One of the primary effects of aircraft noise is its tendency to drown out or "mask" speech, making it difficult or impossible to carry on a normal conversation without interruption. The figure below presents typical distances between talker and listener for satisfactory conversations in the presence of different steady A-weighted background noise levels. As indicated in the figure, satisfactory conversation does not always require hearing every word; 95% intelligibility is acceptable for many conversations. This is because a few unheard words can be inferred when they occur in a familiar context. However, in relaxed conversation, we have higher expectations of hearing speech and require complete 100% intelligibility.
Source: US EPA, Information on Levels of Noise Requisite to Protect the Public Health and Welfare with an Adequate Margin of Safety, March 1974.