Noise map of the New York City Metropolitan Area. Note the darker spots at various airports; aviation traffic is known to be particularly problematic.
The noise levels can vary at any time. For instance, environmental noise may increase due to a single vehicle as it passes by, or it may reach higher, longer levels during rush hour traffic. In general, humans create increased noise levels by day rather than at night, although weather, wind, and seasons impact noise levels as well.
This means that there is some art form to predicting environmental noise at a particular time. However, when the sources are well-defined, such as rail or air traffic, predicting some noise levels may be more predictable. It may also be possible to predict with reasonable accuracy the long-term average levels of environmental noise.
Typically, measuring environmental noise has its challenges and issues. To determine a long-term average, measurements would need to be obtained over a long period of time. In addition, access to private property might be necessary but unavailable; and measurements do not differentiate between sources of environmental noise.
These are the most prevalent reasons that noise mapping is completed with computerized modeling over a region, although measurements may suitable in some instances.
Furthermore, noise models can assess the impact of various transportation and other sources. This means the impact of a new road with ingress and egress to a new housing development can be evaluated to mitigate the impact of the future noise. This is useful in noise action planning, in which cost-benefit analysis of mitigating options may be tested first.