Intermediate 
The tranquil sounds of the natural world might be lost to todays generation as people screen out the noises that surround them, a senior US researcher warns.
Rising levels of background noise in some areas threaten to make people oblivious to the uplifting sounds of birdsong, trickling water and trees rustling in the wind. These sounds can often be heard even in urban centres, said Kurt Fristrup, a senior scientist at the US National Park Service.
The problem was made worse by people listening to music through their earphones instead of tuning in to the birds and other sounds of nature that can easily be drowned out by traf c, music and others noises, he said.
This learned deafness is a real problem, Fristrup told the American Association for the Advancement of Science meeting in San Jose. We are training ourselves to ignore the information coming into our ears.
This gift that we are born with  to hear things hundreds of metres away, all these incredible sounds  might be lost, he said.
The danger is that we are exposed to noise for so long that we stop listening. We are also losing the ability to engage with the environment in the way we were built to, he added.
For the past ten years, the US National Park Service has recorded sound levels at more than 600 places across the US, including Yosemite in California, Yellowstone and Denali in Alaska. All the places were affected by some form of noise from human activity  aircraft, motorbikes, motorboats or tour buses.
Fristrups team combined the sound levels recorded from national parks with similar data from urban settings to create a model of noise levels across the US. They say that noise pollution more than doubles every 30 years.
Its not surprising people are putting on earphones or even noise cancelling headphones to try and create a quieter environment, he said. 
As you raise background sound levels, it has the same effect on your hearing as fog would have on your vision. Instead of having this expansive experience of all the sounds around you, you are aware of only a small area around you, he said. Even in our cities, there are birds and things to appreciate in the environment but the ability to hear them is being lost.
People quickly become used to changes in their environments, including rising noise levels, and, over time, Fristrup fears that we will accept far worse environmental conditions than we should and forget how much quieter the world could be. If  nding peace and quiet becomes too dif cult, many, many children will grow up without the experience and I think its a very real problem, he said.
The warning came as other scientists reported health bene ts from listening to natural sounds. Speaking at the same meeting, Derrick Taff, a social scientist at Pennsylvania State University, described preliminary experiments which suggest that listening to recordings from national parks, of waterfalls, birdsong and wind, helped people recover from stressful events.
In one experiment, Taff told people who visited his lab to give an unplanned talk that would be judged by researchers standing behind a one-way mirror. Measurements of their heart rate and the stress hormone, cortisol, before and after the speech found that people calmed down faster when they listened to nature recordings than when the same soundtracks also contained noises from road traf c, aeroplanes and even normal conversation.
We know that natural sounds are very important to people. They are some of the main reasons people visit protected areas. They want to hear the natural quiet, the birdsong, and the wind and water, Taff said. We may be losing this as people are listening to their iPods all the time. My advice is to go to your protected areas and experience what you are missing.
Why natural sounds might be calming to people is unclear but Fristrup thinks that, over millions of years of evolution, we may have come to associate the more tranquil sounds of the natural world with safety. I suspect theres something about these sounds that reminds our brains of a place thats safe, he said.