Human audio-visual synchronicity tolerance is asymmetric because sound naturally lags light.

Evidence

Audio that is presented too early is noticed when the time shift is shorter than when it is presented after the corresponding video. Different studies propose different numbers for the threshold of detection of the shift: an audio lead of 25–75 ms and lag of 40–90 ms are said to be detected (Levitin et al., 2000). This can be understood from the observation that in nature, for larger distances between the subject and the source, sound arrives at the ears considerably later than light at eyes from the source. Thus, humans are quite accustomed to sound stimuli lagging visual stimuli. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1821

Categorization

• XREF: Connects to the reader’s soundscape/silence work: the same physical fact about sound and light propagation speeds that shapes perception of distance also shapes content-production timing standards.
• SEED: An essay angle on how a natural physical asymmetry (the speed of sound) quietly hard-codes itself into broadcast standards like the ITU recommendation.