Date of this Version
Natural Resource Report NPS/NRSS/NSNSD/NRR 2020/2083 / NPS 368/167035, February 2020
Also available at: https://www.nps.gov/im/publication-series.htm
Please cite this publication as:
E. Brown. 2020. Homestead National Monument of America: Acoustic monitoring report - 2017. Natural Resource Report NPS/NRSS/NSNSD/NRR—2020/2083. National Park Service, Fort Collins, Colorado.
This report presents acoustical data gathered by Student Conservation Association interns and the Natural Resource Specialist at Homestead National Monument of America in 2017. Data were collected at one site to provide park managers with information about the acoustical environment, sources of noise, and the existing ambient sound levels within the monument. This deployment also captured acoustic conditions during the total solar eclipse on 8/21/2017. (Results of the eclipse monitoring effort are provided in Appendix B.)
In this deployment, sound pressure level (SPL) was measured continuously every second by a calibrated sound level meter. Other equipment included an anemometer to collect wind speed and direction and a digital audio recorder collecting continuous recordings to document sound sources. In this document, “sound pressure level” refers to broadband (12.5 Hz - 20 kHz), A-weighted, 1-second time averaged sound level (LAeq, 1s), and hereafter referred to as “sound level.” Sound levels are measured on a logarithmic scale relative to the reference sound pressure for atmospheric sources, 20 μPa. The logarithmic scale is a useful way to express the wide range of sound pressures perceived by the human ear. Sound levels are reported in decibels (dB). A-weighting is applied to sound levels in order to account for the response of the human ear (Harris, 1998). To approximate human hearing sensitivity, A-weighting discounts sounds below 1 kHz and above 6 kHz. For reference, Table 1 provides examples of sound levels measured in parks compared to sound levels of common sound sources.
Table 1. Sound level examples
Park Sound Sources Common | Sound Sources | Sound Level dB*
Volcano crater (HALE) | Human breathing at 3m | 10
Leaves rustling (CANY) | Whispering | 20
Crickets at 5 m (ZION) | Residential area at night | 40
Conversation at 5 m (WHMI) | Busy restaurant | 60
Cruiser motorcycle at 15 m (BLRI) | Curbside of busy street | 80
Thunder (ARCH) | Jackhammer at 2 m | 100
Military jet at 100m AGL (YUCH) | Train horn at 1 m | 120
* dB re 20 µPa A-weighted broadband (12.5 Hz—20 kHz), sound level measured over varied measurement durations and at the distances indicated.
Overall, existing ambient sound levels (LA50) at the monument were measured to be 50.3 dB during the day to 56.5 dB at night. Table 2 reports the percent of time that measured levels at the monitoring locations were above four key sound level values. The first value, 35 dB (LAeq, 1s), addresses the health effects of sleep interruption. Recent studies suggest that sound events as low as 35 dB can have adverse effects on blood pressure in sleeping humans (Haralabidis et al. 2008). This level, 35 dB, 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 dB (LAeq, 1s) (Berglund et al. 1999). The third value, 52 dB (LAeq, 1s), 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 dB (LAeq, 1s), provides a basis for estimating speech interference on normal voice communications at 1 meter. Visitors viewing scenic areas in the park would likely be conducting such conversations.
Sound levels are often measured over narrow frequency bands (typically in one-third octave bands between 12.5 Hz - 20 kHz) because these smaller bands closely represent how humans distinguish between frequencies of sound. In this study, we examine how often sound levels exceeded key values in two frequency ranges. The top value in each split-cell of Table 3 uses the full frequency range (12.5 Hz-20 kHz) collected, whereas the bottom value focuses on frequencies affected by low frequency noise sources (20-1,250 Hz). This Natural Sounds (NS) modification to A-weighting (referred to as ANS weighting, ANSI S3/SC1.100, 2014) eliminates high-frequency sound (leaf rustle, equipment noise, and biologic sounds) allowing for more accurate comparisons of low-frequency ambient sound levels across different land use types (e.g. urban, protected areas; ANSI S3/SC1.100, 2014). This frequency weighting scheme improves ambient sound level measurements in quiet environments. For instance, in the full frequency range, the 52 dB (LAeq, 1s) level was exceeded at HOME008 31 % of the time during the day and 84 % of the time at night, but in the 20-1,250 Hz range, the 52 dB functional sound level value was very rarely or never exceeded in daytime or nighttime. Speech interruption occurs (between two people 1 meter apart) at 60 dB (LAeq, 1s) and this level was never (or very rarely) exceeded at HOME008 during the day.
After data collection was complete, a trained technician calculated how often noise (For the purposes of this document, we will refer to “noise” as any human-caused sound that masks or degrades natural sounds) sources were audible. See Methods section for protocol details, equipment specifications, and metrics calculations. Sound source analysis revealed that noise is audible about 72% of the time at the study site when averaged across all hours of the day (Table 3). This amount of noise audibility is consistent with previous acoustic inventories in the monument, which ranged from 70–91% depending upon site and season. The most common sources of noise during this study period was non-natural unknown, which was likely the nearby fertilizer factory. Natural sources such as wind, rain, thunder, birds, and insects were also commonly audible. Specifically, recordings of coyotes, barred owl, and thunder were collected during this monitoring period. Natural ambient sound levels (LAnat) were measured to be 48.4 dB during the day and 55.2 dB at night. Increased natural ambient sound levels during the night were due to increased insect activity (see Figure 3 in the Results section).
Table 3. Mean time audible for human-caused noise, vehicles, and non-natural unknown, existing and natural ambient sound levels (dB re 20 μPa, A-weighted broadband,12.5 Hz—20 kHz) at HOME008 (where day is 7:00 – 19:00 and night is 19:00 – 7:00)
Site ID: HOME008 | Season: Summer
Mean time audible for noise (% of 24 hour time period), All Noise: 71.8 | Vehicle: 17.3 | Non Natural/Unknown: 32.8
Median Existing Ambient (LA50) in dB. Day: 50.3 | Night: 56.5
Median Natural Ambient (LAnat) in dB, Day: 48.4 | Night: 55.2
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