Frequency Fingerprints

Satellites like NOAA’s GOES instrument monitor frequencies used for personal locator beacons worn by outdoor adventurers and military personnel. Credit: NOAA.

News of radio signals emanating from the bottom of the Indian Ocean provided a much-needed clue to the whereabouts of the missing Malaysian Airlines jet that disappeared from radar screens a little more than one month ago. Over the weekend, ships combing an area one thousand miles west of Australia detected sounds consistent with the ‘pings’ of an airplane’s data recorders, designed to transmit locator signals in the event of an accident. But how can searchers confidently identify the source of these faint echoes from the deep? The answer lies in the pitch and pattern of the pings, and illustrates how frequencies can serve as technological fingerprints.

An airplane data recorder, also known as a black box, emits sound at a frequency of exactly 37.5 kilohertz (one kilohertz equals 1000 hertz, a unit of cycles per second used to measure frequency) and pings once per second. Although whales and other sea life can produce whistles and clicks in the same range of frequencies, their calls vary in pitch and lack the regular timing of a locator ping. To trained rescuers, the sound of a black box is like a barcode that distinguishes it from the cacophony of the submarine soundscape, a place that reverberates with the rumble of undersea earthquakes and volcanoes, the hum of ship motors, the banter of marine mammals and even a handful of mysterious low-frequency shrieks recorded on NOAA microphone arrays dubbed ‘bloops’ that turned out to be the groans of disintegrating ice shelves.

Natural and human activities make the sea a noisy place. Marine mammals also emit sounds at the frequency of black box pings, but their cries have range in pitch and pattern. Credit: Discovery of Sound in the Sea (http://www.dosits.org/tutorials/effects/masking/)

But black boxes are not the only type of technology that uses a unique frequency to set it apart. Above the surface of the ocean, where water does not impede their transmission, many devices use specific frequencies of electromagnetic radiation instead of sound waves. For instance, avalanche transceivers worn by alpine skiers emit radio waves at a standardized frequency of 457 kilohertz. This allows searchers to locate a buried skier because all transceivers emit and search for signals at the same frequency, one that is high enough to penetrate through several feet of snow. Similarly, personal locator beacons worn by outdoor adventurers and military personnel emit at 406 megahertz (one megahertz is one million hertz), a frequency monitored by NOAA satellites and the Air Force. Even your Bluetooth headset has its own electromagnetic niche; it syncs with your phone, your stereo, and your computer at frequencies of about 2.4 gigahertz (a gigahertz is one billion hertz).

The technologies listed above span a wide range of frequencies, and we would not characterize most as producing light, but in fact, they do. That’s because radio waves and visible light are both forms of electromagnetic radiation; in common language, we use the word light to describe the part spectrum we see with our eyes, but from a physical point of view, all electromagnetic waves are the same. Specifically, electromagnetic radiation does not require a carrier like air or water — it can travel through a vacuum but is quickly absorbed in a dense medium like seawater. It behaves both like a wave and a particle (a paradox physicists refer to as the wave-particle duality) and is carried by mass-less photons.

The electromagnetic spectrum includes radio wave, visible light, and x-rays. While electromagnetic radiation cannot travel far through water, it is used in many locating devices on the surface. Credit: NASA.

Sounds waves, on the other hand, like the pings of a black box, are fundamentally different; they are mechanical waves that can only propagate through a medium like water or air. When a sound source like a drumhead vibrates, the membrane compresses the atoms close to it, sending out a pressure wave that moves through the medium until it reaches a sensor like our ears or the receivers used in search and rescue. In fact, sound waves travel farther through a denser substance like water than they would through air, making them ideal for underwater transmissions, but of limited use at the surface. Together, electromagnetic radiation helps us see the world and sound waves help us hear it, and when we harness the power of frequency fingerprints using technology, both allow us to locate all kinds of things we seek to find.

LEARN MORE

Learn more about light and electromagnetism and sound waves in our modules on these topics.

Take a listen to NOAA’s wonderful collection of undersea sounds. 

Check out this humorous cartoon of the electromagnetic spectrum to learn the frequencies of the Death Star laser and slinky waves.

Written by Julia Rosen

Julia Rosen is a freelance science writer and PhD student at Oregon State University. She received a Bachelor’s degree in Geological and Environmental Sciences from Stanford University before beginning her doctoral research on polar ice cores and climate change. In between, she did her “Master's” in backpacking around the world and skiing. Julia is a periodic contributor to Oregon State’s research magazine, Terra, and helps write blog content and develop learning modules for Visionlearning.