Air absorption of high frequencies makes distant voices sound duller

Evidence

The fourth aspect of reverberation, which caught early reverberation unit designers by surprise, is the observation that, as well as suffering many reflections, the sound energy in a reverberant decay will have traveled through a lot of air. In fact, the distance the sound will have traveled will be directly proportional to the reverberation time, so a 1-second reverberation time implies that the sound will have traveled around 344 m by the end of the decay. … In particular, humidity, smoke particles, and other impurities will absorb high-frequency energy and so reduce the level of high frequencies in the sound. This is one of the reasons why people sound duller when they are speaking at a distance. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1943

Categorization

• DEFINE: Explains the mechanism behind why distant speech loses brightness — humidity and impurities absorb high-frequency energy.
• XREF: Connects to Chapter 1’s discussion of speed of sound dependence on temperature. — connects to: Animal choruses concentrate when air is calm and noise minimal — Almo Farina, Soundscape Ecology: Principles,…, 565; Bats retune echolocation calls lower and longer during humid seasons to offset absorption — Almo Farina, Soundscape Ecology: Principles,…, 763; Air absorption must be modeled or artificial reverb sounds overbright — David Howard, Jamie Angus, coustics and Psychoacoustics, 1987