Sound propagation is symmetric between source and receiver when the medium is stationary.

Evidence

The reciprocity theorem states that the acoustic field due to a sound source at point A received at any other point B is the same as that which would be received at A if the same source were placed at B provided the medium carrying the sound is not itself moving. — Eric J. Heller, Why You Hear What You Hear- An…, 590

Categorization

• DEFINE: Defines the reciprocity theorem, one of the core symmetry principles of acoustics.
• XREF: Generalizes beyond sound: reciprocity applies to many wave types — parallels electrical network reciprocity and optical reversibility.