Standing across the street from a friend, I must raise my vo…
Standing across the street from a friend, I must raise my voice to be heard compared to just standing in front of me. This condition roughly approximates a sound field (call the free far field) in which my mouth emits free progressive sound waves. The sound pressure of the speech is diminished so much at the ears of my friend (compared to standing just in front of me) is explained by the law, wherein the sound pressure (or acoustic intensity) decreases as a function of from the sound source.