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Undesirable rumble can result if NC curves are determined mainly by low frequency noise. Similarly, a hissing effect can result from NC level being controlled by higher frequency sounds. To achieve a better balance between low frequency and high frequency components, RC curves have been established for which the objective is to design spectra that meet an RC curve within + 2 dB at all frequencies. A spectrum that exceeds an RC curve by more than 5 dB at frequencies below 250Hz is likely to result in unacceptable rumble. Above 2000Hz, on the other hand, a spectrum more than 5 dB higher than the RC curve might have too much of a hissing quality.

Source: http://www.owenscorning.com/around/sound/glossary.asp

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