OFFICIAL14 Why does the professor mention a bottle?

[00:00.00]NARRATOR: Listen to part of a lecture in an archaeology class. 
[00:04.48]FEMALE PROFESSOR: When we think of large, monumental structures built by early societies, an Egyptian pyramid probably comes to mind. [00:12.51]But there are some even earlier structures in the British Isles also worth discussing. [00:18.16]And besides the well-known circle of massive stones of Stonehenge—which, don't get me wrong, is remarkable enough—well, other impressive Neolithic structures are found there too.
[00:28.66]Oh, yes … we're talking about the Neolithic period here … also called the New Stone Age, which was the time before stone tools began to be replaced by tools made of bronze and other metals. [00:41.20]It was about five thousand years ago— even before the first Egyptian pyramid—that some amazing Neolithic monuments—tombs—were erected at various sites around Ireland, Great Britain, and coastal islands nearby.
[00:56.27]I'm referring, in particular, to structures that, in some cases, look like ordinary natural hills, but were definitely built by humans—well-organized communities of humans—to enclose a chamber, or room, within stone walls and sometimes with a high, cleverly designed ceiling of overlapping stones. [01:15.60]These structures are called “passage graves”—because the inner chamber—sometimes several chambers, in fact—could only be entered from the outside through a narrow passageway.
[01:27.51]MALE STUDENT: Excuse me, professor, but you said “passage graves”. Were these just monuments to honor the dead buried there, [01:35.28]or were they designed to be used somehow by the living.
[01:38.53]FEMALE PROFESSOR: Ah, yes. Good question, Michael. [01:40.63]Besides being built as tombs, some of these passage graves were definitely what we might call astronomical calendars—with chambers that were flooded with sunlight on certain special days of the year … which must’ve seemed miraculous and inspired a good deal of religious wonder. [01:57.45]But research indicates that not just light, but also the physics of sound helped enhance these religious experiences.
[02:06.28]MALE STUDENT: How so? 
[02:07.01]FEMALE PROFESSOR: Well, first, the echoes … [02:08.99]when a religious leader started chanting, with echoes bouncing off the stone walls, over and over again … it must’ve seemed like a whole chorus of other voices—spirits or gods, maybe—joining in.
[02:21.34]But even more intriguing is what physicists call standing waves. [02:28.19]Basically, the phenomenon of standing waves occurs when sound waves of the same frequency reflect off the walls and meet from opposite directions. [02:38.96]So the volume seems to alternate between very loud and very soft. [02:44.75]You can stand quite near a man singing in a loud voice and hardly hear him; [02:50.21]yet step a little further away, and his voice is almost deafening.
[02:54.39]As you move around the chamber, the volume of the sound goes way up and way down, depending on where you are in these standing waves. [03:04.07]And often the acoustics make it hard to identify where sounds are coming from … [03:09.60]it’s as if powerful voices are speaking to you or chanting from inside your own head. [03:16.28]This had to engender a powerful sense of awe in Neolithic worshipers.
[03:21.22]And another bit of physics at play here is something called “resonance”. [03:26.24]I’m no physicist, but … well, I imagine you've all blown air over the top of an empty bottle and heard the sound it makes. [03:32.99]And you’ve probably noticed that, depending on its size, each empty bottle plays one particular musical note …
[03:41.08]Or, as a physicist might put it, each bottle “resonates at a particular frequency.” [03:47.50]Well, that's true of these chambers too. [03:49.96]If you make a constant noise inside the chamber, maybe by steadily beating a drum at a certain rate, a particular frequency of sound will resonate … will ring out intensely, depending on the size of the chamber.
[04:03.70]In some of the larger chambers, though, this intensified sound may be too deep for us to hear—we can feel it, [04:09.52]we're mysteriously agitated by it, but it’s not a sound our ears can hear.[04:17.31]The psychological effects of all these extraordinary sounds can be profound, especially when they seem so disconnected from the human doing the drumming or chanting.
[04:27.86]And there can be observable physical effects on people too. [04:31.95]In fact, the sounds can cause headaches, feelings of dizziness, increased heart rate... that sort of thing, you see?  [04:38.78]Anyway, what was experienced inside one of these passage graves clearly could be far more intense than the everyday reality outside … which made them very special places.
[04:51.70]But back to your question, Michael—as to whether these graves were designed to be used by the living—well, certainly with regard to astronomical or calendar function, that seems pretty obvious, [05:02.29]and I want to go into more detail on that now.