On posted quiet days, visitors were significantly quieter than on other days. A “lost listening area” is an effective way to talk about a noise problem without mentioning decibels, Fristrup said, since many people do not have a good grasp of what decibels mean. (Overnight visitors also expressed concern about sleep interference, he said, but this issue has not been studied in parks.)
Fristrup discussed the necessity of finding an appropriate metric when conducting noise research. The most commonly employed metrics use A-frequency weighting—a standard weighting curve that makes the metric generally representative of human hearing. But for some measurements, sampling should be limited to the frequencies most often produced by a particular source. In other cases, animals may have hearing sensitivities that differ from those of humans. As a workshop participant pointed out, whales have better low-frequency hearing than humans. In those cases, said Fristrup, using a human model may be inadvisable. However, humans have better low-frequency hearing than most other vertebrates, so an A-weighting curve is generally a conservative measurement. “[Human] hearing has also been extremely well studied,” he said. “Someone with healthy hearing can go out in the field and make observations that mean something.”
Researchers sometimes measure the average noise level generated by a given source, but it is difficult to relate this measure to everyday experiences for the public. Knowing how often a noise is present and how loud it is helps with public education. A perceived loudness standard also may be preferable for higher noise levels.
The question of noise metrics was also addressed by George Maling in his brief review of the Technology for a Quieter America report (NAE 2010). Citing the NAE report, Maling noted that human reactions to man-made and natural sounds differ, and that a different metric may be required for the assessment of noise impacts on wildlife. He also observed that the metric used to assess environmental noise depends on the source; for example, aircraft noise is assessed differently from highway noise. For the types of sources discussed at the workshop, the noise metrics will differ but have generally been defined for various noise sources.
Beth Cooper, an acoustical engineer and hearing conservation consultant with the National Aeronautics and Space Administration (NASA),