SPECTROGRAMS ARE A USEFUL TOOL that many National Park Service (NPS) soundscape scientists employ to examine decibel data. For example, they translate decibel data collected in daily or hourly units into a visual format, enabling a quick scan of hundreds of hours’ worth of data for the occurrence of natural and human-caused sound sources at a particular site. Each sound source has its own signature based on frequencies, loudness, and timing, making it easy to identify. Spectrograms are created by converting 1-second decibel and one-third octave band data into a visual format using statistical analysis software, called "R," and an "R" script written by the NPS Natural Sounds Program.
Recently the Soundscape Program at Grand Canyon National Park was tasked with determining the number of fixed-wing aircraft using the Fossil Canyon general aviation corridor over the park (Levy and Falzarano 2007). This corridor is located in a remote part of the park (fig. 1), making an on-site, real-time tally of fixed-wing aircraft unrealistic. Park staff placed an acoustic system in the corridor for one year starting 1 November 2006 (fig. 2). The acoustic system consisted of a Panasonic CF-18 Toughbook laptop, an ANSI type 1 Larson-Davis sound-level meter (model 824), a microphone (GRAS 40AE), and a preamplifier (Larson-Davis 902). Twelve-volt, lead-acid batteries powered the system, recharged by two 80-watt arrays of solar panels. Total system cost was approximately $14,000. Though this may seem cost-prohibitive for some parks, it is important to keep in mind that equipment can be shared between parks, and the data collected could serve a multitude of purposes. Twenty-four-hour spectrograms were created for 365 days of decibel data, enabling program staff to count fixed-wing plane signatures. Spectrograms provided timely, quality information and obviated the need to listen to hundreds of hours of sampled recordings. Instead, park staff were able to identify fixed-wing aircraft on the spectrograms by their “staircase” or “corkscrew” signature shape (fig. 3), which were confirmed by listening to corresponding recordings.
Results show that a total of 1,504 fixed-wing aircraft used the corridor over 10 months with an average of 5 fixed-wing planes per day. Two months of data (August and September) were lost due to equipment failure when an elk knocked over the microphone, which filled with water. Beyond tallying aircraft, park staff used the spectrograms to ensure quality control of collected data, to monitor times of helicopter flights, and to quickly locate recordings of interesting wildlife sounds such as coyotes howling and elk bugling. Spectrograms can be generated in a matter of minutes and display 24 hours of data at a time. Therefore, spectrograms can quickly show data gaps (system failure) or noise interference (e.g., field staff checking the system on site). The park Soundscape Program continues to use spectrograms in a variety of ways to aid management, and this tool has great potential for park interpretive and education programs.
Levy, L., and S. Falzarano. 2010. Case Study: Tools of the trade: An example of using spectrograms to count fixed-wing aircraft. Park Science 26(3):48–49.
Accessed 25 November 2015 from http://www.nature.nps.gov/ParkScience/index.cfm?ArticleID=352.