Environmental Noise Mitigation from Rooftop Exhaust Stacks

Neighboring properties bordering a manufacturing facility complained about a low-frequency noise that they believed was emanating from two rooftop evaporator exhaust stacks. The plant contracted Noise Control Engineering for an environmental noise survey and mitigation solutions if noise levels were determined to be high.  A survey was performed to measure noise levels on the rooftop near the exhaust stacks, around the plant, and near the property lines of the neighbors.  Rooftop noise levels were measured as high as 79 dB(A) with noise levels around the plant measuring 59 dB(A) and 44 dB(A) near the property lines.  Although the absolute noise levels were not extremely high, there was a clear, audible tone in the 315 Hz one-third octave band that was determined to be the source of annoyance to the neighbors. 

To determine the needed attenuation, noise level predictions using a simplified spherical spreading calculation were performed. Approximate distances were determined from Google Maps. The predicted noise levels matched the measured noise levels at all points around the facility as well as points measured near the property line.  Predictions showed that overall noise levels at the neighboring residences (not measured) would be below 40 dB(A). However, because of the pure tone condition and anticipated low background levels, it is likely that this tone would be audible, particularly during evening and nighttime hours.  Noise mitigation mufflers were designed with an 8-10 dB reduction goal for the 315 Hz tone.  These silencers were installed on the exhaust stacks inside the building and significantly reduced the noise levels in the 315 Hz and 400 Hz octave bands from 7 to 13 dB at the rooftop measurement locations.  Overall noise reductions of 7 to 9 dB were observed at these measurement points.  Noise measurements taken near the neighboring property lines showed that the tones at these frequencies were below the background noise.