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Why do preferred noise criteria curves include limits for low-frequency rumble and high-frequency hiss?

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Preferred noise criteria (PNC) curves were developed in 1971 to improve upon the earlier noise criteria (NC) standard by providing more comprehensive design goals for indoor environments. While the original NC curves specified maximum allowable noise levels across the audio spectrum, the PNC curves specifically incorporate limits for low-frequency rumble and high-frequency hiss to better ensure a comfortable working or living environment.1

By establishing these additional constraints, PNC curves help designers manage the full range of sound frequencies that can impact human comfort and environmental quality. If noise levels exceed these established PNC limits, sound-absorptive materials—such as heavy drapery or sound-absorbent tiles—can be introduced into the space to meet the necessary standards.1