The Echoes of Sound: How Balloons, Oceans, and the Atmosphere Share a Hidden Waveguide
The balloon microphone could detect the roar of a rocket launch hundreds of kilometers away. This striking observation comes from a team exploring infrasonic signals and the way sound travels through air and water, as reported in scientific circles.
Back in the 1940s, geophysicist Maurice Ewing identified a phenomenon in the world’s oceans: a natural underwater channel that guides sound over long distances. Sailors used this acoustic hallway to track submarines, and humpback whales communicate with kin thousands of kilometers apart. The shape of this pathway comes from how speed of sound shifts with depth in the ocean. Temperature drops as you descend, causing sound to slow down at first, until a depth where the slowdown levels off into a flat plateau. Beneath that plateau, increasing pressure pushes sound speed higher again. This creates a minimum in the sound-speed profile, flanked by regions where the speed rises. An acoustic beam entering at the right angle can bend downward, then return toward the bottom multiple times, looping through the water in a guided path. That looping is the essence of the ocean being a natural waveguide.
Ewing later pursued a similar idea high in the atmosphere, aiming to locate an analogous channel around the tropopause, the boundary where air meets the stratosphere, roughly ten to eighteen kilometers up. The U.S. military invested in this line of inquiry during the Mogul project, launching balloons equipped with infrasonic microphones to listen for distant nuclear tests. One balloon’s fall near Roswell in 1947 sparked enduring UFO chatter, though the project soon ended after results failed to meet expectations. These early efforts helped frame questions about how far sound can travel and through which layers of air it travels best.
Today, Sarah Albert, a geophysicist at Sandia National Laboratories in New Mexico, revisits infrared balloons that now feature solar power and wireless data links. Computer models had repeatedly suggested a real chance of channeling sound through the atmosphere between roughly ten and forty kilometers up, inspiring renewed experiments. On April 14, 2021, one such balloon succeeded in picking up the sound of Blue Origin’s New Shepard rocket more than four hundred kilometers away. In the recording, three distinct arrivals appeared: the moment of launch, the ascent through the tropopause, and the exit as the vehicle passed through the same atmospheric layer. Albert notes this as the first documented observation of a distant infrasonic source captured by a mobile airborne receiver.
The recordings also featured other infrasonic noises whose origin remains mysterious. A participant in the study described the presence of recurrent, unexplained infrasonic phenomena occurring many times per hour, for which there is no clear explanation yet. Albert suggests that there may be a counterpart to the underwater channel in the atmosphere, though its permanence and capacity to carry as much sound as hoped remain uncertain. The atmosphere is markedly less stable than the ocean, continually altered by temperature shifts and changing winds, which likely complicates repeatability. A separate attempt in September 2021 near Vandenberg Air Force Base to detect a space launch with balloons did not yield the same results, possibly because the distance involved reduced signal strength below detectable levels.
Looking ahead, researchers plan to deploy several balloons at staggered altitudes to map where the atmospheric channel exerts the strongest influence on infrasonic signals. They also intend to explore unusual infrasonic events to better understand their origins. Early interpretations propose that some sounds could stem from events such as spaceborne fireballs or auroral activity. As observed by researchers, the long-standing belief in a waveguide-like atmosphere has found renewed support through new measurements, though scientists acknowledge that there are multiple atmospheric waveguides. The effectiveness of any given waveguide appears tied to the alignment of the sound source and receiver along the axis defined by a minimum in the speed of sound.
According to Sergey Kulichkov, a physicist and head of an atmospheric research institute, there are three distinct atmospheric waveguide types. He notes that the particular atmospheric pathway described by American researchers is not the most advantageous among them. For instance, another type with an upper boundary around one hundred kilometers has historically enabled distant volcanic sounds to be heard in the late nineteenth century and helped establish evidence for the existence of the stratosphere. These insights help frame how scientists understand how sound can travel across great distances, whether through oceans or skies, and why precise conditions matter for detecting faint signals at extreme ranges. | Attribution: based on reports from aerospace and geophysical research teams and historical investigations into acoustic waveguides. |