National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science

Homestead National Monument of America

Acoustic monitoring report - 2017

Natural Resource Report NPS/NRSS/NSNSD/NRR—2020/2083
ON THE COVER

Homestead Heritage Center, NPS Photo
Homestead National Monument of America

Acoustic monitoring report - 2017

Natural Resource Report NPS/NRSS/NSNSD/NRR—2020/2083

Emma Brown

National Park Service

Natural Sounds and Night Skies Division

1201 Oakridge Drive, Suite 100

Fort Collins, Colorado 80525

February 2020

U.S. Department of the Interior

National Park Service

Natural Resource Stewardship and Science 

Fort Collins, Colorado
ii

The National Park Service, Natural Resource Stewardship and Science office in Fort Collins, 

Colorado, publishes a range of reports that address natural resource topics. These reports are of 

interest and applicability to a broad audience in the National Park Service and others in natural 

resource management, including scientists, conservation and environmental constituencies, and the 

public.

The Natural Resource Report Series is used to disseminate comprehensive information and analysis 

about natural resources and related topics concerning lands managed by the National Park Service. 

The series supports the advancement of science, informed decision-making, and the achievement of 

the National Park Service mission. The series also provides a forum for presenting more lengthy 

results that may not be accepted by publications with page limitations. 

All manuscripts in the series receive the appropriate level of peer review to ensure that the 

information is scientifically credible, technically accurate, appropriately written for the intended 

audience, and designed and published in a professional manner. 

Data in this report were collected analyzed using methods based on established, peer-reviewed 

protocols and were analyzed and interpreted within the guidelines of the protocols.

Views, statements, findings, conclusions, recommendations, and data in this report do not necessarily 

reflect views and policies of the National Park Service, U.S. Department of the Interior. Mention of 

trade names or commercial products does not constitute endorsement or recommendation for use by 

the U.S. Government.

This report is available in digital format from the Natural Resource Publications Management 

website. If you have difficulty accessing information in this publication, particularly if using assistive 

technology, please email irma@nps.gov.

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.

NPS 368/167035, February 2020
iii

Contents

Page

Figures....................................................................................................................................................v

Tables...................................................................................................................................................vii

Executive Summary..............................................................................................................................ix

Acknowledgments...............................................................................................................................xiii

Glossary of Acoustic Terms.................................................................................................................xv

Introduction............................................................................................................................................1

National Park Service Natural Sounds and Night Skies Division..................................................1

Soundscape Planning Authorities...................................................................................................1

Study Area .............................................................................................................................................3

Methods..................................................................................................................................................5

Automatic Monitoring....................................................................................................................5

Monitoring Period ..........................................................................................................................6

Calculation of Metrics....................................................................................................................6

Off-Site Listening...........................................................................................................................6

Results....................................................................................................................................................9

Frequency content ..........................................................................................................................9

Sound level: Time Above.............................................................................................................10

Sound Level: Percentile Levels....................................................................................................11

Event duration ..............................................................................................................................12

Conclusions..........................................................................................................................................17

Literature Cited ....................................................................................................................................19

Appendix A: Office Listening..............................................................................................................21

Appendix B. Results of eclipse monitoring .........................................................................................23

v

Figures

Page

Figure 1. Location of monitoring sites...................................................................................................4

Figure 2. SCA intern Kelly Manktelow identifies sound sources using Acoustic 

Monitoring Toolbox (NPS) software .....................................................................................................7

Figure 3. Day and night percentile sound pressure levels for 33 one-third octave bands at 

HOME008............................................................................................................................................10

Figure 4. Median percentile sound levels (LAeq, 1s), in dB re 20 μPa, at HOME008...........................12

Figure 5. Hourly time audible for airplane, vehicle, and all noise sources at HOME008. ..................15

Figure 6. Screen shot of Listening Center. Three ten second samples are displayed side by 

side. ......................................................................................................................................................21

Figure 7. Locations of acoustic monitoring stations deployed during the solar eclipse. .....................23

Figure 8. Bioacoustic monitoring station in Homestead N.M. ............................................................24

Figure 9. (A) One-third octave sound levels before/after and during eclipse, and (B) 

change in types of sounds between eclipse and non-eclipse days .......................................................25

vii

Tables

Page

Table 1. Sound level examples .............................................................................................................ix

Table 2. Time above metrics for HOME008. ........................................................................................x

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 ..................................................................................................................xi

Table 4. Metadata for each season of acoustical monitoring.................................................................3

Table 5. Time above metrics for HOME008. ......................................................................................11

Table 6. Mean hourly time audible (%) for each natural sound source at HOME008, n=8 

days......................................................................................................................................................13

Table 7. Mean hourly time audible (%) for each noise source at HOME008, n=8 days.....................14

ix

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 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 8/21/2017.1

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 5m (ZION) Residential area at night 40

Conversation at 5m (WHMI) Busy restaurant 60

Cruiser motorcycle at 15m (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 

1 Results of the eclipse monitoring effort are provided in Appendix B. 
x

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.

Table 2. Time above metrics for HOME008.

Site ID

Frequency 

(Hz)

Time above sound level

(% of daytime hours, 07:00 to 19:00)

Time above sound level (% of 

nighttime hours, 19:00 to 07:00)

35

dB*

45

dB*

52

dB*

60

dB*

35

dB*

45

dB*

52

dB*

60

dB*

HOME008 

12.5-20,000 100.0 97.7 31.0 0.0 100.0 100.0 84.1 18.9

20-1,250 30.3 0.8 0.1 0.0 7.6 0.2 0.0 0.0

* dB LAeq, 1s re 20 μPa

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 lowfrequency 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 noise2 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 

2

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

natural sounds
xi

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 Season

Mean time audible for noise

(% of 24 hour time period)

Median 

Existing 

Ambient (LA50)

in dB

Median 

Natural 

Ambient 

(LAnat) in dB

All Noise Vehicle

Non Natural 

Unknown Day Night Day Night

HOME008 Summer 71.8 17.3 32.8 50.3 56.5 48.4 55.2

xiii

Acknowledgments

Acoustic data for this report was gathered by Jesse Bolli. Audibility analysis completed by D. 

Hanselmann.

xv

Glossary of Acoustic Terms

Glossary of acoustic terms and definitions

Term Definition

A-weighting

A-weighting is applied to sound levels in order to account for the sensitivity of 

the human ear (Harris, 1998). To approximate human hearing sensitivity, Aweighting discounts sounds below 1 kHz and above 6 kHz.

Acoustic Environment

A combination of all the physical sound resources within a given area. This 

includes natural sounds and cultural sounds, and non-natural human-caused 

sounds. The acoustic environment of a park can be divided into two main 

categories: intrinsic and extrinsic.

Acoustic Resources Includes both natural sounds like wind, water, & wildlife and cultural and historic 

sounds like tribal ceremonies, quiet reverence, and battle reenactments.

Amplitude The relative strength of a sound wave, described in decibels (dB). Amplitude is 

related to what we commonly call loudness or volume.

ANS Weighting

The Natural Sounds (NS) modification to A-weighting 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.

Audibility

The ability of animals with normal hearing, including humans, to hear a given 

sound. It can vary depending upon the frequency content and amplitude of 

sound and by hearing ability of individual animals.

Decibel (dB)

A unit of sound energy. 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 as decibels (dB). Every 

10 dB increase represents a tenfold increase in energy. Therefore, a 20 dB 

increase represents a hundredfold increase in energy. 

Existing ambient sound level 

(LA50)

sound level (LAeq, 1s) exceeded 50% of the time (50th percentile) for a specified 

duration. This level is referred to as the existing ambient sound level and the 

preferred metric for chronic conditions, as it is insensitive to infrequent loud 

events.

Frequency

Related to the pitch of a sound, and defined as the number of times per second 

that the wave of sound repeats itself and is expressed in terms of hertz (Hz). 

Sound levels are often adjusted ("weighted") to match the hearing abilities of a 

given animal. In other words, different species of animals and humans are 

capable or hearing (or not hearing) at different frequencies. Humans with 

normal hearing can hear sounds between 20 Hz and 20,000 Hz, and as low as 

0 dB at 1,000 Hz. Bats, on the other hand, can hear sounds between 20 Hz and 

200,000 Hz.

Percentile sound levels (LA10, 

LA50, LA90)

Metrics used to describe A-weighted sound pressure levels (L), in decibels, 

exceeded 10, 50, and 90 percent of the time, respectively. Put another way, 

half the time the measured levels of sound are greater than the LA50 value, 

while 90 percent of the time the measured levels are higher than the LA90 value,

and 10 percent of the time measured levels are higher than the LA10 value.
xvi

Glossary of acoustic terms and definitions (continued)

Term Definition

Day-Night average sound levels 

(Ldn)

Day-Night Average Sound Level. Average equivalent sound level over a 24-

hour period, with a 10-dB penalty added for sound levels between 10 p.m. and 

7 a.m.

Energy Equivalent Sound Level 

(LAeq)

The sound energy level averaged over the measurement period. Generally, 

refers to A-weighted 1-second time averaged sound levels measured between 

12.5 Hz - 20 kHz. This is a standard measurement collected using NSNSD 

acoustic monitoring protocol for sound level meters. Sound levels measured 

over 1 second intervals are used to calculate summary statistics, specifically 

percent of the time a sound level of interest is exceeded.

Natural Ambient Sound Level 

(LAnat )

The natural sound conditions in parks, which would exist in the absence of any 

human-caused noise sources. LAnat is the preferred metric to represent baseline 

or reference conditions.

Noise Free Interval (NFI) The length of the continuous period of time during which no human-caused 

sounds are audible.

Time Above

Within a defined time period, the percent of the time sound levels (LAeq, 1s) are 

above a specified sound level (LAeq, 1s). Commonly used levels are 35, 45, 52 

dB (LAeq, 1s). 

Time Audible

The amount of time that various sound sources are audible to humans with 

normal hearing, commonly expressed in percent of day, or percent of daytime 

hours and nighttime hours. A sound may be above natural ambient sound 

pressure levels, but still not audible. Similarly, some sounds that are below the 

natural ambient can be audible. Time Audible is useful because of its simplicity. 

It is a measure that correlates well with visitor complaints of excessive noise 

and annoyance. Most noise sources are audible to humans at lower levels than 

virtually all wildlife species. Therefore, time audible is a protective proxy for 

wildlife. These data can be collected either by a trained observer (on-site 

listening) or by making high-quality digital recordings for later playback (off-site 

listening).

Sound Exposure Level (SEL) The total sound energy of the actual sound during a specific time period. SEL is 

usually expressed using a time period of one second.

Sound Pressure Minute change in atmospheric pressure due to passage of sound that can be 

detected by microphones.

Sound vs. Noise

Sound and noise are often used interchangeably to describe an acoustic 

source. A common definition of noise is unwanted sound or sounds that 

interfere with a signal of interest (Harris 1998; Templeton 1997). However, 

noise is not a purely subjective designation. Any sound that serves no function 

is noise. Most sounds produced by human transportation and other machinery 

are unintended and serve no function, therefore are noise regardless of the 

attitudes of the listener. While there are unintended sounds in nature, like the 

footfalls of an animal, these sounds provide vital cues for some receivers and 

are therefore considered sounds to the receiver, yet noise from the perception 

of the producer. 

Soundscape The human perception of physical sound resources.
1

Introduction

A 1998 survey of the American public revealed that 72 percent of respondents thought that providing 

opportunities to experience natural quiet and the sounds of nature was a very important reason for 

having national parks, while another 23 percent thought that it was somewhat important (Haas &

Wakefield 1998). In another survey specific to park visitors, 91 percent of respondents considered 

enjoyment of natural quiet and the sounds of nature as compelling reasons for visiting national parks 

(McDonald et al. 1995). Acoustical monitoring provides a scientific basis for assessing the status of 

acoustic resources, identifying trends in resource conditions, quantifying impacts from other actions, 

assessing consistency with park management objectives and standards, and informing management 

decisions regarding desired future conditions.

National Park Service Natural Sounds and Night Skies Division

The Natural Sounds and Night Skies Division (NSNSD) helps parks manage sounds in a way that 

protects park resources and the visitor experience. The NSNSD addresses acoustical issues raised by 

Congress, NPS Management Policies, and NPS Director’s Orders. The NSNSD works to protect, 

maintain, or restore acoustical environments throughout the National Park System. Its goal is to 

provide coordination, guidance, and a consistent approach to soundscape protection with respect to 

park resources and visitor use. The program also provides technical assistance to parks in the form of 

acoustical monitoring, data processing, park planning support, and comparative analyses of 

acoustical environments.

Soundscape Planning Authorities

The National Park Service Organic Act of 1916 states that the purpose of national parks is "... to 

conserve the scenery and the natural and historic objects and the wild life therein and to provide for 

the enjoyment of the same in such manner and by such means as will leave them unimpaired for the 

enjoyment of future generations." In addition to the NPS Organic Act, the Redwoods Act of 1978 

affirmed that, "the protection, management, and administration of these areas shall be conducted in 

light of the high value and integrity of the National Park System and shall not be exercised in 

derogation of the values and purposes for which these various areas have been established, except as 

may have been or shall be directly and specifically provided by Congress."

Direction for management of natural soundscapes3 is represented in 2006 Management Policy 4.9:

The Service will restore to the natural condition wherever possible those park soundscapes that have 

become degraded by unnatural sounds (noise), and will protect natural soundscapes from 

unacceptable impacts. Using appropriate management planning, superintendents will identify what 

levels and types of unnatural sound constitute acceptable impacts on park natural soundscapes. The 

frequencies, magnitudes, and durations of acceptable levels of unnatural sound will vary throughout a 

3 The 2006 Management Policy 4.9 and related documents refer to “soundscapes” instead of “acoustic resources.” 

When quoting from this authority, it is advisable to note that the term often refers to resources rather than visitor 

perceptions.
2

park, being generally greater in developed areas. In and adjacent to parks, the Service will monitor 

human activities that generate noise that adversely affects park soundscapes [acoustic resources], 

including noise caused by mechanical or electronic devices. The Service will take action to prevent 

or minimize all noise that through frequency, magnitude, or duration adversely affects the natural 

soundscape [acoustic resource] or other park resources or values, or that exceeds levels that have 

been identified through monitoring as being acceptable to or appropriate for visitor uses at the sites 

being monitored (NPS 2006a).

It should be noted that “the natural ambient sound level—that is, the environment of sound that exists 

in the absence of human-caused noise—is the baseline condition, and the standard against which 

current conditions in a soundscape [acoustic resource] will be measured and evaluated” (NPS 

2006b). However, the desired acoustical condition may also depend upon the resources and the 

values of the park. For instance, “culturally appropriate sounds are important elements of the national 

park experience in many parks” (NPS 2006b). In this case, “the Service will preserve soundscape 

resources and values of the parks to the greatest extent possible to protect opportunities for 

appropriate transmission of cultural and historic sounds that are fundamental components of the 

purposes and values for which the parks were established” (NPS 2006b).

Further guidance is provided in 2006 Management Policies 4.1.4 Partnerships, 4.1.5 Restoration of 

Natural Systems, 8.2 Visitor Use, 8.2.2 Recreational Activities, 8.2.3 Use of Motorized Equipment, 

and 8.4 Overflights and Aviation Uses (NPS 2006).

Directors Order 47, Preservation of the Acoustic Environment and Noise Management (2015) builds 

on the principles set out in Management Policies, but goes on to direct how and when to consider 

acoustic resources in park management. Through this order, parks are guided to manage noise by: 

identifying noise sources, minimizing noise from park operations, considering the acoustic 

environment in park planning documents, and promoting park sounds and noise management through 

communication, education, and outreach.

National Parks Air Tour Management Act (NPATMA) was passed on April 5, 2000 to regulate 

commercial air tour operations for each unit of the National Park System, or abutting tribal land, 

where such operations occur or are proposed. The Act required the Federal Aviation Administration 

(FAA), in cooperation with the NPS, to develop an Air Tour Management Plan (ATMP) for each unit 

of the National Park System to provide acceptable and effective measures to mitigate or prevent the 

significant adverse impacts, if any, of commercial air tour operations upon natural and cultural 

resources and visitor experiences. In 2012, NPATMA was amended to allow the FAA and NPS to 

enter into voluntary agreements with a commercial air tour operator as an alternative to an ATMP.
3

Study Area

This report covers results from an acoustic inventory conducted in 2017 at HOME008, but the same 

acoustic monitoring system was deployed seven times at 4 different sites in the park during 2011 and 

2012 (Table 4). Sites HOME001 and HOME003 were chosen because of potential plans for an 

outdoor education classroom and for their proximity to Highway 4. Site HOME002 was chosen due 

to its proximity to the historic Freeman School and Highway 4. Site HOME004 was chosen as a 

representative of the woodland area located further from Highway 4. HOME008 was an ideal 

location for collecting data during the 2017 eclipse because it was located far from centers of human 

activity like the visitor centers. Figure 1 shows the location of acoustic inventory sites in map format. 

Table 4. Metadata for each season of acoustical monitoring

Site Site Name Dates 

Deployed

Vegetation Elevation

(m)

Latitude Longitude

HOME001 South of fire ring 8/11/2011-

9/9/2011

Riparian area, 

deciduous forest

376 40.29030 -96.83520

HOME001 South of fire ring 12/21/2011-

1/12/2012

Riparian area, 

deciduous forest

376 40.29030 -96.83520

HOME002 Freeman School 9/10/2011-

10/12/2011 Grass 384 40.29278 -96.84254

HOME003 Picnic Area 11/3/2011-

12/02/2011

Riparian area, 

deciduous forest

391 40.29129 -96.83483

HOME003 Picnic Area 6/22/2012-

7/15/2012

Riparian area, 

deciduous forest

391 40.29129 -96.83483

HOME004 Cottonwood Tree 3/28/2012-

4/12/2012 Deciduous forest 381 40.28622 -96.83797

HOME004 Cottonwood Tree 9/14/2012-

10/9/2012 Deciduous forest 381 40.28622 -96.83797

HOME008 South Woodland 8/12/2017-

9/11/2017 Riparian 388 40.28526 -96.83923
4

Figure 1. Location of monitoring sites
5

Methods

Automatic Monitoring

A Larson Davis 831 sound level meter (SLM) was deployed at this monitoring site. The Larson 

Davis SLM is a hardware-based, real-time analyzer which constantly records sound pressure level 

(SPL) and one-third octave band data. This Larson Davis-based site met American National 

Standards Institute (ANSI) Type 1 standards. The sound level meter provided the information needed 

to calculate metrics described below in Calculation of Metrics.

Acoustical monitoring equipment is used by many industries to determine noise levels in different 

environments, both indoors and outdoors. NPS uses equipment that is similar to the equipment used 

by other industries, but has developed a unique configuration that stands up to the potentially hash 

environment encountered in national parks. The microphone with environmental shroud was set up 

on a tripod at 1.5 m, which is an approximation of the average height of the human ear. The digital 

audio recorder recorded continuous audio through the entire monitoring period. An anemometer was 

attached to a tripod and placed approximately 10 feet from the microphone, to capture local wind 

conditions without recording possible sound from anemometer wind cup rotation.

The sampling station consisted of:

• Type 1 sound level meter

• Microphone with environmental shroud

• Preamplifier

• 12 V battery pack

• Anemometer (wind speed and direction)

• Digital audio recorder

The sampling station collected:

• A-weighted 1 second time averaged sound level (LAeq, 1s) in dB re 20 μPa

• Continuous digital audio recordings

• One-third octave band data every second ranging from 12.5 Hz – 20,000 Hz

• Continuous meteorological data for wind speed
6

Monitoring Period

The monitoring period lasted approximately 29 days. NSNSD has determined that 25 day monitoring 

periods during opposing seasons allow the data to capture seasonal difference that occur at each site

within a reasonable margin of error (NPS 2005).

Calculation of Metrics

The status of the acoustical environment can be characterized by sound level (LA50, LAnat, LA90, LA10, 

LAeq) and frequency content, and event durations (through off-site listening). NPS uses descriptive 

figures and metrics to interpret these characteristics. 

Two fundamental descriptors are existing ambient (LA50) and natural ambient (LAnat) sound levels. 

These are both examples of percentile levels, where each Lx value refers to the sound level that is 

exceeded x% of the time. The LA50 represents the median sound level, and is drawn from a full 

dataset (removing data with wind speed > 5m/s to eliminate error from microphone distortion). The 

LA50is the preferred metric to represent prevailing acoustic conditions. The natural ambient (LAnat) is 

an estimate of what the sound levels for a site would be if all human-caused noise sources were 

removed. LAnat is the preferred metric to represent baseline or reference conditions.

For a given hour (or other specified time period), LAnat is calculated to be the sound level exceeded x 

percent of the time, where x is defined by equation (1):

xx = 100−PPHH

2

+ PPHH,

PH is the percentage of samples containing noise for the hour. For example, if human caused sounds 

are present 30% of the hour, x = 65, and the LAnat is equal to the L65, or the level exceeded 65% of the 

time. To summarize and display these data, the median of the hourly LAnat values for the daytime 

hours (0700-1900) and the median of the hourly LAnat values for the nighttime (1900-0700) are 

displayed in Figure 4 in the results section. 

Off-Site Listening

Off-site listening is normally done by listening to an audio recording and simultaneously visually 

analyzing a spectrogram (for more information on Listening Center software, see Appendix C). 

Auditory analysis was used to calculate the audibility of sound sources at the monitoring locations. 

Trained technicians at HOME (Figure 2) listened to a subset of .mp3 audio samples (10 seconds 

every two minutes for eight days of audio) in order to identify durations of audible sound sources. 

The total percent time noise was audible was then used to calculate the natural ambient sound level 

(LAnat) for each hour (see Equation 1 above for more information). Bose Quiet Comfort Noise 

Canceling headphones were used for off-site audio playback to minimize limitations imposed by the 

office acoustical environment. For the complete results of this thorough audibility analysis, see Table 

6 and Table 7.

(1)
7

Figure 2. SCA intern Kelly Manktelow identifies sound sources using Acoustic Monitoring Toolbox (NPS) 

software

9

Results

Frequency content

In order to determine the effect that noise has on the acoustical environment, it is useful to examine

percentile metrics across a frequency range. High frequency sounds (such as a cricket chirping) and 

low frequency sounds (such as flowing water) often occur simultaneously, so the frequency spectrum 

is split into 33 smaller ranges, each encompassing one-third of an octave. These smaller bands 

closely represent how humans distinguish between frequencies of sound. For each one-third octave 

band, sound level (Leq, 1s) was recorded once per second for the duration of the monitoring periods. 

The percentile sound levels for 33 one-third octave band frequencies over the day and night periods 

are shown in Figure 3. 

Examining the sound energy in each one-third octave band (combined with digital audio recordings) 

allows acoustic technicians to determine what types of sounds are contributing to the overall sound 

levels at a site. The grayed area of Figure 3 represents sound levels outside of the typical range of 

human hearing. The percentile levels (Lx) are also shown for each one-third octave band. They 

represent the sound levels exceeded x percent of the measurement period. For example, L90 is the 

sound level that has been exceeded 90% of the time, and only the quietest 10% of the samples can be 

found below this point. On the other hand, the L10 is the sound level that has been exceeded 10% of 

the time, and 90% of the measurements are quieter than the L10. The bold portion of the column 

represents the difference between L50 and Lnat. The height of this bold portion is a measure of the 

contribution of anthropogenic noise to the existing sound levels at this site. The size of this portion of 

the column is directly related to the percent time that human caused sounds are audible. When bold 

portions of the column do not appear the natural and existing sound levels were either very close to 

each other, or were equal. The typical frequency levels for transportation, conversation, and 

songbirds are presented on the figure as examples for interpretation of the data. These ranges are 

estimates and are not vehicle-, species-, or habitat-specific.
10

Figure 3. Day and night percentile sound pressure levels for 33 one-third octave bands at HOME008

Sound level: Time Above 

To understand how acoustic conditions in the park might affect visitors, measured sound levels are 

compared to established sound levels. Specifically, Table 5 reports the percent of time that measured 

sound levels (LAeq,1s) were above four key functional values during the monitoring periods (daytime 

and nighttime). The top value in each split-cell uses the full frequency range, whereas the bottom 

values report the percent of time ANS-weighted sound levels (20-1,250 Hz) are above functional 

values. 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. ANS weighting

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 (Table 5). 

The first functional value in Table 5 is 35 dB (LAeq,1s), which is designed to address the health effects 

of sleep interruption. 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 is also the desired background 

sound level in classrooms (ANSI S12.60-2002). The second sound level value, 45 dB (LAeq,1s),

addresses the World Health Organization’s recommendations that noise levels inside bedrooms 

remain below 45 dB (LAeq,1s) (Berglund et al. 1999). The third sound level value, 52 dB (LAeq,1s), is 

based on the EPA’s speech interference threshold for speaking in a raised voice to an audience at 10 

meters (EPA 1974). This threshold addresses the effects of sound on interpretive presentations in 
11

parks. The final value, 60 dB (LAeq,1s), 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. 

Table 2. Time above metrics for HOME008.

Site ID

Frequency 

(Hz)

Time above sound level

(% of daytime hours, 07:00 to 19:00)

Time above sound level (% of 

nighttime hours, 19:00 to 07:00)

35

dB*

45

dB*

52

dB*

60

dB*

35

dB*

45

dB*

52

dB*

60

dB*

HOME008 12.5-20,000 100.0 97.7 31.0 0.0 100.0 100.0 84.1 18.9

20-1,250 30.3 0.8 0.1 0.0 7.6 0.2 0.0 0.0

* dB LAeq, 1s re 20 μPa

Sound Level: Percentile Levels

To understand the range of acoustic conditions at the park, percentile sound levels are reported 

(Figure 4). In Figure 4, the A-weighted percentile sound levels (LA90, LAnat, LA50, and LA10) for 

HOME008 are shown. The hourly percentile sound levels are calculated from the broadband (12.5 

Hz - 20 kHz) A-weighted, 1-second time averaged sound levels (LAeq, 1s) within each hour of the day. 

For instance, in Figure 4, the LA50 (median) sound level for HOME008 during the 6:00 hour is 52.5

dB. On the other hand, the sound level exceeded 10% of the time (LA10) for the same hour at this site 

is 54 dB, meaning 90% of the measurement period is quieter. In other words, 90% of the 

measurements are quieter. High percentile sound levels in the late evening and early morning hours 

are largely due to insect activity (Figure 4). Hours where the LA50 and the LAnat differ the most are 

usually hours with the most human-caused noise (in this case, 6:00 – 20:00). 
12

Figure 4. Median percentile sound levels (LAeq, 1s), in dB re 20 μPa, at HOME008

Event duration

Through off-site listening analysis, event duration for all audible sounds is calculated. Table 6 and 

Table 7 list audible natural sounds and noise sources at HOME008. Mean hourly audibility was 

calculated over eight days of analysis. See Appendix A for more information on analysis procedures. 

Figure 5 displays hourly audibility for all non-natural sources, as compared to audibility of two noise 

sources of interest: aircraft and vehicles. 
13

Table 3. Mean hourly time audible (%) for each natural sound source at HOME008, n=8 days

Sound 

Source

00h 01h 02h 03h 04h 05h 06h 07h 08h 09h 10h 11h 12h 13h 14h 15h 16h 17h 18h 19h 20h 21h 22h 23h

Wind 45.0 52.5 51.3 47.1 39.6 43.3 32.5 30.0 35.8 47.9 51.7 65.0 68.8 64.6 84.6 75.4 81.7 69.6 72.5 62.1 45.8 15.0 31.7 41.7

Water 0.0 0.0 3.8 0.4 0.0 3.8 12.1 18.3 11.3 11.7 11.7 11.3 10.4 11.7 7.1 5.0 2.9 0.4 0.8 0.0 0.0 0.0 0.0 0.0

Rain 0.0 0.0 2.5 0.0 0.0 0.0 0.8 12.5 12.5 12.5 7.9 10.4 7.9 2.9 0.4 0.0 0.0 0.0 0.0 0.0 0.4 0.0 0.0 7.5

Thunder 4.6 5.0 2.9 1.2 0.4 2.1 4.6 6.7 2.1 0.0 0.4 2.9 0.8 4.2 0.4 2.9 1.7 0.8 0.8 0.0 0.0 0.0 8.7 11.3

Mammal 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.8 0.4 0.0 0.0 0.0 0.0 0.0

Squirrel 0.0 0.0 0.0 0.0 0.0 0.0 4.2 7.5 4.2 4.6 5.0 2.1 4.2 1.7 4.2 3.3 3.3 7.9 10.0 12.5 4.6 0.0 0.0 0.0

Coyote 0.0 0.0 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.8 0.0 0.0

Bird 12.5 5.8 9.6 10.0 12.1 17.9 92.5 87.5 88.7 96.2 83.7 78.3 71.7 64.6 59.6 60.0 54.6 52.1 52.9 60.4 35.0 6.2 5.4 3.3

Amphibian 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0 0.0 0.0 0.0

Insect 100 100 100 100 100 100 100 87.5 87.1 97.9 100 99.2 99.6 97.1 97.5 99.2 100 100 99.6 99.6 100 100 100 99.6

Natural 

Unknown 1.7 0.8 2.9 1.7 2.9 1.7 2.9 2.1 4.2 2.9 7.1 5.8 5.8 2.5 3.3 3.8 3.3 2.9 2.5 3.3 4.2 3.8 3.3 2.1
14

Table 4. Mean hourly time audible (%) for each noise source at HOME008, n=8 days

Sound 

Source 00h 01h 02h 03h 04h 05h 06h 07h 08h 09h 10h 11h 12h 13h 14h 15h 16h 17h 18h 19h 20h 21h 22h 23h

Aircraft 3.3 12.1 8.7 5.4 12.1 10.0 7.5 11.3 13.8 12.9 15.8 18.3 25.0 24.6 17.1 12.9 14.2 15.0 12.1 12.9 9.6 0.4 2.5 3.8

Jet 2.5 4.6 3.3 3.3 6.7 3.8 5.0 7.9 10.4 16.3 15.0 14.6 19.2 16.3 19.6 15.8 20.4 17.9 22.9 22.5 16.7 7.1 8.7 8.3

Propeller 

Aircraft 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.7 0.4 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Vehicle 12.1 12.1 6.2 15.0 11.3 19.6 31.2 35.4 31.7 18.3 22.5 16.3 14.2 15.8 12.9 12.1 15.4 22.1 12.9 22.9 17.1 10.4 14.2 14.6

Automobile 6.2 0.4 5.0 3.8 1.7 0.4 1.2 2.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Alarm, Horn 0.0 0.0 0.0 0.0 0.0 0.0 1.2 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0 0.0 0.4 0.0 0.4 0.0 0.0 0.0 0.0

Motorcycle 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.8 0.0

Truck 2.5 1.7 2.1 1.7 0.4 5.0 15.4 23.7 10.0 9.6 8.3 8.7 11.7 7.9 6.2 9.6 7.9 8.7 6.2 5.0 3.3 1.7 0.4 0.4

Heavy 

Equipment 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Train 

Whistle 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Lawnmower 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.8 2.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Voices 0.0 0.0 0.0 0.0 0.0 0.0 0.4 1.7 12.9 12.5 11.7 10.0 11.7 10.4 4.6 7.5 2.5 0.0 0.8 0.0 0.0 0.0 0.0 0.0

Dog 0.0 0.0 0.0 0.0 0.0 0.0 0.0 3.3 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Cow 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.2 2.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Non-Natural 

Unknown 17.5 22.5 26.2 28.3 32.1 28.7 24.2 17.9 30.4 40.4 38.3 35.4 23.3 28.3 42.9 45.4 43.3 35.8 40.8 39.2 41.7 32.9 34.2 36.7

All non 

natural 43.3 50.8 50.0 56.7 60.8 65.4 78.3 80.8 87.9 85.8 83.3 82.9 82.9 80.0 82.1 81.2 82.1 83.7 81.2 82.5 77.5 49.2 57.9 57.1
15

Figure 5. Hourly time audible for airplane, vehicle, and all noise sources at HOME008.

17

Conclusions

Acoustic monitoring allows parks to gain insight into levels of extrinsic noise and biologic activity. 

The results can help estimate the effects of noise on park visitors and wildlife alike. The study was 

successful in determining the acoustical conditions at Homestead National Monument of America 

(HOME) in a wooded area near Cub Creek in 2017. 

Results from HOME008 included measures of existing ambient sound levels, calculations of sound 

source audibility through off-site listening, and estimates of natural ambient levels. 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. In previous monitoring efforts at HOME, noise was audible on average between 

70 – 91 % of the time, depending upon year, season, and site location. A non-natural unknown noise 

source was the most commonly heard noise during the 2017 monitoring period at HOME008, with a 

24-hour mean audibility of 32.8 % (Table 7). It was not described by the listener in the analysis files, 

but it was likely related to the nearby fertilizer factory, which was a prominent noise source in the 

2011-2012 study period, and was categorized as “non-natural other” during that analysis process.

Vehicles were the second most commonly heard noise source during the 2017 monitoring period at 

HOME008, with a 24-hour mean audibility of 17.3 %, but was as high as 35% during the 7:00 hour. 

During the 2011 – 2012 monitoring period, vehicles were the most frequently heard noise source 

across all sites and seasons. During summer daytime hours (8 am to 5 pm) at HOME001 and 

HOME003, music from the education center was often audible, but its presence was not noted in 

2017 because HOME008 is about 0.45 miles (0.72 km) from the education center. Natural sources 

such as wind, rain, thunder, birds, and insects were commonly audible at HOME008 in 2017. 

Specifically, recordings of coyotes, barred owl, and thunder were collected during this monitoring 

period. 

During the study period, natural ambient sound levels were measured to be 48.4 dB (LAnat) during the 

day and 55.2 dB (LAnat) at night. Insects contributed a significant amount of energy to the acoustic 

environment as night, as shown in Figure 3, as particularly high sound levels above 1.6 kHz. Existing 

sound levels (encompassing natural and noise sources) were measured to be 50.3 dB (LA50) during 

the day and 56.5 dB (LA50) at night. During the 2011-2012 monitoring period across all sites, existing 

sound levels were measured to be 38.4 dB – 53.6 dB (LA50) during the day, and 34.9 dB – 59.0 dB

(LA50) at night. 

For a broader context for the acoustic conditions at HOME, a comprehensive 1982 study of noise 

levels in residential areas found that nearly 87% of US residents were exposed to day-night sound 

levels (Ldn) over 55 dB, and an additional 53% were exposed to Ldn over 60 dB (EPA 1982). Note 

that noise levels have increased nationally with population growth since the EPA study (Suter 1991; 

Barber et al. 2010). Additionally, a nationwide study modeling daytime summer sound levels 

indicated that only 23 % of the continental United States was predicted to have an existing ambient 

sound level above 40 dB (LA50, 12 hr), and only 1% of the continental U.S. was predicted to have an 

existing daytime ambient sound level above 50 dB (LA50, existing) (Mennitt 2013). Consider, though, 
18

that daily sound levels reported in this report for HOME008 can be influenced by both natural and 

non-natural sources. 

Based on the results of this study, visitors to HOME are unlikely to experience a significant noisefree interval near the monitoring site, though noise is audible less in the early morning and late 

evening hours (Figure 5, Table 7). Noise has the potential to affect a visitor’s experience in parks by 

causing annoyance (Rapoza et al. 2015), reducing the perceived scenic beauty (Weinzimmer et al. 

2014), or simply by limiting opportunities for solitude. Increased sound levels may also have wide 

ranging effects on wildlife such as reduced predatory success (Mason 2015), changes in vocal 

communication, or increased vigilance by keystone species (Shannon et al. 2014). In a review of 

literature addressing the effects of noise on wildlife published between 1990 and 2013, wildlife 

responses to noise were observed beginning at about 40 dB (LAeq, x).4 Of the papers reviewed, 20% 

showed impacts to terrestrial wildlife at or below noise levels of 50 dB (LAeq, 1s) (Shannon et al. 

2015).

The information presented in this report will be used to inform park managers and planners when 

they make management decisions, but it will also serve as a permanent record of what the park 

sounded like in 2017. Sound level data as well as continuous digital audio recordings will be stored 

at the Natural Sounds and Night Skies Division office in Fort Collins, Colorado for archiving 

purposes. In addition, during the 2017 monitoring period a separate monitoring system was deployed 

as part of a nationwide eclipse monitoring project. This unique data set measured acoustic conditions 

before, during, and after the total solar eclipse on August 21, 2017. Results of that study are provided 

in Appendix B. 

4 This metric is a composite of multiple metrics with varying time averaging metrics.
19

Literature Cited

Acoustical Society of America. 2014. Methods to Define and Measure the Residual Sound in 

Protected Natural and Quiet Residential Areas, Melville, NY. ANSI/ASA S3/SC1.100-2014.

Barber J.R., C. Crooks, and K. Fristrup. 2010. The costs of chronic noise exposure for terrestrial 

organisms. Trends in Ecology and Evolution 25:180–189.

Berglund, B., T. Lindvall, and D. H. Schwela (Eds.). 1999. HWO. Guidelines for community noise. 

World Health Organization, Geneva.

Environmental Protection Agency. 1974. Information on Levels of Noise Requisite to Protect the 

Public Health and Welfare with an Adequate Margin of Safety.

Environmental Protection Agency. 1982. National Ambient Noise Survey. Office of Noise 

Abatement and Control, Washington, DC.

Haas, G.E., and T. J. Wakefield. 1998. National parks and the American public: A national public 

opinion survey on the national park system. Washington D.C. and Fort Collins, CO.: National 

Parks and Conservation Association and Colorado State University.

Haralabidis Alexandros, S., et al. 2008. Acute effects of night-time noise exposure on blood pressure 

in populations living near airports. European Heart Journal Advance Access. Published online 

February 12, 2008.

Harris, C. M. 1998. Handbook of Acoustical Measurements and Noise Control, 3rd ed. McGrawHill, New York.

McDonald, C. D., R. M. Baumgarten, and R. Iachan. 1995. Aircraft management studies: National 

Park Service Visitors Survey. HMMH Report No. 290940.12; NPOA Report No. 94-2, National 

Park Service, U.S. Department of the Interior.

Mennitt, Daniel, et al. 2013. Mapping sound pressure levels on continental scales using a geospatial 

sound model. Proceedings of Inter-Noise. 1-11.

Mennitt, D., K. Sherrill, and K. Fristrup. 2014. A geospatial model of ambient sound pressure levels 

in the contiguous United States. Journal of the Acoustical Society of America 135:2746-2764.

National Park Service. 2005. Acoustic and soundscape studies in National Parks: Draft, Fort Collins, 

Colorado.

National Park Service. 2006a. Management Policy 4.9: Soundscape Management.

National Park Service. 2006b. Management Policy 8.2.3: Use of Motorized Equipment.
20

Rapoza, A., Sudderth, E., & Lewis, K. 2015. The relationship between aircraft noise exposure and 

day-use visitor survey responses in backcountry areas of national parks. The Journal of the 

Acoustical Society of America, 138(4), 2090-2105.

Shannon G, Angeloni LM, Wittemyer G, Fristrup KM, Crooks KR. 2014. Road traffic noise modifies 

behaviour of a keystone species. Anim Behav. 94:135–141.

Suter, A. H. 1991. Noise and its Effects. Administrative Conference of the United States. Available 

at: http://www.nonoise.org/library/suter/suter.htm (accessed 17 January 2014).

Templeton, D. (Ed.). 1997. Acoustics in the built environment: advice for the design team. 

Architectural Press, Oxford

Weinzimmer, D., Newman, P., Taff, D., Benfield, J., Lynch, E., & Bell, P. 2014. Human responses to 

simulated motorized noise in national parks. Leisure Sciences, 36(3), 251-267.
21

Appendix A: Office Listening

Office listening is a way to characterize the length and type of noise events occurring at a monitored 

site (Figure 6). The NSNSD protocol calls for 8 days of analysis per monitoring period. The 

Acoustical Monitoring Toolbox splits the audio files in to 10 second clips every two minutes per one 

day. This results in 16 hours worth of data being analyzed per site. Each sound is assigned a number 

which is then put into the Listening Center program each time the listener hears the sound. These 

numbers are eventually used to calculate the LAnat for the site.

Figure 6. Screen shot of Listening Center. Three ten second samples are displayed side by side. Audible 

sound sources and annotations are recorded in the spreadsheet cells below.

23

Appendix B. Results of eclipse monitoring 

Background

A total solar eclipse swept across the US on August 21, 2017 and awed millions of viewers. The 

path of totality crossed 21 national park units and 7 national trails. The Natural Sounds and Night 

Skies Division (NSNSD) of NPS partnered with 16 park units to deploy 29 acoustic recording 

systems before, during, and after the total solar eclipse on August 21, 2017. This national effort 

captured the sounds of the eclipse and offered an unique opportunity to measure how the eclipse 

changed the soundscape. The 14 parks (18 sites) with data spanned the continental US and included 

a diversity of habitats– from the plains of the midwest to the swamps of Congaree (Figure 7). Two 

parks (3 sites) were not in the path of totality. 

Figure 7. Locations of acoustic monitoring stations deployed during the solar eclipse.

Data Collection

At HOME, one acoustic monitoring sites was deployed (40.29N 96.83W) from 18 August to 24 

August (Figure 8). This was a frontcountry site in a temperate broadleaf/mixed habitat. At this site 

totality began at 18:02 GMT and lasted for 2min 34sec. 
24

Figure 8. Bioacoustic monitoring station in Homestead N.M.

Data Analysis

Acoustic data during the time of totality for all days was extracted using automated scripts. To see if 

the soundscape changed, sound levels (at different frequencies) during totality were compared to all 

other days at the same time (Figure 9A). Google Form surveys were developed by NSNSD in which 

NPS participants listened to five different unidentified sound clips to determine what types of sounds 

were present or not present during the eclipse. A total of 312 sound clips were listened to by at least 

two different people. The sound clip identity was unknown to remove bias. The participants marked 

down how often they heard different types of sounds (birds, mammals, insects, voices, human 

activity, weather) in each clip. Changes in the presence of the sound types were analyzed by 

comparing what was heard during the eclipse verses what happened on days before and after the 

eclipse (Figure 9B). Link to sound clips

(https://drive.google.com/open?id=1HCHDiHe6IlQdCD034l-bRFIJcWbPz3Ld).
25

Figure 9. (A) One-third octave sound levels before/after and during eclipse, and (B) change in types of 

sounds between eclipse and non-eclipse days

Results

At the site in HOME, sound levels around 1,000 Hz increased above the standard deviation of noneclipse days (A), and likely relates to an increase in human voices (B). All types of biological sounds 

decreased (B) and at the higher frequencies (A: 10 kHz), a decrease in insects sounds likely caused 

this change (B).

The Department of the Interior protects and manages the nation’s natural resources and cultural heritage; provides scientific 

and other information about those resources; and honors its special responsibilities to American Indians, Alaska Natives, and 

affiliated Island Communities.

NPS 368/167035, February 2020 
National Park Service

U.S. Department of the Interior

Natural Resource Stewardship and Science

1201 Oakridge Drive, Suite 150

Fort Collins, CO 80525

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