Sones to Decibels: Conversion and Fan Noise

Sones to decibels: use the right relationship

For loudness N of one sone or more, the conventional relationship is loudness level L_N = 40 + 10 log₂(N), expressed in phons. A loudness level in phons is the level of a 1 kHz reference tone judged equally loud.

That last step is the limitation: phons equal the sound-pressure level in dB of the reference tone, not the dBA level of every real sound. A fan produces a spectrum of sound, so its sone rating cannot be converted into a universal receiver dBA figure using this formula alone.

AMCA 301 defines the reference and provides a frequency-band method for calculating fan sound ratings under specified conditions.

A conversion table with its assumptions

The following values are loudness-level equivalents for N ≥ 1. The final column applies only to the corresponding 1 kHz reference tone. It is not a product comparison chart or a room-noise prediction.

For example, 3 sones gives about 55.8 phons using the logarithmic relationship. Do not extend this simple formula below one sone; the low-loudness relationship needs the applicable method.

Sone vs decibel: why doubling is different

The sone scale is intended to express loudness proportionally: two sones represents twice the loudness of one sone on that scale. A decibel is logarithmic and requires a reference quantity. Doubling acoustic power increases sound-power level by about 3 dB, which is a different statement.

Frequency content affects loudness. Two sounds with the same overall physical level can have different spectra and perceived loudness. Our dB and dBA guide explains frequency weighting, while sound power and sound pressure separates the source from the receiver.

Reading a fan's sone rating

Look for the rating standard, operating point and installation category. Fan ratings can be calculated from frequency-band sound data under defined assumptions about propagation and listening conditions. A value measured or calculated for one arrangement is not automatically a prediction for another.

Compare two fans only when airflow, pressure and the rating basis align. A small domestic fan's sone label is not directly comparable with an industrial extraction fan's A-weighted sound-power declaration. Ask for matching frequency-band data if the acoustic design depends on the result.

From a rating to a quieter installation

For chemical extraction, identify the receiver location and the sound path before choosing a component. The system needs to maintain its specified airflow while controlling noise. A PVC duct silencer can be considered for duct-borne sound using the appropriate frequency data and pressure allowance.

Send the original acoustic sheet rather than a converted number alone. Include the fan duty, duct arrangement and where the noise matters. That preserves the assumptions needed for a useful design discussion.