Sonic impact: lowering the boom of supersonic flight - research efforts reported at the May 1995 Acoustical Society of America meeting in Washington, D.C - Cover Story

Science News, Sept 23, 1995 by Ivars Peterson

One design has the slim, sharply pointed profile of an arrow. Another looks like a flying wing, with no fuselage or tail. A third features a sleek, streamlined aircraft body with wide, triangular wings.

Though these designs represent dramatically different visions of future supersonic travel, they all share a common goal. Flying at 1,600 miles per hour (more than twice the speed of sound), any one of these proposed aircraft would carry as many as 300 passengers from Los Angeles to Tokyo in less than 5 hours, cutting the usual travel time by more than half.

But the supersonic speedway from the drawing board to commercial flight is studded with obstacles. To sell more planes, aircraft makers want a supersonic transport to fly as many routes as possible, over land and sea.

The trouble is that supersonic flight is inevitably accompanied by ear-splitting, window-shattering sonic booms. Even planes flying at 60,000 feet or more lay down a discernible sonic track across any landscape they traverse. This means that any future high-speed airliner would have to be considerably quieter than the Concorde, the only supersonic airplane now in commercial service.

In recent years, airplane manufacturers, NASA, and others throughout the world have been taking a fresh look at the prospects for commercial faster-than-sound flight. Since 1990, researchers have expended considerable effort on developing the knowledge needed to design a new generation of supersonic jets cleaner and greener than the Concorde (SN: 10/26/91, p.270).

One component of this effort has focused on ways to soften sonic booms--to reduce their impact by altering the plane's design or the way it's flown. But the right formula for achieving this goal has proved elusive.

Moreover, though researchers have made considerable progress in understanding and predicting the effects of sonic booms, recent studies and community surveys strongly suggest that people find even occasional sonic booms much more disturbing than loud, continuous noise such as that of an airport.

Such findings indicate that "any commercial, overland supersonic flight is highly unlikely within the near future," says Christine M. Darden of NASA's Langley Research Center in Hampton, Va.

Research continues to probe ways to reduce the peak intensity of sonic booms generated by aircraft and to make more accurate predictions of how these shock waves travel through the air and how disturbingly loud they sound at ground level.

Other studies likely to affect the development of supersonic transport involve ongoing assessments of the potential impact on marine life and wild birds of a projected 500 or more daily ocean crossings by supersonic airliners.

Researchers presented progress reports on various aspects of this work at a meeting of the Acoustical Society of America held in Washington, D.C., this May.

Like a ship plowing through water, an aircraft has to push air aside as it flies. When a plane travels faster than the speed of sound, the air cannot move quickly enough to get completely out of the way, so a tremendous pressure wave builds up, trailing behind the plane like a ship's wake.

This shock wave reaches to the ground and lays down a broad track that marks the plane's flight path. A stationary observer on the ground hears a sharply defined noise like a thunderclap and feels the vibration as the pressure wave roars by.

Supersonic transports have distinctive sonic signatures that depend on the airplane's shape, speed, and motion. The needle nose of the Concorde, for example, cuts drag and allows the jetliner to slice through the air efficiently. But this configuration also generates a particularly strong shock wave.

The shape of the resulting pressure waves can also be quite complex. A supersonic aircraft typically sheds at least two shock waves, including one from the plane's nose and another from its tail.

Although it's impossible to eliminate these pressure waves, aircraft designers have sought to reduce the boom's intensity by altering the plane's shape, thereby changing its signature. For example, a plane that gradually widens from front to back produces a sonic boom more like distant, rolling thunder than a sharp, intense crash.

Aeronautical engineers have also found that a thunderclap boom can be broken up into smaller bangs by spreading the lift-generating surfaces more evenly around the aircraft. But such modifications hurt the airliner's performance and decrease its fuel efficiency.

How a plane is flown also affects the sonic boom it generates. For example, accelerating an aircraft already moving in a straight line at supersonic speeds can intensify and focus the resulting boom.

Such maneuvers interest the U.S. Air Force, which would like to develop techniques for directing sonic booms, either to inflict psychological harm or structural damage at specific sites or to avoid causing such disturbances.

In April 1994, researchers from the Armstrong Laboratory at Wright-Patterson Air Force Base in Ohio conducted a series of flight tests at Edwards Air Force Base in California to collect data on the ability of air crews to focus sonic booms and control the placement of them.