Sound not only weakens but dulls with distance due to air absorption

Evidence

Equation 1.30 shows that the sound intensity for a sound wave that spreads out in all directions from a source reduces as the square of the distance. Furthermore, this reduction in intensity is purely a function of geometry and is not due to any physical absorption process. In practice, there are sources of absorption in air, for example impurities and water molecules or smog and humidity. These sources of absorption have greater effect at high frequencies and, as a result, sound not only gets quieter but also gets duller as one moves away from a source. The amount of excess attenuation is dependent on the level of impurities and humidity and is therefore variable. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 303

Categorization

• XREF: Connects to the reader’s broader listening-and-silence project in One Square Inch of Silence: explains physically why distant natural sounds arrive muffled in a way that matters for questions of quiet preservation.
• corroborated: 2 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 303