A data centre's hum is low and tonal. Two things follow, and this tool lets you see both at once. First, low frequencies are barely absorbed by the air, so the hum carries far while higher-pitched sound fades. Second, A-weighting — the filter behind the ordinary decibel reading — discounts exactly those low frequencies, so the same tone that reaches a distant house is quiet on the meter. Set a source and a distance and watch the gap between what arrives and what the meter records.
Model and assumptions. A single tonal point source over reflective ground. Level at the receiver = source sound power − geometric divergence (20·log₁₀ d + 11 − 10·log₁₀ Q, with Q = 2 for hemispherical radiation) − atmospheric absorption (ISO 9613-1 analytical formula at 20 °C, 70 % relative humidity, 101.325 kPa). The A-weighted value applies the IEC 61672-1 weighting at the tone's frequency. Ground effect, screening, meteorological refraction and indoor transmission are omitted, so real values vary; the frequency dependence the tool illustrates is robust to those omissions. All arithmetic was verified in Node.js before publication.