Why did we pursue this work?
National Park Service policy and legislative mandates require the agency to conserve acoustic and night sky environments unimpaired for the enjoyment of future generations. Director's Order #47 specifically addresses soundscape preservation and noise management. Park visitors and wildlife interact with each other and park resources through their senses, so conservation or restoration of these physical resources creates many benefits for the integrity of ecosystems and the quality of visitor experience.
What metric is used to describe the sound level?
The quantity in both the existing and natural map is the L50 sound pressure level, dBA re 20μPa. The L50 is the sound pressure level exceeded half of the time. A-weighting is an adjustment that reflects how the human ear perceives sound. All conditions are predicted for a typical daytime hour during the summer with calm weather conditions. Sound levels are often lower at night and during the winter.
To be more specific, we make one-second A-weighted time-averaged sound level measurements (LAeq, 1s), collect all of the summer daytime seconds for a given site, sort these values, and report the center value (the median). This means that for any given second at that site, there is a 50% chance that the sound level will be greater than or equal to the reported value. A-weighting is the commonly used method for combining sound energy across the entire audio spectrum;it accounts for the decreased sensitivity of human hearing at low (<1 kHz) and very high (>6 kHz) frequencies.
A few notes about decibels (dB), which are measured on a logarithmic scale. A 3 dB increase corresponds to a doubling of sound energy, and a 10 dB increase corresponds to a tenfold increase in sound energy. Those pairings of numbers have another interpretation. A 3 dB increase in background sound levels will reduce the area from which you could previously have heard a natural sound by 50%;a 10 dB increase in background sound level decreases your "listening area" by 90%.
What are "existing" and "natural" conditions?
The existing sound level is the condition as measured;it accounts for all of the acoustic energy including anthropogenic, biotic and abiotic sources. Predictions from the geospatial sound model are expected conditions based on relationships between observed conditions and many explanatory variables. A natural scenario was synthesized by systematically minimizing human factors, leaving only physiographical sources of sound like wind, flowing water, precipitation, animals, and geological events.
Where were measurements made?
NPS initiated a system-wide effort to inventory sound levels in national park units. Measurements are long term, often 25 days or more, to capture a range of activity and weather conditions at each site. Additional measurements were obtained from urban and suburban areas across the country.
This revised map uses more empirical data, both acoustical measurements and explanatory geospatial data. The model included 479 unique site locations. Some sites were sampled in more than one season, for a total of 966 observations. There are about 1,500,000 hours of acoustical data overall.
How accurate is the map?
In general, urban areas can be predicted more accurately than natural areas. Whereas urban areas are dominated by human activity, natural soundscapes are a complex mix of many sources including human activity. Because they are quieter, sources can be heard from much farther away and a natural soundscape can encompass a very large area.
Accuracy was evaluated rigorously using an exhaustive "leave-one-out" cross validation. At half of the natural sites, levels are predicted within 3.1 dB whereas urban sites are predicted within 1.7 dB (this is the median absolute deviation). Errors are larger at other sites. The largest errors arise from inadequacies of the geospatial data to describe powerful acoustic sources like nearby rivers and roads. For example, at one site in Olympic National Park, levels were much lower than expected because a nearby road was closed during the time coinciding with the measurement period.
There are more places we haven't sampled than we have and no doubt some of them have extreme sonic environments. These maps describe the expected long term conditions in most places.
Are the data available?
The dataset is publicly available for download through the NPS Data Store. Available products include georeferenced raster data for the contiguous United States, Alaska, and Hawaii as well as explanatory data and related metadata.
D. J. Mennitt and K. Fristrup, "Influential factors and spatiotemporal patterns of environmental sound levels," Proceedings of INTER-NOISE 2015 San Francisco, CA (2015). [PDF]
D. J. Mennitt, K. Fristrup, and K. Sherrill, "A geospatial model of ambient sound pressure levels in the contiguous United States," The Journal of the Acoustical Society of America, 135, pp. 2746-2764, 2014. http://dx.doi.org/10.1121/1.4870481
D. J. Mennitt, K. Fristrup, K. Sherrill, and L. Nelson, "Mapping sound pressure levels on continental scales using a geospatial sound model," Proceedings of INTER-NOISE 2013, Innsbruck, Austria (2013). [PDF]
D. J. Mennitt, K. Fristrup, and L. Nelson, "Mapping the extent of noise on a national scale using geospatial models," Presented at the 166th meeting of the Acoustical Society of America, San Francisco, USA (2013).