What other factors go into the acoustical design of noise barriers?
Factors that must be considered include current and future levels, which will dictate required effectiveness (noise reduction), site topography, actual land uses and types of receptors. Design parameters are primarily the height (how much the line-of-sight is broken), length (the "angular coverage" provided), and location between the source (highway) and receiver.
What kinds of impact assessment do you do? How does that work?
Impact assessment considers two factors: actual level, relative to the FHWA impact thresholds, and increase in noise level over existing levels resulting from a proposed project.
What are you able to do with the data you collect on transportation noise analysis?
Acoustic data collection serves two purposes; one as baseline information for impact assessment, and responding to citizen inquiries or complaints, and two as a method to validate the computer models.
What technologies and tools do you most frequently use?
For measurement, sound level meters are used to obtain existing noise levels. Computer prediction programs like FHWA's Traffic Noise Model (TNM) provide forecasts of future levels and include design tools for developing noise barrier designs.
How do you ensure that you have accurate data input and accurate computer models?
If we have a candidate area that we’re looking to assess, either just for noise impact or an actual noise barrier design study, the first aspect is to go out and physically measure the noise level at various locations, and at the same time, document environmental conditions that exist simultaneously with your measurement. You have to do classified counts of the traffic that’s going by as you do your measurements, documenting its speed, and hopefully you have reasonably neutral environmental conditions — no wind or rain. Once you take that real-world existing data, then when you go back and construct a computer model for that particular area, you, in a perfect world, can take that traffic, the associated speeds, all the trucks and other types of vehicles that you encountered in the field, plug that into your model, and ideally you will get the same noise level back from the model that you measured in the field. That’s a measure of your validation of your model.
That’s probably the biggest challenge of that whole process and procedure: getting things validated, getting reliable results out of your model. You physically develop the model, looking at terrain conditions, ground cover, and whether you have intervening buildings, or if you have existing barriers of any sort in the environment.
Any other challenges?
Obtaining accurate and complete data inputs for the computer models. TNM utilizes traffic parameters such as vehicle volumes, truck mixes, and speeds to arrive at noise level predictions. As with any computer model, the results are only as reliable as the input — recognizing, of course, the skill of the "modeler" in effectively and accurately constructing the model.