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AUDIE CORNISH, Host:

If a tree falls in the forest and nobody is around to hear it...

(SOUNDBITE OF TREE FALLING)

CORNISH: Yes, it does make a sound. That recording comes from Crater Lake National Park in Southern Oregon. This soundscape is a natural resource the park is supposed to protect. But as Amelia Templeton of Oregon Public Broadcasting's EarthFix project reports, engine noise is making it harder to hear the natural quiet.

(SOUNDBITE OF CRUNCHING FOOTSTEPS)

AMELIA TEMPLETON: Scott McFarland is looking for a place to set up a microphone. He starts hiking away from the main attraction of this park, a deep blue lake in a volcano that blew its top off 7,000 years ago. His feet crunch across a field of pumice and ash.

SCOTT MCFARLAND: Anything you could possibly think of hearing, we probably have a recording of it - everything from badgers and porcupines grunting to the wings of a butterfly.

TEMPLETON: McFarland has set up solar-powered recording stations at 20 different sites in this park. One of his microphones caught this conversation between two endangered spotted owls.

(SOUNDBITE OF OWLS HOWLING)

TEMPLETON: The owls are pretty easy to identify, but McFarland spends hours puzzling over less obvious sounds.

(SOUNDBITE OF ANIMAL CHEWING)

MCFARLAND: A lot of times if it's just a branch breaking or somebody just rustling around on the ground, you can't tell what it is. And what we've found was elk and deer like to chew on the windscreens for the microphones.

TEMPLETON: On this day, McFarland decides to set up his recording station in a place where small trees have taken root and we listen.

(SOUNDBITE OF SILENCE)

TEMPLETON: Did you hear anything I didn't?

MCFARLAND: Prop plane. Propeller aircraft going by.

TEMPLETON: The park is less quiet than it sounds at first.

CHRIS WAYNE: This project kind of ruins your ability to hear silence.

TEMPLETON: Chris Wayne is another scientist who's helping McFarland today. He also heard the plane.

WAYNE: And once you start paying attention, you can always hear the road, wherever you are. Always hear the planes.

TEMPLETON: Even in the most remote parts of the park, McFarland's recordings pick up human-caused noise about 15 percent of the time. It may not sound serious, but species like those spotted owls need quiet in order to find their prey.

MCFARLAND: Think about how quiet a mouse would be under the snow. Owls have the ability to hear that. So, just a small increase in noise can really limit the area that they can actually hunt.

TEMPLETON: The problem, McFarland says, is that human noise, like climate change, is a problem the national parks can't just fence out. For NPR News, I'm Amelia Templeton in Medford, Oregon.

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