The impact of tones on NC ratings

Historically, the NC rating of steady-state background noise is determined by comparing the measured octave band sound pressure levels with the NC curves, where the rating is determined by the lowest curve such that no measured octave band level exceeds the NC curve. The 2023 version of the ASHRAE Applications Handbook presents the NC curves in Figure 5 of Chapter 49, and the supporting text states: “In HVAC systems that do not produce excessive low-frequency noise and strong discernable pure tones, the NC rating correlates relatively well with occupant satisfaction if sound quality is not a significant concern.” At present, NC curves are defined only in 5-dB increments and only in octave bands, but an ANSI S12 working group is currently working on a revision of ANSI S12.2 that may also define the NC curves in 1-dB increments and in one-third octave bands. ASHRAE Research Project 1707 studied the annoyance of steady-state tones in background noise dominated by HVAC systems applicable to an office environment. One of the results of this research project is a software program that predicts the annoyance of HVAC background noise that may or may not contain tones. This presentation examines several steady-state background noise signals that contain steady-state tones of varying frequency and tone to noise ratio to illustrate how tones in the spectrum can impact the NC rating and the overall annoyance of the background noise containing tones.