Theodore Roosevelt National Park: Acoustic monitoring report

McFarland, Scott

In 2012, the Natural Sounds and Night Skies Division (NSNSD) received a request to collect baseline acoustical data at Theodore Roosevelt National Park (THRO). Between July 2012 and August 2015, seven acoustical monitoring systems were deployed for approximately 30 days each. The goal of the study was to establish a baseline inventory of the soundscape at THRO. This inventory will be used to establish indicators and standards of soundscape quality that will support the park and NSNSD in developing a comprehensive approach to protecting the acoustic environment through soundscape management planning. Results of this study will help the park evaluate the potential effects of various man-made sound sources, especially unprecedented oil and gas development throughout the region. For the purposes of this document, we will refer to “noise” as any human-caused sound that masks or degrades natural sounds (Lynch et al. 2011). The most common sources of noise at THRO include vehicles, aircraft, and motors (distant low frequency sounds, likely associated with oil and gas machinery). Table 1 displays percent time audible values for each of these common noise sources during the monitoring period, as well as ambient sound levels. Ambient sound pressure levels were measured continuously every second over the 30 day monitoring period by a calibrated, Type 1, Larson Davis 831 sound level meter. Percent time audible metrics were calculated by trained technicians after monitoring was complete. See Methods section for protocol details and equipment specifications. Median existing (L50) and natural (Lnat) ambient metrics are also reported for daytime (7 am – 7 pm) and nighttime (7 pm – 7am). See Methods section for detailed information on how these metrics are calculated. In determining the current conditions of an acoustical environment, it is informative to examine how often sound pressure levels exceed certain values. Table 2 reports the percent of time that measured levels were above four key values. The first value, 35 dBA, is designed to address the health effects of sleep interruption. Recent studies suggest that sound events as low as 35 dB can have adverse effects on blood pressure while sleeping (Haralabidis et al. 2008). This is also the desired background sound level in classrooms (ANSI S12.60-2002). The second value addresses the World Health Organization’s recommendations that noise levels inside bedrooms remain below 45 dBA (Berglund et al. 1999). The third value, 52 dBA, is based on the EPA’s speech interference level for speaking in a raised voice to an audience at 10 meters (EPA 1974). This value addresses the effects of sound on interpretive presentations in parks. The final value, 60 dBA, provides a basis for estimating impacts on normal voice communications at 1 meter. Visitors viewing scenic areas in the park would likely be conducting such conversations. The top value in each split-cell focuses on frequencies affected by transportation noise (20-1250 Hz) whereas the bottom values use the full frequency range (12.5-20,000 Hz) collected. Most motorized human-caused noise is confined to the truncated, lower-frequency range, while many natural sounds, including insects and birds, are higher in pitch. Therefore, the truncated range (20-1250 Hz) is more appropriate for identifying impacts from anthropogenic noise in parks (Acoustical Society of America 2014).

Theodore Roosevelt National Park: Acoustic monitoring report

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