I’m going to discuss traffic noise evaluations. Whenever a street is extended, moved, altered, or another one is constructed and there are homes, schools, hospitals, places of worship adjacent, etc, a sound study is needed. The study looks to check whether sound mitigation should be installed.
Sound walls are the typical noise reducing measure utilized to reduce traffic. In a few places, reduction from RAC (rubber asphalt concrete) is permitted. You can move the street up, down or to one side or another but that does not commonly produce sufficient noise reduction. Now and again were sound alleviation is justified and sound walls are not plausible, purchasing properties or enhancing outer surface of a house is done. By and large by enhancing the windows.
Generally concrete, tilt up sound walls are used to reduce noise.
Traffic Noise Study Procedure
In a sound study you begin by assessing the site to recognize the noise sensitive locations (homes/schools/parks/libraries/houses of worship). In the event that there is a group of homes, you need to verify you assess the most affected home, if there is no noise issue at that home, then the remaining homes ought to be fine. If there is a noise problem at the most impacted home in a group, then you have to assess others in the group to determine the degree of the impact.
The sound study methodology is as follow:
- predict the current sound levels from the current road at homes/parks/schools/hospitals during the loudest hour (generally rush hour). This needs to be carried out utilizing free program from Federal Highway Administration (FHWA) called Traffic Noise Model (TNM).
- measure the sound levels at the homes during rush hour to verify the model is right. Adjust the noise model if necessary.
- predict the future loudest sound hour condition using more TNM.
- compare the future sound levels with the criteria – to check whether sound mitigation needs to be assessed.
If mitigation needs to be evaluated, determine the needed size of the barrier and see if it is reasonable
In the first place you have to figure out who is financing the project change. It could have federal financing, state financing, county or city. Knowing this will allow you to determine what regulations you have to use.
Most all traffic sound regulations utilize the Federal Highway Administration’s (FHWA) Noise Abatement Criteria (NAC) as the premise for their regulation. For homes, holy places, parks, schools, clinics and so forth, the FHWA sound limit is approaching 67 dba (for the loudest one hour equivalent sound level – rush hour) – so 66 dba or less. There is a second criteria that the sound level can not increase by more than 5-15 dba.
Each one state, region, and city will have their own particular meaning of methodology and permissible increment. In Arizona, ADOT says that the maximum is 64 dba – however the county I live in 66 dba is the maximum. The county here says that you can give a 3 dba reduction for utilizing RAC. The state does not permit you to take any reduction for RAC. They both consider a significant increase as 15 dba. So you have to know whose rule you have to take after.
I have tried RAC, it can significantly reduce the wheel-street interaction sound level yet does nothing for sound from the motor. It is effective on level streets or down slopes and better if a large portion of the vehicles are autos. It is less effective on up hills and on trucks which have a large amount of sound from the motor.
When you know whether there are homes that will be affected (have high soundlevels), you have to check whether sound mitigation (normally sound walls) are feasible and sensible.
Every location with have distinctive guidelines, but here are some general rules:
- The sound wall will be limited to 10-20 feet high
- If only one home/church/park/school/and so forth is helped, the sound wall is unreasonable
- There will be an expense limit for a sound wall. You have to ascertain the tallness and length of the proposed sound wall that will give sound mitigation to these affected homes. The state/county/city will tell you the limit per square foot (for example, $35/ft2). At that point you decide the number of homes are being helped and you ascertain the expense every benefitted home. The rule will state the maximum allowable cost, for example, $49k every benefited home.
- A receiver with a benefit is one that gets no less than 5 dba sound attenuation from the sound wall.
- A few regulations will require a reduction of no less than 7 dba sound decrease
- Alleviation is for just the first floor of multiple story homes
- No alleviation will be accommodated undeveloped properties unless building grant issued before the last EAMR record.
In the event that the sound wall is considered reasonable, the majority of the affected property holders must favor the wall.
Here are a few general guidelines.
- In the event that you increase the volume of traffic by two, you get a 3 dba increment in commotion.
- In the event that you increase your separation from the street by two, you get around 3 dba lower.
- Blocking the path between the sound source (vehicle) and the receiver (person’s ear), you will get 5 dba of noise diminished.
- If you totally block 50% of the street, you will get a 3 dba sound level reduction.
- Typically a break in an obstruction, (for example, access from the road) will render it ineffective (giving short of 5 dBA reduction)
The TNM program takes the road geometry, vehicle speeds, the mix of vehicles (cars, medium trucks, heavy trucks, motorcycles), topography, ground type (grass, hard soil, …) and the location of the receivers.
Thank you for visiting Noise Expert Acoustical Consulting – Please let us know if you have any questions or if you would like our assistance with acoustical evaluations. 480-332-9325, firstname.lastname@example.org, www.noiseexpert.com
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