Always use the frequency-specific dB HL-to-SPL reference offset from the ISO 389 standard rather than a single flat number — the human ear's sensitivity genuinely varies by pitch.

Keep the converter link nearby for the next time the same debate shows up in a group chat. Re-typing the factor from memory is how the wrong cousin of a unit sneaks back in.