For the more information about natural sounds and night skies in the National Park Service, please visit http://www.nature.nps.gov/sound_night/.
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 Managing Soundscapes and Noise Pollution), and must be collected in a reliable and systematic manner.
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.
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.
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.
Measurement season and duration
Ideally, acoustical data are collected for all seasons. However, two seasons are usually adequate. When assessing specific sound sources, 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, aircraft are a sound source of interest for the development of air tour agreements. 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. 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.
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.
Last Updated: September 14, 2015