Low-frequency sound: Method to identify tones masked by high background noise

Often, noise measurements are performed under non-ideal scenarios, such as high background noise levels or high traffic noise levels, which is a problem when the goal is to analyze low-frequency noise from HVAC, industrial plants, wind turbines, etc., not only to study its impact but also to determine its immission level for legal purposes. Since the introduction of the A-weighting curve, acousticians have been aware that noise measured in dBA levels is not the best tool for assessing low-frequency annoyance, and for this reason some standards and some regulations include a correction factor to consider its presence, in order to add some penalties due to it. Following the theoretical concept of “sound designation” presented in the ISO 1996-1 standard, the authors have developed a method to find low-frequency tones from steady noise sources masked by background noise, not by filtering but by removing the unwanted sound levels from the value file as if they were outliers. Since standards and legislation require the evaluation of the specific sound of sources with emissions containing low frequencies, and not the total sound recorded by SLMs, this paper explains an understandable heuristic mathematical method for doing this, along with a statistical demonstration to validate these algorithms.