National Park Service



Date of this Version



Natural Resource Report NPS/NRSS/NSNS/NRTR 2020/2078 / NPS 638/166866, February 2020: xiii, 30 pages

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Please cite this publication as:

Manktelow, K. and E. Brown. 2020. Homestead National Monument of America: Acoustic monitoring (2011 - 2012). Natural Resource Report NPS/NRSS/NSNS/NRTR—2020/2078. National Park Service, Fort Collins, Colorado.


United States government work. Public domain material.


Executive Summary

This report presents acoustical data gathered by Student Conservation Association interns and the Natural Resource Specialist at Homestead National Monument of America in 2011 and 2012. Data were collected at four sites to provide park managers with information on the acoustical environment, sources of noise, and the existing ambient sound levels within the monument. The data will also inform the park managers with information regarding the potential impact of traffic on Highway 4.

Monitoring occurred at each site during two different seasons (except HOME002) in order to document seasonal variations. In each deployment, sound pressure level (SPL) was measured continuously every second by a calibrated sound level meter. The setup also 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), 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.

Overall, existing ambient sound levels (LA50, 24 hr) at the monument ranged from 36.5 dB to 50.2 dB in fall/winter and 34.9 dB to 59.0 dB in spring/summer. Table 2 reports the percent of time that measured levels at the monitoring locations were above four key sound level values. The first value, x 35 dB (LAeq, 1s), 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 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). 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 (20-1,250 Hz) is more appropriate for identifying impacts from anthropogenic noise in parks (Acoustical Society of America 2014). For instance, in the full frequency range, during the day, the 52 dB level was exceeded at HOME001 in the summer approximately 74% of the time and at HOME002 51% of the time. Speech interruption occurs (between two people 1 meter apart) at 60 dB and this level was exceeded 15% of the time at HOME002.

After data collection was complete, a trained technician calculated how often noise1 sources were audible. See Methods section for protocol details, equipment specifications, and metrics calculations. Sound source analysis revealed that noise is audible between 70-91% of the time at the study sites, depending upon season and location. The most common sources of noise during all seasons were vehicles and the nearby fertilizer factory. Natural sources such as wind, birds, and insects were also commonly audible. Natural ambient sound levels ranged from 33 to 58 dB (LAeq, 12hr), and were generally quieter in the winter than the summer, likely due to an absence of biologic sounds such as insects in the winter.

For the purposes of this document, we refer to “noise” as any human-caused sound that masks or degrades natural sounds