听写训练营Day26

1. We've been talk about how sea animals find their way under water, how they navigate, and this brings up an interesting puzzle, and one I'm sure you'll all enjoy. I mean, everybody loves dolphins, right? And dolphins, well, they actually produce two types of sounds, uh, one being the vocalizations you are probably all familiar with, which they emit through their blowholes.

2. But the one we are concerned with today is the rapid clicks that they use for echolocation, so they can sense what is around them, these sounds, it has been found, are produced in the air-filled nasal...  of the dolphin, and the puzzle is how does the click sounds get transmitted into the water? It's not as easy as it might seem, you see, the denser the medium, the faster sound travels, so sound travels faster through water than it does through air.

3. You've got a sound wave traveling merrily along through one medium, when suddenly, it hits a different medium, what does gonna happen then? Well, some of the energy is going to be reflected back, and some of it is going to be transmitted into the second medium. And…and…and if the two media have really different densities, like air and water, then most of the energy is going to be reflected back, very little of it will keep going, uh, get transmitted into the new medium. 

4. So, how did the dolphin's clicks get transmitted from its air-filled nasal... into the ocean water? Because given the difference in density between the air in the nasal cavity and the seawater, we'd expect those sounds to just kind of go bouncing around inside the dolphin's head, which will do it no good at all. If it's going to navigate it, needs those sounds to be broadcast and bounced back from objects in its path.

5. It turns out that the jaw is primarily responsible for capturing and transferring returning sound waves to the dolphin's inner ear, so these rapid clicks that are sent out bounce off objects, maybe a group of fish swimming over here, a boat coming from over there, the sounds bounce off them and the lower jaw captures the returning sounds, making it possible for the dolphin to sense what's in the surrounding water and decide where to swim.