Sound Solutions (noise engineers) regularly works on environmental noise projects. We generally measure the noise from a noise source and compare it with the applicable noise regulation. If the noise levels exceed the noise guideline, noise mitigation measures are recommended. We were hired to work on a project in Payson, Arizona. The noise source was a wood working facility that makes doors. They have large blowers located outside the buildings. There is a small airport nearby. The closest residences are over a half mile away.
The biggest difficulty the acoustical consultants had with this project was that neither the state of Arizona, Gila County, nor the city of Payson have a noise standard that specifies an actual dBA limit. The noise criteria just states that disturbing noises should not radiate from other properties.
Sound Solutions made sound level measurements in the vicinity of the residential properties for several hours during the day and night without the blowers from the door manufacturer operating. This gave us a baseline of the background noise level from aircraft and traffic. Noise levels were then measure at the closest residence in each direction of the facility with the blowers operating.
We compared the measured noise levels with the blowers operating with the background noise levels and with typical noise limits in the United States. Noise regulations can vary greatly, but in general nose levels below an hourly Leq of 55 dBA during daytime hours (7 AM to 10 PM) and 50 dBA at night (10 PM to 7 AM) are considered acceptable. Another general guideline that appears in some limits is that the noise from the facility should not increase the background noise level by more than 10 dBA.
At large distances, the atmospheric conditions can greatly influence the measured noise levels. Hence, we also made noise measurements close to the blowers. We used some simple formulas (in a spreadsheet) to calculate the noise levels at the closest residences. We used the noise measurements to help support our calculations.
We found that in one direction the hourly Leq noise level was above 50 dBA (at times the facility operates during nighttime hours). In addition, during some nighttime hours, the blower noise exceeded the background noise level by more than 10 dBA.
We recommended applying noise mitigation. To determine what would be the best form of noise mitigation, we needed to know how much noise was coming from what part of the system. The primary noise sources were the motor, noise coming out the top of the exhaust stack and the square metal stack vibrating and radiating noise. A large percentage of the noise was coming from both the motor and out the top of the sack. We determined this by measuring the noise levels around the blower to determine the contribution of each source. We also to spectral (FFT) data to help us identify what source contributed what portion of the spectrum.
To address the motor noise, we discussed: installing a partial enclosures, barriers, changing the motor speed (and flow velocity), and lagging the motor. To address the noise from the top of the stack we discussed: adding a passive silencer, adding an 90 degree elbow to the discharge, extending two sides of the discharge, active noise control, and reducing the velocity of the discharge air.
We worked with the manufacturing facility to come of with acceptable noise mitigation and then predicted approximately how much total noise reduction would be provided. Because of the cost of installing the noise mitigation options, they wanted to have a clear understanding of how much noise reduction would be provided and to sure that would be adequate.
They developed an agreement with the neighbors and Sound Solutions measured the noise levels after the mitigation was installed.
Sound Solutions is an acoustical consulting firm that works throughout the US conducting noise measurements, analysis, and reporting for environmental and architectural noise issues. Please contact us for a free proposal.