OFFICIAL7 According to the professor, why does a pine tree produce a "smooth" echo?

[00:00.00]Narrator: Listen to part of a lecture in a biology class.
[00:06.36]FEMALE PROFESSOR: So, that’s how elephants use infrasound… [00:09.80]Now let's talk about the other end of the acoustical spectrum—sound that's too high for humans to hear: ultrasound. [00:18.47]Ultrasound is used by many animals that detect—and, some of them, send out—very high-frequency sounds. [00:27.82]So, what's a good example?[00:30.47]Yes, Carol?
[00:31.88]FEMALE STUDENT: Well, bats—since they're all blind, bats have to use sound for—uh, y'know—to keep from flying into things.
[00:41.69]FEMALE PROFESSOR: That’s echolocation. [00:45.24]Echolocation is pretty self-explanatory: Using echoes—reflected sound waves—to locate things… [00:55.10]As Carol said, bats use it for navigation and orientation… [00:59.53]and what else? Mike?
[01:03.60]MALE STUDENT: Well, finding food is always important—and, uh, I guess, not becoming food for other animals…
[01:12.33]FEMALE PROFESSOR: Right on both counts. [01:14.19]Avoiding other predators—and locating prey—uh, typically insects that fly around at night.  [01:21.19]Now, before I go on, let me just respond to something Carol was saying—this idea that bats are blind… [01:27.93]actually, there are some species of bats—the ones that don't use echolocation—that do rely on their vision for navigation but, it is true that, for many bats, their vision is too weak to count on.
[01:41.56]OK, so: quick summary of how echolocation works. [01:46.68]The bat emits these ultrasonic pulses—very high-pitched sound waves that we can't hear—[01:52.85]and then: they analyze the echoes—how the waves bounce back. [01:57.86]Uh, here, let me finish this diagram I started before class…[02:01.99]So the bat sends out these pulses—very focused bursts of sound, and echoes bounce back…  [02:12.47]Y'know, I don't think I need to draw in the echoes. [02:18.66]Your-your reading assignment for the next class—it has a diagram that shows this very clearly—
[02:22.96]so anyway as I was saying… By analyzing these echoes, the bat can determine, say, if there's a wall in a cave that it needs to avoid… and—how far away it is. [02:35.77]Another thing it uses ultrasound to detect, is the size and shape of objects. [02:41.49]For example, one echo they’d quickly identify is the one they associate with a moth, which is common prey for a bat—particularly, a moth beating its wings.
[02:51.15]However, moths happen to have a major advantage over most other insects: [02:56.01]they can detect ultrasound. This means that, when a bat approaches, the moth can detect the bat's presence… [03:03.85]so it has time to escape to safety… or else they can just remain motionless—[03:09.44]since, um, when they stop beating their wings, they’d be much harder for the bat to distinguish from, oh, a-a leaf… or-or some other object… 
[03:18.90]Now, we've tended to underestimate just how sophisticated the abilities of animals that use ultrasound are. [03:26.16]In fact, we kind of assumed that they were filtering a lot out—uh, the way a sophisticated radar system can ignore echoes from stationary objects on the ground. [03:37.08]Radar does this to remove “ground clutter”—information about, um, hills or buildings that it doesn’t need.
[03:44.76]But bats—we thought they were filtering out this kind of information because they simply couldn't analyze it. [03:51.81]But it looks as if we were wrong. [03:54.40]Recently, there was this experiment with trees and a specific species of bats—[04:00.46]a bat called the lesser spear-nosed bat.
[04:03.90]Now a tree should be a huge acoustical challenge for a bat, right? [04:09.61]I mean, it's got all kinds of surfaces, with different shapes and angles… [04:14.28]So, well, the echoes from a tree are going to be a mass of chaotic acoustic reflections, right? Not like the echo from a moth.
[04:23.26]So, we thought, for a long time, that bats stopped their evaluation at simply “that's a tree.” [04:30.63]Yet, it turns out that-that bats, or at least this particular species, can not only tell that it’s a tree, but can also distinguish between, say, a pine tree and a deciduous tree—like, a maple, or an oak tree: just by their leaves—[04:47.24]an-and when I say “leaves,” I mean pine needles, too. [04:50.85]Any ideas on how it would know that?
[04:54.50]MALE STUDENT: Well… like with the moth—could it be their shape?
[05:02.05]FEMALE PROFESSOR: You're on the right track. It’s actually the echo off all the leaves—as a whole—that matters. [05:09.05]Now, think: A pine tree—with all those little, densely packed needles… [05:15.19]those produce a large number of faint reflections in wh-what's called a-a “smooth” echo—
[05:21.12]the waveform is very even … but an oak—which has fewer but bigger leaves with stronger reflections—produces a jagged waveform—or what we call a “rough” echo. [05:34.03]And these bats can distinguish between the two—and not just with trees, but with any echo that comes in a smooth or rough shape.