What is the difference between dB(A) and sones in fan noise specs?

dB(A) is a logarithmic level; sones are a linear loudness scale. Doubling the sone value means roughly double the perceived loudness, while a 10 dB(A) increase is perceived as roughly a doubling. Fan catalogues use both — always compare within one scale, at the same measurement distance (typically 1 m), and at the same operating point.

The two scales

dB(A) applies an A-weighting curve that mimics human hearing's sensitivity across frequencies, then expresses the result on a logarithmic scale: +3 dB(A) ≈ double the sound power, +10 dB(A) ≈ perceived twice as loud. Sones re-express loudness linearly: 2 sones is twice as loud as 1 sone. A common engineering anchor: 40 dB(A) at 1 m ≈ 1 sone (band-dependent). Because the mapping is frequency-dependent, precise conversion needs spectrum data — approximate for broadband fan noise.

Why the operating point changes everything

Catalogue noise is measured at a stated condition (commonly free air or a nominal back-pressure at 1 m). At your installed operating point — against real enclosure resistance — the fan may run faster or slower than the catalogue state, and noise follows speed strongly. A fan that wins on the datasheet can lose in the box. For tone-sensitive products, also ask for octave-band data: two fans with equal dB(A) can differ sharply in tonal character, and tonal noise annoys far beyond its level.

• Request noise at a stated pressure, not only free air.
• Ask for octave bands when the product has a human within arm's reach.
• Beware 'quiet' claims at reduced voltage: the same fan at full duty tells the truth.

Practical comparison workflow

Normalize candidates: same scale, same distance, same pressure state. Convert mentally with the anchors above. Then weigh character: a fan 2 dB(A) higher but with no discrete tone usually sounds better than a 'quieter' fan with a strong blade-pass or PWM tone. Validate on the unit — structure-borne transmission through panels can dominate what the customer hears, independent of the fan's airborne number.