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Noise Curves Preferred Noise Criteria PNC (in accordance with ASA 1971) PNC curves represent a more stringent method based on an extension of the basic Noise Criteria system. They have been used in the past to judge the acceptability of ventilation and other background broadband noise. PNC curves are less often used than Noise Criteria curves because they are more stringent at lower frequencies than the Noise Criteria curves, but also because the latest (2008) version of Noise Criteria curves also includes an extended frequency range somewhat mitigating the original reasons for PNC. Noise Curves Result Page 1 12 2 11 3 4 5 6 10 9 7 108 8