What Exactly Is A “Sonic Boom”?
By Aerion Supersonic on Nov 26, 2019
4 min read
Here at Aerion, we’re focused on supersonic flight. We think that innovation in high-speed flight will bring people closer together by eliminating distance as a barrier to love, friendship, and shared experiences. But we recognize that innovation in mobility can’t come at the cost of our planet, that’s why one of the things that sets Aerion apart is our vision for sustainable supersonic flight. Transportation, especially high-speed transportation, can have a number of impacts on the environment, and we are fully committed to not only understanding those impacts, but mitigating them.
One of the more infamous environmental impacts of supersonic travel is the “sonic boom.” And, yes, some airplane nerds, like us, think hearing a sonic boom can be cool (and seeing them is even cooler). But if you’re in your home sleeping, trying to quiet your kids, or just enjoying a relaxing Sunday afternoon, we are fully aware that sonic booms are not so cool. For that reason, we are working diligently to equip our aircraft with technology that will enable it to avoid ever creating a sonic boom that reaches the ground – this is done by taking advantage of a phenomenon called “Mach cut-off”. But before we tell you more about that, let’s talk about what causes a sonic boom in the first place.
The Doppler effect is an everyday phenomenon you would probably recognize from hearing a police car or ambulance drive by, and it’s the reason why supersonic aircraft create a “sonic boom.” Basically, the sound of the siren changes as it approaches, and subsequently passes, you. As it approaches, you hear one sound, and then the frequency of the sound seems to change as it drives away (a car horn is another great example of this). That’s because your ear is hearing the sound waves from the siren, but those waves are influenced by the object emitting them, and the movement of that object. As the object approaches, the sound waves are closer together which provide a higher frequency noise or higher pitch sound. As the emitter moves away, the sound waves get farther apart as they are generated and result in a lower frequency noise or lower pitch sound. The image below illustrates the relative “bunching” and “separating” of the sound waves as the source (the red dot) moves in the direction indicated by the arrow (right to left).
A sonic boom is generated when an object moves faster than the speed of sound. It is an extreme version of the Doppler effect because now the object is moving so fast that, instead of the waves propagating in front of the object they are left behind as soon as they are emitted. This effect applies to generic pressure waves, not just sound waves, which is why a “silent” object like a bullet can still create a sonic boom. These waves will bunch up to form a “shock cone” (the area where the boom is heard, shown below), resulting in all the waves reaching the observer (that’s you or me) simultaneously. Instead of hearing the sound over time, it is as if all that sound is bottled up into one instant, resulting in a much louder sound.
The strength of the sonic boom and the corresponding sound that someone on the ground would hear are dependent on a number of factors, including the speed and shape of the airplane, the characteristics of the atmosphere, and how far away the listener is from the object. As we mentioned above, Aerion is taking advantage of those factors combined with natural temperature variations in the atmosphere to reflect the sonic boom away from the ground and change the shape of the shock cone. This technique, called Mach cut-off, is something we’ll explain in more detail in a future blog post, but we are studying it carefully because we believe that the only acceptable sonic boom is no boom at all.