Noise Impact Assessment

Chapter Summary The two most common types of noise are point source and line source. Natural factors such as topography, vegetation, and temperature can reduce noise over distance. A hard site exists where sound travels away from the source over a generally flat, hard surface such as water, concrete, or hard-packed soil. When ground cover or normal unpacked earth is present between the source and receptor, the ground becomes absorptive to sound energy and is called a soft site. Topography, vegetation, and atmospheric factors can also affect the rate of sound attenuation. Existing ambient noise levels can serve as a baseline from which to measure potential disturbance caused by project activities. Baseline (ambient) noise levels vary greatly and depend on site-specific factors. Most transportation projects involve traffic noise. Identifying the amount and type of traffic helps to determine the baseline (ambient) noise conditions. One of the hardest things to quantify is noise associated with construction activities. Although noise from multiple sources at the same location results in louder levels than a single source alone, the decibel is on a logarithmic scale, so sound levels cannot be added by standard addition. For transportation projects, traffic noise typically determines the baseline noise level in the project area. In the absence of traffic, community or environmental noise levels may be important in project noise analysis. Defining the zone of noise impact requires the following steps: 1. Estimate the equipment noise level for the project. 2. Estimate the baseline (ambient) noise level. In most cases this can be done by defining traffic noise in the project area. 3. Determine whether hard or soft site conditions exist.