Hear and see how air absorbs sound as a function of distance, temperature, humidity, and pressure. ISO 9613-1 / Bass 1995.
Sound is absorbed by air through several mechanisms, parameterized in ISO 9613-1 as a sum of three terms: a classical term (viscous and thermal-conductive losses plus rotational relaxation, all roughly proportional to f2 at audio frequencies); the vibrational relaxation of O2; and the vibrational relaxation of N2. The two vibrational terms have distinct relaxation frequencies and Arrhenius factors; they dominate above about 1 kHz and depend strongly on humidity because water vapor catalyzes both transitions, shifting their relaxation frequencies up into the ultrasonic.
The result is a smooth absorption coefficient α(f), in dB per unit distance, that grows with frequency: about 5 dB/km at 1 kHz, 160 dB/km at 10 kHz, and 3 dB/meter at 100 kHz under typical conditions (20 °C, 50% RH, 1 atm). α(f) is plotted as the gray dashed curve above and depends only on the air (temperature, humidity, pressure), not on the listener's distance from the source.
The total attenuation at frequency f over a propagation distance r is then simply A(f, r) = α(f) · r, which is the solid blue curve. Doubling the distance shifts the blue curve up by 6 dB at every frequency (since multiplying r by 2 adds 20 log10 2 ≈ 6 dB everywhere); the shape of the curve never changes with r, only its overall vertical position. Over a few kilometers, every frequency above a few kHz accumulates so much A(f, r) that it is effectively gone: this is why distant thunder is a rumble, not a crack.
The audio path applies a frequency-dependent filter whose magnitude response approximates 10−A(f, r)/20 (the linear-amplitude equivalent of the dB attenuation). Specifically, a cascade of 10 octave-spaced peaking-EQ biquads (31.5 Hz to 16 kHz, Q ≈ 1.4) is gain-matched per band to the target A(f, r) curve, so the wet path's magnitude response tracks the blue plot to within a couple of dB everywhere in the audible band. Biquads' smoothly-automatable AudioParams make the response interpolate cleanly as you drag, with no clicks.
Adjust temperature, humidity, pressure, and distance above and listen to the change. Move the distance slider continuously while a sound plays to hear the timbre roll off. You can also grab the blue curve directly with the mouse and drag it up or down on the plot — the distance slider follows.
Temperature. Air temperature. The vibrational relaxation frequencies depend on T via Arrhenius factors exp(−Θ/T) with Θ = 2239 K (O2) and 3352 K (N2). Cold air generally absorbs more at audio frequencies than warm air, though the dependence is not large (about a factor of two between −20 °C and +20 °C at 1 kHz).
Relative humidity. Water vapor catalyzes vibrational relaxation. The absorption at a given frequency does not increase monotonically with dryness; it peaks at a specific moderate humidity that depends on frequency (about 4% RH for 1 kHz, about 10% for 4 kHz, about 19% for 10 kHz at 20 °C, 1 atm). Both much-drier and much-wetter air absorb less than the peak. This non-monotonic dependence is one of the more counterintuitive features of atmospheric acoustics.
Pressure. Lowering pressure at fixed RH has a surprisingly weak effect on absorption (less than 1% change at 1 kHz between 80 and 101 kPa). This is because lowering pressure at fixed RH also raises the molar concentration of water vapor in a compensating way. The pressure slider becomes interesting when combined with a low RH: a dry, low-pressure stratosphere is a much more transparent acoustic medium than dense low-altitude air with the same RH.
Distance. Range from source to receiver. The total attenuation is α(f) · r, with α in dB/m or dB/km and r in matching units. The 1/r geometric spreading from a point source is not applied by default; toggle the checkbox to include it.