In anticipation of the first flights of its supersonic X-59 Quiet SuperSonic Technology (QueSST) X-plane, NASA recently tested a microphone that's 30 miles (48 km) long. Spread out on the Mojave Desert outside the space agency's Armstrong Flight Research Center in Edwards, California, the giant array will measure the acoustic signature of the next-generation aircraft that replaces the sonic boom with a soft sonic "thump."
One of the major hurdles facing the return of commercial supersonic flight is the infamous sonic boom caused by the build-up of a shock wave at the nose of an aircraft flying faster than the speed of sound. This produces a noise similar to that of a thunderclap or an explosion that's so loud and disturbing that commercial supersonic aircraft like the Concorde were restricted to flying only over the oceans and empty deserts.
The X-59 currently being built by Lockheed Martin is meant to change that by demonstrating technologies to make commercial supersonic aircraft quiet enough to fly over populated areas. Capable of cruising at an altitude of 55,000 ft (17,000 m) at a speed of Mach 1.27 (940 mph, 1,512 km/h), its new wing and hull design will produce a sonic boom of only 75 Perceived Level decibels (PLdB), or about as loud as a car door closing.
Part of this project will involve flying the X-59 over selected areas of the United States to demonstrate the quieter technology, gather scientific data and gauge public opinion that will be used to rewrite outdated regulations that were drafted in the 1960s and were often prejudiced against commercial supersonic flight.
But before that can happen, the X-59 needs to go through an acoustic validation phase, hence the portable 30-mile-long microphone array that measures at 50,000 samples per second an aircraft's on-the-ground sound profile or "carpet." The Carpet Determination In Entirety Measurements (CarpetDIEM) flight series was set up in the Mojave between Edwards Air Force Base and Nevada, where an F/A-18 aircraft flew over at supersonic speeds at high altitudes, allowing engineers to test both the array and the logistics of deploying it over such a large area.
The collected data is now undergoing analysis in the run-up to later tests.
"We have a series of microphones set up, and the various sites have different configurations," says Juliet Page, a physical scientist from the Volpe National Transportation Systems Center in Cambridge, Massachusetts. "We have microphones oriented in different configurations, including inverted, vertical, horizontal, some with different windscreens, and we’re evaluating the acoustic performance and the difference between the different configurations in preparation of the X-59. It’s cool to come out in the desert and just do all these measurements and just kind of geek out with this technology."
The second phase of the CarpetDIEM testing is scheduled to take place in the summer of 2020, while the X-59 is slated to begin flight tests in 2022.
Source: NASA
