Miami - Human noise is polluting more than half of natural protected areas in the United States to such an extent that it could interfere with animals' ability to hunt and survive, researchers said on Thursday.Sounds were recorded at 492 sites meant as havens for biodiversity across the country, said the report in the journal Science.Researchers found that background noise exceeded three decibels (dB) in 63% of protected areas, it said.In 21% of areas, background noise was 10 dB higher than it would have been without the influence of humans.This meant between "a doubling and tenfold increase in sound levels above natural", said lead author Rachel Buxton, a conservation biologist at Colorado State University.Another way to look at it, Buxton said, is that human-caused noise has reduced the area where natural sounds can be heard by 50- and 90%."So if you could have heard something at 30m away, now you can only hear it from 3- to 15m away," she said.A computer algorithm was used to estimate the baseline for natural sounds in an area, based on its unique features.The sources of the extra noise include oil and gas exploration, fracking for natural gas, timber and mining activities, motorcycles and vehicle traffic, and just general human development.The impact of the added noise could be vast."Birds may have a harder time finding a mate, or a prey species may not be able to hear the approach of a predator and may be more likely to be eaten," Buxton said."Even if only one species is actually being affected by noise directly, these impacts may cascade through an ecological community."Researchers said the study shows that human noise pollution, while often considered a city problem, is much more far-reaching than previously understood.