Mysterious audio channel: How to hear a rocket launch from hundreds of kilometers away

The microphone on the balloon could hear the sound of a rocket launch 400 kilometers away. About informs journal Science.

In the 1940s, geophysicist Maurice Ewing discovered an underwater sound channel in the world’s oceans, through which sound can propagate over very long distances. Sailors use it to track submarines, and humpback whales communicate with their relatives thousands of kilometers away.

It occurs due to the fact that the speed of sound in the ocean depends primarily on two factors – temperature and pressure. The temperature decreases with the speed of sound with depth, but flattens out and remains constant below a certain level. Below this plateau, the pressure continues to rise and increase the speed of sound. Thus, there is a minimum sound velocity region above and below which the velocity increases. Therefore, if an acoustic beam goes up through this region at a certain angle without bursting, it will bend downward due to refraction and eventually return to the bottom. Since the speed of sound also drops below the minimum level, the beam will return for the same reason and therefore cyclically many times over. This is how an ocean waveguide is formed.

Ewing devoted further work to searching for a similar channel in the tropopause region, the boundary between the atmosphere and the stratosphere, located at an altitude of about 17 kilometers. The US military hoped to use this channel to eavesdrop on Soviet nuclear tests, and therefore launched the Mogul project, in which balloons with infrasonic microphones were launched into the sky. One of them fell near Roswell, New Mexico in 1947, creating one of the largest. famous UFO conspiracy theories. Soon the project was closed due to unsatisfactory results.

Today, Sarah Albert, a geophysicist at Sandia National Laboratories in New Mexico, decided to retest infrared balloons, which are already equipped with solar panels and wireless data transmission. The scientists were motivated by the fact that computer models consistently showed the formation of sound channels at altitudes between 10 and 40 kilometers.

Albert launched one of the balloons on April 14, 2021, and the attempt was successful: The team was able to hear the sound of Blue Origin’s New Shepard rocket. The rocket was launched about 400 kilometers from the balloon, and in the recording, the scientists saw three clear signals: during launch, as it ascended the tropopause, and descended through it. According to Albert, this is the first observation of such a distant infrasonic source using a receiver on an airplane.

To the surprise of the scientists, there were other sounds in the recording. “Infrasonic phenomena of unknown nature occur several times per hour and there is no good explanation for them,” said one of the participants in the study.

“I believe the Underwater Sound Channel has an atmospheric counterpart,” Albert says. “But I’m not sure it exists permanently and can send as much sound as we think.”

Especially because the atmosphere is much less stable than the ocean, it is affected by temperature changes, changes in winds. And indeed: when a team of scientists tried to repeat the success in September 2021 and hear about the space launch at Vandenberg Air Force Base, they failed. Perhaps this is because the balloon is 1300 kilometers from the rocket.

Going forward, the researchers plan to listen for space launches with several balloons staggering at different altitudes to find where the impact is strongest. They also deal with the nature of unusual infrasonic sounds. So far, the authors suggest they were produced by fireballs falling from space or the northern lights.

“Researchers from America write that for a long time they believed in the existence of such a waveguide in the atmosphere and they finally found it… Although it is well known that there are many areas of sound speed minimums in the atmosphere. So there are many waveguides in the atmosphere similar to ocean waveguides. Note that such waveguides arise effectively only if the sound source and receiver are located on the axis of the waveguide – in the horizon of the minimum speed of sound, ”says Sergey Kulichkov, Doctor of Physics and Mathematical Sciences, Head of the Atmospheric Institute. Physics. AM Obukhov RAS.

Kulichkov states that there are three different types of waveguides in total in the atmosphere, and that the waveguide described by American scientists is not the most useful of them. For example, with the help of a different type of waveguide with an upper limit of 100 kilometers, the sound of distant volcanoes was recorded in the late 19th century and the existence of the stratosphere was discovered.

Waveguides transmit sound (or light) a great distance, allowing you to wrap them as if you were in a narrow corridor. One of these waveguides was recently discovered by American researchers who heard infrared sound coming from a rocket hundreds of kilometers from the launch. The Institute of Atmospheric Physics of the Russian Academy of Sciences told socialbites.ca that this phenomenon has long been predicted theoretically, and its practical use is not so great.



Source: Gazeta

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