A brief guide: Supersonic flight without the boom
A brief guide: Supersonic flight without the boom
Supersonic flight without the boom? Coming right up—either through ingenious aerodynamics or with the help of Mach cutoff, a sophisticated physical effect.
author:<br>Andreas Spaeth
5 mins reading time
published on:<br>08.05.2025<br>Share article
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©NASA / James C. Jensen
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What causes the sonic boom?
When an aircraft flies at less than the speed of sound, the sound it generates spreads evenly in all directions in the form of waves. This threshold, which depends on the environment but is generally around 1,200 kilometers per hour, is referred to as Mach 1. As the aircraft approaches this speed, the sound can no longer propagate forward. Instead, it builds up as a pressure wave in front of the aircraft. The sound waves coalesce into what is known as the sound barrier. This acts like a solid body, as the air ahead of the aircraft can no longer equalize pressure differences. Once the aircraft reaches supersonic speed, it breaks through this barrier.
As the aircraft passes through this air mass, a shock wave appears at its nose and spreads out behind the jet in a funnel shape. Wherever this shock wave hits the ground, it is perceived as a loud boom of up to 120 decibels—close to the human pain threshold of 130 decibels. The passengers on board don’t notice the boom, because they are traveling faster than the sound itself. Often, a second boom can be heard at ground level shortly after the first. This is because shock waves are generated not only at the aircraft’s nose but also at its tail. Depending on how far away the aircraft is from the listener, they will hear a double boom.
How can Mach cutoff prevent the boom?
Mach cutoff is a physical phenomenon that causes sound waves to refract and bend back up into the atmosphere, away from the ground. This means a supersonic aircraft emitting sound waves in flight that would normally be audible as a boom on the ground can pass overhead without any noticeable effect for humans. To do this, the aircraft must adapt precisely to the prevailing conditions of the area it is overflying. Blake Scholl, CEO of Boom Supersonic, explains: “Just as a beam of light refracts when passing through a glass filled with water, sound waves will also change direction depending on which layers of air they’re traveling through and how the speed of sound varies there.”
©Pennsylvania State University Acoustical Model of Mach cutoff Flight
©Pennsylvania State University Acoustical Model of Mach cutoff Flight
The speed of sound depends largely on the temperature—and this decreases with increasing altitude. In colder air, sound waves refract and bend back upward, forming a U-shaped trajectory that keeps the sound away from the ground. “How high up this U is depends on several factors,” Scholl says, “including the airspeed, the temperature gradient in the atmosphere, and the prevailing wind conditions.”
The key factors here are the atmosphere and the nature of the terrain. “The aircraft’s cruising speed must be constantly adjusted so that the sonic boom waves don’t extend below a desired height—the cutoff altitude,” says Bernd Liebhardt, a scientist at the German Aerospace Center (DLR) in Hamburg who studies commercial supersonic flight. Achieving what the company calls “Boomless Cruise” is certainly within reach:
“It will always work, provided you have the exact weather data, precise ground contours, an exact calculation of the maximum possible flight speed, and precise flight control,” Liebhardt says. “Today’s technologies make the combination of these capabilities achievable at reasonable effort.” The maximum possible airspeed for Mach cutoff is Mach 1.3, but depending on the weather conditions it will usually lie in a range between Mach 1.1 and 1.2 (1,358 and 1,481 km/h).
©Boom Supersonic
©Boom Supersonic
©Boom Supersonic
A small sensation occurred here on January 28, 2025: the Boom XB-1 single-seater test aircraft managed to break the sound barrier several times, reaching a top speed of Mach 1.18.
Can Boomless Cruise work for supersonic passenger jets?
The XB-1, Boom Supersonic’s test aircraft, proved for the first time in January 2025 during test flights over the Mojave Desert that Mach cutoff, and thus Boomless Cruise, works. So far, however, this has been shown only for shorter flights over land, where the capacity for boomless flight is also crucial. The only way speeds can be increased from today’s Mach 0.94 to anywhere up to Mach 1.3 in the future is if there is no boom. “But in this speed range, air resistance and hence fuel consumption are at their highest,” Liebhardt points...