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Krista: This is from your interview with Gorden Hempton called "The Last Quiet Places:"

Mr. Hempton: But you have brought up something really important to me and that is about our ancient past. When I go to a quiet place, I get to challenge assumptions. And one of the major assumptions is that the human ear is tuned to hear the human voice. If that were true, that's an assumption that audiologists, scientists who study human hearing, have believed for a long time, that our ears evolved to hear the human voice.

Ms. Tippett: Right.

Mr. Hempton: But if, if — yeah, I know. But if that were true, we'd be the first species on planet Earth, OK, to have evolved so separate and protected from the rest of nature.

So my natural curiosity was to look at the range of human hearing and these equal-loudness contours. And we have a very discreet bandwidth of supersensitive hearing and that's between 2.5 and 5 kilohertz in the resident frequencies of the auditory canal. Is there something in our ancestors' environment that matches our peak hearing human sensitivity? Because most of what I'm saying right now, except for the "s" sounds and the high-pitched sounds, falls well below that range. And, indeed, there's a perfect match: birdsong. Birdsong [laugh].

Ms. Tippett: Mm-hmm.

Mr. Hempton: Why would it have any benefit to our ancestors to be able to hear faint birdsong? Why would our ears possibly have evolved so that we could walk in the direction of faint birdsong? Birdsong is the primary indicator of habitats prosperous to humans. Isn't that amazing? Now when you're in a quiet place, what is the listening horizon? If you ask a person that lives in a city, they might take a wild guess and say, "Oh, you can listen for a mile." Right they know it's a trick question, so they're going to pick something really big. You can listen for a mile. You ask somebody in the country? Oh, you can listen for three or four miles. And I've heard sounds 20 miles away. If you do the math, that is the size of 1,276 square miles. Do you know what it's like to listen to 1,276 square miles when the sun is rising?

(Sound bite of birdsong)