Breaking the sound barrier means that an aircraft or any object must exceed a term known as Mach 1, which refers to the local speed of sound. The speed of sound varies depending on the local temperature, and at higher temperatures, the speed of sound is much quicker, meaning it takes more energy and speed to fly faster. When an aircraft approaches the speed of sound, it approaches an invisible pressure barrier set up by the sound waves just ahead of the plane, and the compressed air in front of the plane exerts a much larger than usual force on the plane. There is a noticeable increase in the aerodynamic drag on the plane at this point, hence the notion of breaking through the "sound barrier". When a plane exceeds the speed of sound, it is said to be supersonic, and anything exceeding the speed of sound creates a "sonic boom," not just airplanes. Breaking the sound barrier creates a shock wave known as a “sonic boom,” which occurs when an aircraft moves faster than the speed of sound. When pilots break the sound barrier, they don’t hear the boom because they are traveling faster than sound.