The questions kids ask about science aren’t always easy to answer. Sometimes, their little brains can lead to big places adults forget to explore. With that in mind, we’ve started a series called Science Question From a Toddler, which will use kids’ curiosity as a jumping-off point to investigate the scientific wonders that adults don’t even think to ask about. The answers are for adults, but they wouldn’t be possible without the wonder only a child can bring. I want the toddlers in your life to be a part of it! Send me their science questions and they may serve as the inspiration for a column. And now, our toddler …
Q: I want to hear what the loudest thing in the world is! — Kara Jo, age 5
No. No, you really don’t. See, there’s this thing about sound that even we grown-ups tend to forget — it’s not some glitter rainbow floating around with no connection to the physical world. Sound is mechanical. A sound is a shove — just a little one, a tap on the tightly stretched membrane of your ear drum. The louder the sound, the heavier the knock. If a sound is loud enough, it can rip a hole in your ear drum. If a sound is loud enough, it can plow into you like a linebacker and knock you flat on your butt. When the shock wave from a bomb levels a house, that’s sound tearing apart bricks and splintering glass. Sound can kill you.
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Consider this piece of history: On the morning of Aug. 27, 1883, ranchers on a sheep camp outside Alice Springs, Australia, heard a sound like two shots from a rifle. At that very moment, the Indonesian volcanic island of Krakatoa was blowing itself to bits 2,233 miles away. Scientists think this is probably the loudest sound humans have ever accurately measured. Not only are there records of people hearing the sound of Krakatoa thousands of miles away, there is also physical evidence that the sound of the volcano’s explosion traveled all the way around the globe multiple times.
Now, nobody heard Krakatoa in England or Toronto. There wasn’t a “boom” audible in St. Petersburg. Instead, what those places recorded were spikes in atmospheric pressure — the very air tensing up and then releasing with a sigh, as the waves of sound from Krakatoa passed through. There are two important lessons about sound in there: One, you don’t have to be able to see the loudest thing in the world in order to hear it. Second, just because you can’t hear a sound doesn’t mean it isn’t there. Sound is powerful and pervasive and it surrounds us all the time, whether we’re aware of it or not.
In general, our world is much more crowded than we think it is. We all live life like we’re Maria von Trapp, swinging our arms around in an empty field. In reality, we’re more like commuters on the subway at 5 p.m. — hemmed in in every direction by the molecules that make up the air around us. Snap your fingers and you jostle the particles right next to you. As they wiggle, they bump into the particles next to them, which, in turn, nudge the particles next to them.
These wiggles are what the world’s barometers were measuring in the wake of the Krakatoa eruption. Again, think of being on a crowded train car. If you were to hip check the person standing next to you — which I do not recommend — they would tense up and scoot away from you. In the process, they’d probably bump into the next person, who would tense up and shimmy away from them. (There would also be words exchanged, but that is neither germane to our thought experiment nor child friendly.) Meanwhile, though, that original person you bumped into has now relaxed. The pattern travels through the crowd — bump-tense-wiggle-sigh, bump-tense-wiggle-sigh.
That’s what a sound wave looks like. It’s also why you can’t hear sounds in space. Being in a vacuum is like being in an empty subway car — there’s no molecular medium for the pattern of movement, tension and release to travel through. Likewise, sound travels a bit differently in water than it does in air, because the molecules in water are more tightly packed — a Tokyo subway car compared to one in New York.
For instance, the loudest animal on Earth might, in fact, live in the ocean. Sperm whales use echolocation to navigate, similar to what bats use — they make a clicking sound and can figure out what’s around by the way that sound wave bounces off objects and returns to them. A sperm whale’s click is 200 decibels, the unit used to measure the intensity of a sound, said Jennifer Miksis-Olds, associate professor of acoustics at Penn State. To give you a sense of the scale, the loudest sound NASA has ever recorded was the first stage of the Saturn V rocket, which clocked in at 204 decibels.
But the whale is not really as loud as the rocket, she told me. Because water is denser than air, sound in water is measured on a different decibel scale. In air, the sperm whale would still be extremely loud, but significantly less so — 174 decibels. That’s roughly equivalent to the decibel levels measured at the closest barometer, 100 miles away from the Krakatoa eruption, and is loud enough to rupture people’s ear drums. Suffice to say, you probably don’t want to spend a lot of time swimming with the sperm whales.
|A mosquito from 20 feet away||0|
|Conversation at home||50|
|A motorcycle from 25 feet away||90|
|Chelyabinsk meteor from 400 miles away||✓||90|
|Mine crushing machine from 328 feet away||✓||127|
|Deck of an aircraft carrier||140|
|NASA’s acoustic testing chamber for satellites||163|
|Krakatoa from 100 miles away||172|
|Sperm whale echolocation||174|
|Saturn V Rocket||204|
Because sound is all about the motion of invisible objects, it’s also possible for that motion to happen and for you not to hear it. That’s because the molecules have to wiggle just right when they hit our eardrum. If the motion is going through the crowd of molecules too slowly or too quickly, our body can’t transfer that motion into signals our brains understand. This is called frequency, and it’s measured in hertz. Humans can hear a pretty broad range — 64 hertz to 23,000 hertz.1
But hertz and decibels are independent of one another. A sound can be extremely loud and still be at a frequency that we can’t hear. That’s what traveled all the way to England and beyond after Krakatoa erupted: sound waves that were inaudible to humans. Because extremely low frequency sound waves can travel much, much farther than higher frequencies, it’s specifically low-frequency sounds that can make these kinds of epic journeys. Scientists call this infrasound, and they’re listening for it, for a whole host of reasons. The Comprehensive Nuclear-Test-Ban Treaty Organization has 60 monitoring stations in 35 countries and uses infrasound to spot illegal nuclear detonations. The USArray, which is managed by a consortium of universities and government agencies, measures infrasound across the North American continent as a way of learning about seismology. Both these networks use microbarometers and low-frequency microphones, tracking modern infrasound similarly to the way scientists once tracked the infrasound from Krakatoa.
And there are many, many sounds to track, said Michael Hedlin. He and his wife, Catherine de Groot-Hedlin, run the Scripps Institution of Oceanography’s Laboratory for Atmospheric Acoustics and studies infrasound data. Hedlin can process that data — essentially just speeding it up — so that it becomes audible to human ears. Ghost sounds made flesh.
Hedlin’s sensors hear thunderstorms rolling through hundreds of miles away. They hear the sounds of coal mining as it happens in the next state. And then there are the more constant sounds. The wind blows. Waves on the ocean slap at each other. The inaudible signals travel hundreds of miles, sometimes thousands. When I called him from landlocked Minneapolis, Hedlin told me, “You’re probably immersed in sounds from the ocean you can’t hear.”
Milton Garces, the director of the Infrasound Laboratory at the Hawai’i Institute of Geophysics and Planetology, agreed. In particular, he told me that two sounds interfere with the Nuclear Test-Ban Treaty network, because they are so constant, so pervasive and so loud. First are microbaroms, which happen on the edges of storms at sea, when two ocean waves traveling in opposite directions meet, amplifying each other into a wave that’s bigger than either was alone. The other is just the sound of the wind — which can reach infrasound decibel levels equivalent to those of a motorcycle. “We developed our hearing threshold so we don’t go nuts,” Garces told me. “If we had hearing perception in that band it would be difficult to communicate. It’s always there.”
Even with that protection, extremely loud infrasounds can still have an impact on our bodies. Humans exposed to infrasounds above 110 decibels experience changes in their blood pressure and respiratory rates. They get dizzy and have trouble maintaining their balance. In 1965, an Air Force experiment found that humans exposed to infrasound in the range of 151-153 decibels for 90 seconds began to feel their chests moving without their control. At a high enough decibel, the atmospheric pressure changes of infrasound can inflate and deflate lungs, effectively serving as a means of artificial respiration.
And that, Kara Jo, is why I don’t want to answer your question without also telling you about the loudest sound you cannot hear. That would be the Chelyabinsk meteor, which exploded in the sky over southern Russia, near the border between Europe and Asia, on Feb. 15, 2013. Test-Ban Treaty sensors picked up the infrasound more than 9,000 miles from the source and the sound waves circled the globe. The nearest sensor was 435 miles away, Garces told me, and even at that distance the infrasound decibel level reached 90. Turns out, things don’t have to say “boom” to go boom.