In the Field

National park units from across the country can request technical assistance from the Natural Sounds Program to conduct sound monitoring. Acoustical data may be needed for a variety of reasons (see Sounds We Protect and Noise). We are therefore constantly innovating new methods to address the diverse projects that come to our attention. Meanwhile, our standard system for acoustic monitoring follows rigorous protocols to collect data in a consistent and systematic manner.

Sampling Scheme

To define sampling areas within a park, we identify areas with similar vegetation, land cover, topography, elevation, and climate that typically contain similar animals, physical processes, and other sources of natural sounds. 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.

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.

Installation of sound equipment at Granite Lake
Installation of sound equipment at Granite Lake, Yosemite National Park

NPS

Site Selection

In most situations, the principal consideration in selecting measurement locations is to ensure that representative data are collected for all primary sampling areas of the park. Additional considerations include, in rough order of priority:

  • Park management zones and their soundscape management objectives
  • Specific sound-sensitive areas
  • Specific acoustical data needs
  • Proximity to natural and human-caused sounds
  • Equipment considerations (security, solar exposure, visibility, etc.)
Acoustic technicians also consider site access, equipment availability and capability, and availability of personnel to deploy and maintain the equipment.

Measuring by season

The time of year when acoustical data are collected depends upon the question being asked. Often, two seasons of data will be useful: a high season and a low season for a specific sound source of interest. This approach allows comparison of a park's soundscape during the season(s) in which the activity occurs as well as the season when the activity occurs least or not at all. For example, aircraft are a sound source of interest for many parks. Air tour companies typically operate during the summer months; few tours, if any, are conducted in the winter. In this case, measurements are taken during both the summer and winter seasons.

Two images of sound monitoring equipment on tripods with mountains in the background. One image is green summer, the other snowy winter.
This acoustic site in Denali National Park & Preserve was monitored in both winter and summer seasons.

NPS Photo

Duration

To determine how long to record, acoustic technicians consider the daily and seasonal variability of acoustic conditions in parks. Measurements taken at a particular site need to be long enough to ensure statistical confidence in the data. However, practical and other resource considerations are also important. Research shows that measurement periods of at least 25 days limit the uncertainty of ambient data to less than three decibels. For some situations or environments, shorter or longer measurement periods may be needed.

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