In this blog, I will discuss environmental noise projects.
I will use cell towers as an example because we have had a lot of these projects in the last couple of year but these principles apply to any environmental noise project. The primary noise sources at a cell tower are AC units and generators. Generators generally need to be operated once a week for about an hour.
The process usually involves the following.
- Visiting the site to evaluate the conditions as they pertain to sound propagation and measure existing noise levels at the nearest noise sensitive receivers (residences, park, schools, churches, etc).
- Evaluate the noise limits where the site is located or develop a noise guideline based on the existing noise levels and typical noise regulation.
- Possibly measure the noise level from an existing facility in all directions at know distances.
- Model the noise radiating from the proposed facility to the nearest noise sensitive receivers.
- Provide noise mitigation options with approximate costs.
- Measure the noise levels after the operation and/or mitigation is in place.
- Generate a report that summarizes our findings.
The goal is to determine the noise levels from the air conditioning units on the cell towers at closest residences. At the closest residences, there are many other noise sources including: traffic, residential air conditioning units, and aircraft.
In many states there are no uniform noise limits. If you are working in a state were there are limits that that is what you use. If not you need to check with the county and with the city to determine the limits. Some noise regulations are very vague stating that you can not cause a ‘nuisance’ or ‘disturbance’, that ‘excessive’ or ‘unreasonable’ noise is not allowed, or that noise can not be audible on a residential property. In Tucson, we have a maximum noise limit of 70 dBA. So someone could make 70 dBA of noise day and night as long as they do not exceed 75 dBA. That is not a reasonable criteria. The city of Mesa had a 24 hour Leq noise limit of 60 dBA. Someone could produce a very high noise level for a parts of several hours as long as they are quieter the remaining hours.
When there are no clearly defined applicable noise limits we generally look to nearby regulations. Typical noise limits in the United Sates are 55 dBA during daytime hours (7 AM to 10 PM) and 50 dBA during nighttime hours (10 PM to 7 AM). Some noise regulation, such as in Oregon and Massachusetts, restrict the noise to no more than 10 dBA above the ambient (existing background) noise level.
These levels do not insure that the noise will be acceptable (that will also depend on the background noise, time of occurrence, duration, and frequency content); however, these are considered reasonable limits.
The following is a summary of a few noise regulations in the region:
- City of Phoenix, Arizona
The City of Phoenix Zoning Ordinance (Section 627.E.) states that the average noise level, measured at the property line, shall not exceed 55 dBA.
- Town of Marana, Arizona
The Marana Town Code (Section 11-5-3 (D)) states that the maximum noise level measured at a property line can not exceed 55 dBA during daytime hours (7 AM to 10:30 PM) and 50 dBA during nighttime hours (10:30 PM to 7 AM).
- Town of Gilbert, Arizona
The Town of Gilbert Noise Ordinance (Section 42-61) limits noise radiating to residential receivers to 55 dBA during daytime hours (5 AM to 10 PM) and 45 dBA during nighttime hours (10 PM to 5 AM).
- City of Salt Lake City, Utah
The Salt Lake City Health Regulations (#21 Section 9.1) limits noise radiating to residential property to 55 dBA during daytime hours (7 AM to 10 PM) and 50 dBA during nighttime hours (10 PM to 7 AM).
- State of Colorado
The State of Colorado Statutes (Chapter 25, Article 12, Section 103 (1) and (2)) limit the noise radiating to a residential property to 55 dBA during daytime hours (7 AM to 7 PM) and 50 dBA during nighttime hours (7 PM to 7 AM).
- State of Oregon
The Oregon Department of Environmental Quality (DEQ) noise regulations (OAR 340-35-035) limit noise radiating to residences to an L50 (about the same as an Leq) of 55 dBA during daytime hours (7 AM to 10 PM) and 50 dBA during nighttime hours (10 PM to 7 AM). In addition, new industrial and commercial operation cannot increase the existing ambient noise level by more than 10 dBA.
- Tempe, Arizona
Section 20-6 of the Tempe City Code (allowable noise levels) limits noise to residential zones to 55 dBA during daytime hours (7 AM to 10 PM) and 45 dBA during nighttime hours (10 PM to 7 AM). If the ambient noise level is 40 dBA or less between 10 PM and 7 AM then the actual ambient noise level will be the noise limit.
- City of Peoria, Arizona
The City of Peoria Ordinance (Section 13-92(b)) states that no one shall create any noise that would exceed 65 dBA during daytime hours (6 AM to 10 PM) and 55 dBA during the nighttime hours (10 PM to 6 AM) as measured fifty (50) feet from the property line where the noise is generated exceeding the following community noise standards
- City of Tucson, Arizona
The Tucson Code (Section 16-31) states that the maximum noise allowed to radiated beyond a person’s property line will be no more than 70 dBA during daytime hours (7 AM to 10 PM) and 62 dBA during nighttime hours (10 PM to 7 AM).
- U.S. Department of Housing and Urban Development (HUD)
HUD uses the day-night average sound level, abbreviated DNL, to evaluate external noise environments at a site. A DNL of 65 dBA or less is considered acceptable.
Mitigation measures can include barriers (and/or berms), orientation of the buildings, ducting the intake, getting quieter AC units.
Barriers should be solid (>4 pounds per square foot) and airtight. However, the transmission loss of the barrier does not need reduce the noise by more than 10 dBA below the level attenuated by diffraction effects over and around the barrier. Examples of sound wall materials include: 4″ thick poured in place or pre-cast concrete panels, 6″ thick CMU wall, 1-½” thick board and batten, tongue and groove wood on one side of post, or cementitious wood panels. An earthen berm alone or in conjunction with a wall can be used.
A sound level meter app on a smart phone can give you an approximate noise level. They are best in a range from 50 to 70 dBA. An inexpensive meter can be purchased through Amazon (here is a link to a sound level meter and calibrator). It is best to get a calibrator with the sound level meter and be sure to measure on an A-weighted scale. We have seen these meters are not in line with certified sound level meters at lower and higher frequencies and higher that 80 dBA and lower then 40 dBA. However, they can help get an idea what the levels are.
Noise Engineers provides information and resources to help people address acoustical issues. In these episodes my goal is to provide resources, inexpensive tools, rules of thumb when dealing with acoustical issues. I would like to explain basic acoustic principles and answer any questions. I will describe actual projects to make this as practical as possible.