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Around the world, ancient people built incredible structures, as performance venues, sites of worship, and lots of other things. And these structures can be visually stunning, but often, there’s more to them than just their looks.

In a field of science known as acoustic archaeology, engineers and archaeologists have shown that many ancient sites also had some truly amazing effects on sound waves. And by studying the acoustics of these structures, we can learn new things about the experiences people had there, hundreds or even thousands of years in the past. One of the most famous examples is the theater built in the ancient Greek city of Epidaurus in the fourth century BCE.

It’s made of a circular stage with rows of steps that rise from the center to the back, wrapping around the stage. It has a capacity for around 15,000 spectators. Despite being totally open and outdoors, even today, it’s famous for having excellent acoustics.

Somehow, the structure manages to cut out low-level noises like a murmuring audience or the sound of wind while still carrying human voices clearly from the stage. Scientists think a lot of this has to do with the design of the steps and the way they interact with sound waves. See, when a sound wave encounters a surface, it often gets scattered into different directions as it reflects off the surface and goes through a process called diffraction.

Part of the wave may bounce off the surface and collide with another part of the wave, canceling itself out in a given direction. But in other directions, the reflected sound waves may line up in such a way that they reinforce each other, so the echo amplifies the sound in certain directions. How exactly this plays out all depends on the geometry of the obstacle.

At Epidaurus, sounds between 100 and 500 hertz tend to interfere with each other as they bounce off the steps and somewhat cancel each other out. But higher-pitched sounds above 500 hertz reinforce themselves as they scatter. Conveniently, a lot of background noise, like whispers, shuffling, and wind, tends to be fairly low-pitch, while the performers’ voices would have been higher pitched. 500 hertz is still pretty high for a human voice, but scientists believe that higher tones of performers’ voices would have carried through, and people’s brains would have filled in the missing tones.

Now, we don’t know how well the Greeks understood the physical mechanisms that made this theater a success. But even if the effect was just a happy accident, researchers think it may have inspired later Greek and Roman theaters, which were built with similar designs. Diffraction doesn’t always just amplify or muffle sounds, though.

In some cases, it can distort sounds to make them seem like something else altogether. You can hear an example of this at El Castillo pyramid in the ancient Mayan ruins of Chichén Itzá. Like the theater at Epidaurus, it has rows of evenly-spaced steps that climb up to the top on all four of its sides.

And it’s been known for a while that if you stand near the bottom of the pyramid and clap your hands, the sound echoes off one face of the pyramid and comes back to you. But that echo doesn’t sound like a clap. It sounds more like a chirp that falls off in pitch.

The acoustic engineer who first studied the effect in the late 1990s, David Lubman, realized that it had to do with the way sound was diffracting off the steps. And in 2004, researchers in Belgium used computer simulations to show exactly what was going on. By simulating the sound of a clap echoing off the pyramid, they found that, just like the theater at Epidaurus, the steps reflected higher frequencies more than lower ones.

That still didn’t really explain the chirp sound, but there was more to the story. Since the clap comes from a person closer to the base of the pyramid than the top, the sound reflects off the lower steps sooner than it reflects off the top. As a result, the echo gets stretched out a little bit, so it sounds longer than the original clap.

But it also drops off in pitch. The exact reason for that involves a lot of math, but ultimately, thanks to the geometry of the pyramid, sound waves diffract in such a way that you hear the higher pitches first, followed by the lower ones. One detail that has made this echo especially intriguing is the fact that it sounds remarkably similar to the call of the quetzal, a bright green bird with long feathers that was sacred to the Maya.

Researchers have even found that there are specific frequencies in the sound of a clap that diffract in just the right way to return the frequencies in a quetzal call. Combining that with cultural evidence, some researchers have argued that the pyramid was intentionally designed to mimic the sound of this sacred bird, but so far, that’s still up for debate. Then there is Stonehenge, the iconic prehistoric monument in England that’s famous for lots of reasons, but not so much for its effect on sound.

Yet, something unusual does happen to sound within Stonehenge. Researchers learned this by exploring the acoustics of a concrete reconstruction of Stonehenge in Washington state, complete with all the stones that were missing from the remaining structure in England. They found that, within the structure, although each other’s voices were clear, they couldn’t pin down where they were coming from; the voices seemed to come from everywhere at once.

Using computer simulations, the same researchers figured out that the original Stonehenge would have approximately created what’s called a diffuse field. That’s what happens when lots of little echoes reach a listener at around the same time, making it hard to tell where the original sound came from. Normally, if you just make a sound within a ring of rocks, it’ll hit all those rocks and bounce back all at once like an ordinary echo.

So the energy in the sound wave goes out and comes back in, and as it hits your ear, your brain can tell with direction it came from. But within its outer ring of rocks, the original Stonehenge had many other rocks arranged in various patterns. Simulations show that sound waves produced on the inside would have scattered off those inner rocks first, before they hit the outer ring, so the echoes would seem to have come from many places at once.

That’s likely why the researchers at the replica seemed to hear the sounds of each other's voices coming from all directions. As with the other examples, it’s hard to know if this was a deliberate design with a specific acoustic purpose, but it does tell us something about what visitors to the real Stonehenge probably experienced three thousand years ago. Around roughly the same time, in what is now Peru, ancient civilizations were using a totally different phenomenon to manipulate sound.

High in the Andes, a hillside known as Chavín de Huántar is scattered with ancient ruins from the Chavín people, who lived there as early as 1200 BCE. At the top, there’s a temple with a maze of tunnels underneath it that forms part of the complex where the Chavín conducted worship rituals. Underneath one tunnel known as the Lanzón gallery, archaeologists unearthed a number of conch shells called pututus.

It was clear that they were meant to be played as instruments, they were carved like horns, and ancient carvings near the gallery even showed priests playing them. So, out of curiosity, researchers at Stanford University played a pututu in the lab and they found that the shell produced clearly defined tones around 300 hertz. To explore how that and other sounds would have traveled within the Lanzón gallery, the same team of researchers used a loudspeaker to play different tones from inside the tunnels.

From there, sound traveled through air ducts carved into the structure that opened out into a plaza. Of all the tones, researchers found that those with a pitch of 300 hertz, the natural frequency of the pututus, came out about two to four times louder than other noises! And that is because, for the pututus, the ducts were acting like waveguides.

Acoustic waveguides are specifically shaped tunnels that can amplify certain sounds. Specifically, if the distance the wave travels inside the tube is some multiple of the distance from a sound wave’s crest to its trough, so, say it’s two or 20 times the wavelength, that wave will reinforce itself as it bounces off the walls. Basically, that echoed sound retraces a path similar to the original wave.

As a result, specific sounds will be louder than others when they come out the other end. It’s the same physics that helps musical instruments project sound. And by acting as waveguides, the air ducts from the gallery became an extension of the pututu itself, turning the entire temple into a kind of instrument that channeled sounds perfectly to the plaza below.

Later studies confirmed that the ducts were pretty much perfect for amplifying the pututu’s sounds, which strongly suggests that the ducts could have been made that way by design. No one knows exactly what role this sound played at the site, but researchers have speculated that the pututu may have been used to project sound out to worshippers in the plaza from inside the gallery. Another incredible example of a structure built to amplify sound is the.

Hypogeum of Hal Saflieni, an underground temple on the island of Malta. The Hypogeum was built by removing material from the limestone ground to create intricately designed chambers beneath its surface. In one of those chambers, known as the Oracle Chamber, human voices create echoes that last up to seven or eight seconds.

Part of the reason for this is that the chamber’s curved ceilings, as well as one curved wall, reflect human voices across the space really well. But for certain frequencies, the structure doesn’t just efficiently reflect sound; the spacing between the walls sets up what’s called a standing wave. A standing wave is created when the spacing between two surfaces is just right, so that when one surface reflects a sound, it travels through the air and hits the other wall right at the peak of the wave.

As it’s reflecting back, it lines up with the incoming wave, amplifying the sound and sustaining the echo. According to a 2014 study on the Oracle Chamber, the specific sound waves that line up with the spacing of the structure to create that standing wave have frequencies of 70 and 114 hertz. And those frequencies may not be totally random.

The authors pointed out that similar frequencies have been found to resonate in other Stone Age sites in Europe. Now, we don’t know exactly why various ancient people built chambers that resonate at these precise frequencies. But we know that these are pitches male voices can reach while chanting.

And research has also shown that our brains may have a specific response to hearing them. An early 2008 study by researchers at UCLA measured brain activity in 30 adult participants while they listened to various frequencies found in ancient cave-like structures. At around 110 hertz, researchers found that the regions of the brain responsible for language processing became less active, while the regions responsible for emotional processing became more active.

Now, it’s worth bearing in mind that these are small studies and the original 2014 study of the Oracle Chamber hasn’t been replicated. So while there’s plenty of reason to be cautious about interpreting these results, the authors point out that early cultures may have built spaces to evoke certain sensation, and resonances at Oracle Chamber and some other ancient sites may have put people in a more spiritual state of mind. Of course, it’s hard to know exactly how to interpret any of these findings.

But the idea and ability to use sound as a way of gaining insight into people who lived long before us is still fairly new, and sites like these show us that at the very least, it is a fascinating way to explore the past. Thanks for watching this episode of SciShow! And a special thank you to our patrons, whose support and curiosity about the world makes episodes like this possible.

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