Monitoring and Data
Sound monitoring, carried out by the Natural Sounds Program and other park staff, teaches us about the acoustical environment within the national parks. From a management perspective, sounds that characterize the acoustical environment of a park can be divided into two main categories: intrinsic and extrinsic. Through monitoring efforts we learn about the acoustical environment as a whole, and we can identify desirable and appropriate sound sources, as well as sounds that are undesirable in the park setting.
The current sound intensity of an area is referred to as the existing ambient sound level. In addition to calculating the existing ambient level, staff estimate what the acoustical environment would be like in the absence of extrinsic sounds. The natural ambient sound level is used as a starting point, or baseline, for park management purposes. Monitoring of the natural ambient and the existing ambient sound conditions allow managers to assess the impact of potential intrusions against a standard. Acoustic data, associated metrics, and sound-source information can be used to model sound levels and identify the potential impacts of non-natural sounds and proposed developments or actions that may affect the acoustical environment. Information on the acoustical environment becomes especially helpful during park planning efforts. 
Once the baseline condition is established, scientists and managers continue to monitor the acoustical environment to ensure protection of the resource. Long-term monitoring efforts take into account the primary acoustical resources identified in General Management Plans and/or more detailed Soundscape Management Plans. Monitoring protocols use standardized methodology to produce scientifically defensible data that is comparable to other studies. Changes or degradation of the resource may require action to ensure managers meet the standards for protection set by laws, policies, and management documents for the park.
Data collected during monitoring informs the decision-making process by providing metrics, reference standards, and thresholds for:
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However, acoustic monitoring can also support:
- Biological studies (ex: peregrine falcon nesting patterns)
- Evaluations of internal park operations (ex: construction projects or grounds care)
- Impact analysis for specific sound sources (ex: snowmobiles, airboats, ORV's, or vehicles)
- Visitor Experience and Resource Protection (VERP) studies
Data Collection: Planning and Preparation
Identifying Sampling Areas | Measurement Location | Measurement Season | Measurement Duration
Preparation for data collection includes identifying sampling areas, selecting representative acoustic areas and specific measurement locations, and defining the season and period of time for taking measurements. Data collection focuses on the primary sampling areas of each park. Each monitoring effort considers specific management areas and soundscape management objectives, as well as acoustically sensitive areas, such as endangered species habitat or cultural areas.
Identifying Sampling Areas
Areas of similar vegetation, land cover, topography, elevation, and climate that typically contain similar animals, physical processes, and other sources of natural sounds are used to define sampling areas in a park. Areas with similar attributes have similar natural sound sources, sound levels, propagation and attenuation properties, and other acoustic qualities. Once the primary sampling areas have been identified, measurement locations are selected to ensure that all of the primary sampling areas of the park are sampled.
In developing park sampling areas, land cover and climate regions are the two greatest factors influencing ambient sound levels. Acoustic specialists/technicians consider specific measurement locations within sampling areas relative to other factors such as park resources, park management zones, visitor-use, and wildlife habitats.
Measurement Location
In most situations, the acoustic technician’s principle consideration in selecting measurement locations is to ensure data are collected in all of the primary sampling areas of the park. Additional considerations include, in rough order of priority:
- Park management zones and soundscape management objectives of those zones;
- Specific sound-sensitive areas;
- Specific acoustic data needs;
- Proximity to natural and human-caused sounds; and
- Equipment considerations (security, solar exposure, visibility, etc.).
Final selection of measurement locations is made by screening potential sites considering all of the above factors. Acoustic technicians also consider site access, equipment availability/capability, and availability of personnel to deploy and service the equipment.
Measurement Season
Ideally, acoustic data is collected for all seasons. However, two seasons, summer and winter, are usually adequate. When assessing specific sound sources, such as aircraft, for the development of Air Tour Management Plans (ATMPs), acoustic technicians collect data during the season(s) in which the activity occurs as well as the season when the activity occurs the least or not at all. For example, air tours companies typically operate tours during the summer months. Few, if any, are conducted in the winter. In this case, measurements are taken during both the summer and winter seasons. For those parks at which regular air tours occur year-round, acoustic technicians collect data from different seasons to adequately model potential air tour impacts.
Measurement Duration
To determine adequate measurement periods, acoustic technicians consider the daily and seasonal variability of acoustic conditions in parks. Measurements taken at a particular site need to be of sufficient duration to ensure statistical confidence in the data, but also be reasonable in light of practical and other resource considerations. Acoustic literature shows that a minimum 25-day measurement period limits the measurement uncertainty of ambient data collected in various national parks to less than three decibels. For some situations or environments, shorter or longer measurement periods may be needed.
Data Collection Types
Acoustic
At each site, monitoring equipment takes frequency and sound pressure level readings. The sound pressure level (intensity) is recorded in decibels (dB), and the frequency, or pitch, of a sound is recorded in hertz (Hz). Currently, sound equipment used in the parks allows us to record sounds from 20 to 20,000 Hz, which approximately represents the human hearing range. High frequency sounds (a cricket chirping) and low frequency sounds (water flowing in a river) often occur simultaneously, so we split the frequency spectrum into 33 smaller ranges, each encompassing one-third of an octave. For each one-third octave band, the dB level is recorded once every second for the duration of the monitoring period. Recording the sound intensity of each one-third octave band (combined with digital audio recordings described below) allows sound technicians to determine what types of sounds are contributing to the overall sound pressure level of a site. At sites where the sound pressure levels are very low, it is ideal to monitor with low-noise, high-sensitivity microphones, whenever possible. For some types of analysis, the dB levels are A-weighted (dBA), to more closely represent the sensitivity of the human ear to different frequency ranges.
Meteorological
Previous studies in national parks show a strong correlation between land cover, wind speed, and ambient sound level. Sounds travel differently in cold or hot temperatures making meteorological data important to understanding acoustical environments. Acoustic technicians collect data about wind direction, air temperature, and humidity as well as continuous, one-second wind speed data. Wind data is particularly important because a substantial change in noise level can occur as wind speeds increase. Wind speed is recorded at 10-second intervals throughout the monitoring period. High wind speeds (> 5 m/s) exceed the limitations of some of the recording equipment. So, to ensure the most accurate data, recordings taken during high wind events are excluded from analysis
Digital Audio Recordings
Advancements are continuously being made in acoustic data analysis.Therefore, it is crucial to obtain high-quality archival recordings that can be used to compute any conceivable metric for future analysis. It is almost certain that metrics specified today will be inadequate to meet all future needs, thus making high-quality digital audio recordings important. Digital recordings provide an archival record of the soundscape of the area.
To provide an accurate characterization of the natural and non-natural acoustic conditions in a park, it is important that we know not only how loud the park is, but also the source of sound. Digital recordings provide us with one way to identify specific sound sources. This can help park staff when determining noise intrusions and how to manage them.
Source identification/On-site Listening
In addition to providing a way to determine sound sources, observer data logging provides us with a way to discern timing and duration of different sound sources. During observer logging sessions, acoustic technicians or park staff will note the sounds they hear (using a palm pilot) over a one-hour period. The Natural Sounds Program uses a palm pilot program that logs the sound source, and time and duration of the event.