The following list gives a rough idea of what the various STC ratings mean*:
25—normal speech easily understood
30—normal speech audible, but unintelligible
35—loud speech understood
40—loud speech audible, but unintelligible
45—loud speech barely audible
50—shouting barely audible
55—shouting inaudible
* For more, see U.S. Department of Commerce National Bureau of Standards’ Handbook 119, Quieting: A Practical Guide to Noise Control, written by Raymond Berendt et al (1976).
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