Radiolab - The Walls of Jericho

Episode Date: October 4, 2010

Jad and Robert pit physics against a bible story with this simple question: could a team of trumpeters really bring down the walls of Jericho?

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W. N. Y. C. C. C. Yes. And NPR. Hello, I'm Jad. I'm Robert.

This is Radio Lab, the podcast. So today, uh, slightly offbeat, uh, exploration of acoustic warfare. And we're going to do it by looking at the. a famous... Do you know this story? I know what we're doing. Should I remind you of this story? I've heard the phrase Walls of Jericho, and I know nothing else about it. It's most famous as a song, Joshua foot the Battle of Jericho, Jericho, Jericho, Joshua

foot the Battle of Jericho and the walls came tumbling down. Don't know it. Ooh, that plenty moaning, who that titty-de-de-de. No idea. You don't know that? No. Before my time. Really before my time. No, no, no, no. It's a Negro spiritual. It's a Black American spiritual.

It looks like that. That's a moon. Anyway. So tell us the story so we can get started. Well, as you know, the Hebrew people crossed the Red Sea and then wandered around in the desert for a while. We'll just say I knew that. 40 years.

40 years. Now we're up to, we're almost into the promised land, but there is this city called Jericho. Who's inside Jericho? Well, the Jerichoans. Really? The Jerichoans. I don't know much about them, actually.

Why aren't they friendly? Are they against God? No, there's this. just, they just, I think they have all these 40,000 people who are these people? I get them out of here. So they were just looking after their property? I suppose. Or maybe they just didn't like what they saw. Okay. Now, Jericho. You're reading now. I'm reading from the Bible. Now, Jericho was tightly shut because of the sons of Israel. So I guess God said you've got to take Jericho. Maybe that was it. I'm a little fuzzy here on the cause of the thing. It had to be done. Had to be done. However,

now quoting the Bible, Jericho was tightly shut. Because it had a wall. So read the part about how they knock down the wall. Here's the formula. From the mouth of God. And you shall march around the city all the men of war circling the city once, and you shall do so for six days. And on the seventh day, you shall march around the city seven times, and seven priests shall carry seven trumpets of rams horns.

It's a shofar, actually, is a rams horn. And the priest shall blow the trumpets. And it shall be. The wall of the city will fall flat down. And they do this with seven trumpets, you say. Yes. All right, so here we go. The question we have then for this podcast is,

what would it really take? To do this. Yeah. And I'm talking without God, only puny physics. Your headphones? Independent left and right volume control should be fairly comfortable. Is it just in principle possible for sound to blow down a wall? That's our question.

We actually call up a guy who's thought about this. I'm an acoustical consultant. David Lubman. Is his name? And an acoustical scientist. And if you think about the nature of sound, it's a fluctuating pressure. He explained that when sound hits an object, the waves actually push the object but also pull at it. Many times per second.

And theoretically, he says, if you can get enough of those pushes and pulls on the wall. Eventually, it will begin to crumble. So the first question he asked naturally is, what kind of wall are we dealing with? Well, looking at the construction of Bronze Age walls in the Middle East, there were mud-brick walls. The question then was how much sound would be necessary, what volume, to topple a wall like that? Yeah. And he came up with a number. This is a technical number for the strength of the sound.

You'd need to produce 177 dB. 177 decibels. Yes. That would knock down the wall? Yes. How many decibels just for a scale is my voice right now? now, roughly. Your voice is probably about 60 or 65 decibels. Oh. So we're already a third of the way there just talking? Well, no. Here's where the issues start. Turns out, he says, the sound doesn't add up

the way that you would think. And there's the rub. Assume that your voice level was 60 decibels. In order to get 70 decibels, you would have to produce 10 times as much power. made much sense to us until we ended up doing what he did and recruiting some experienced shofar blowers from from synagogues yes we went up to all souls a unit what is it all souls unitarian church a mini times a month that church actually is a host to a synagogue oh my god I hear them did you hear that like animals dying hey there so so far's were the horns that apparently not down the wall, so we wanted to measure how loud they could be.

And we were lucky enough to find this guy. Cantor Daniel Pinkus. Who got together about 10 people, everybody with their ramshorns. Okay. This is a Yemenite shofar. Though Daniels was not quite a ramshorn. Probably from either an animal called a kudu, perhaps from an antelope. I'm not exactly sure.

It's quite big. Quite big. It's about two feet plus a few inches, in a somewhat corkscrew shape. corkscrew shape. So in any case, he got us started, and we asked him to blow his shofar as loud as he could, and we were going to measure the decibel level. So that was our baseline, 96 decibels for one shofar player.

But interestingly, when we doubled it, had two shofar players. Listen to what happened. We only got up to 98, just shy in 99. And when we doubled that, went from two to four shofar players, we only bumped it up three more decibels. Turns out, and this is actually a rule of thumb, anytime you want to bump up your overall volume by 3 decibels, you've got a double the amount of shofar players.

So if you want to go from 101 dB to 104, that means going from four shofar players to 8. If you want to go from 104 to 107, that means eight shofar players become 16. And if you want to go from there all the way up to our target, 177 dB. Well, then you're going to have to double yourself a lot. But here's the question.

How many in the end, how many shofars would you need to make the walls of Jericho come tumbling down? The number I calculated is 407,380. What? And it might take a while, too. Well, wait a second. Seven is what the Bible says, and you just said 407,380. Yes, and that would be a minimum number. That's five rose bowls full of trumpet players.

But of course, if it was a miracle, all bets are off. But what if you could get that number of people together? Could you still do it? Could you knock down the wall? Well, with the puny physics... According to David, we still have a problem, unfortunately. The problem I had was getting a very large number of men so close to the wall that we could produce the necessary pressure.

And as I added men, I'd have to put them further and further back. Imagine, he says, you've got all these hornblowers, hundreds of thousands at this wall. you've got to organize them, put them in rows, and that creates a little bit of a situation. Yes, well, the people in the front row would have their heads blown off by the blasts of the people behind them. That's a sort of problem if you're a musician, I think. If you'd like, we could do an experiment. Do you volunteer? No. Okay, but what if you could put the people in the front row in helmets to protect them?

Then could we do it? You know, actually we have another problem? That's right. The sound, according to Dave, has to be focused. It has to actually sort of point at one spot on the wall. When you put that many people together in front of a wall, some of them are going to be way behind the ones in front. We're going to lose focus.

And there's the problem. But then we found Woody. Yeah, my formal name is Alwood Norris. I go by Woody. We called Woody because, well, he's an inventor. President, chairman, and CEO of a brand new company called Parametric Sound Corporation. And he may be able to help us with our focus problem because he's invented a technology that can beam sound in a direct line, like a laser. Which I'm going to demonstrate for your friend here.

Let's check some of these things out. That's reporter Kirk Conan. Okay, first I'm going to play you this guy. To demonstrate, he pulls out his sound beamer. This is an ultrasonic emitter. It's like a mini satellite dish, but kind of in a square. Stand over there. He and Kurt get on opposite sides of the room very far apart.

And then Woody shoots a concentrated beam of sound. In this case, the sound of rushing water right at Kurt's head. Now, if I shine it at you, see the difference? Turn it towards me. Just aim it at my chest. Almost 100% gone. Wow. Magic.

Does your invention allow us to take the sound and put it into a beam such that it will hit a spot on the wall of Jericho? Absolutely with a caveat. What's the caveat? There is no known loudspeaker on the planet that can put out 170 decibels. Really? My company makes some of the loudest speakers on the planet. They're known quite popularly around the world as an L-Rour. red, long-range acoustic device. They're sold to the military, police departments, and the loudest

unit the company sells, which can be over $100,000 for one unit. Whoa. Puts out about 155 decibels. That's not enough to knock down our wall, though. No, not at all. There's another issue. This will be caveat number two. Uh-oh. When you get about 155, 165 decibels, you get close to causing cavitation. in the air

where the air turns into a plasma. You mean the sound won't travel through the air? Only for a few millimeters. We weren't able to a thousand percent confirm this, but according to Woody, even if we were able to make the necessary amount of noise,

we would not be able to get that noise to the wall. The sound would just go Yeah. We are just, there's an Anglo-Saxon where they would go Right there, but we're talking about the Hebrews.

But we are that thing, and I don't know what to do. Well, I had an alternate theory that could make the story plausible. What I've imagined is that the attackers would try to undermine the wall by digging underneath it. And the defenders, figuring that the attackers would do that, would send spies out to find out where the digging is so they can use countermeasures such as boiling oil. But then the – Wait, wait, let him finish this. I'm enjoying this.

Okay. But then the attackers would say they'll probably send out spies to find out where we're digging. So let's issue orders that nobody is to know where we are digging. And we won't tell those blabbermouthed Israelites because it's sure to be picked up by one of the spies. And the next thing you know, we'll have boiling oil on our head. In the meantime, the diggers, in order to keep the wall from falling in on them, they would prop it up with timbers. Then when the digging was about complete,

they would pull out whatever was the equivalent of a zippo lighter in the Bronze Age and alight the timbers and then run like heck. As the fire burned through, eventually that part of the wall would fall straight down. No, no, you're rewriting the whole thing. But where does the horn blowing come into the equation? Well, in the meantime, the spies report back

we can't figure out where those Israelites are digging. So the king of Jericho probably says, well, we'll have to use the old hole in the shield trick to find out. And so they take their bronzed shields with a hole in the middle, and they place them on the ground and put the ear to the hole. And they do this all around the perimeter of Jericho trying to hear the digging. But then the attackers say the defenders will probably use the old hole in the shield trick. We know that. So we'll have to use acoustic warfare, make noise, to use. prevent them from being able to hear where the digging is.

So let's send out a bunch of priests with shofars to make noise. So your shofars are there to keep the shield-listening Jerichoans from overhearing the digging Hebrews. And the horns are just a way to mask the digging Hebrews' location. Yes. This is very unsatisfactory. Thanks to Daniel Lubman, Elwood Woody Norris, Daniel Pinkis, and his show far all-stars.

Rachel Kelk. Anna Levy. Adam Jonah Hammett's Burner. Richard Shiner. Bob Wine. Ed Carson. Danielle Drakler. Miriam Frank. I'm Chad. I boomrod. I have Robert Krollwitch.

Bye. My name is Kion Wolfe and I'm a radio lab listener in Hartford, Connecticut. And an announcer for WNPR, Connecticut Public Radio. The Radio Lab podcast is founded in part by the Sloan Foundation. Thanks. I hope that helps. You guys seriously made my whole month. End of message.