Supersonic aircraft are not inherently quiet; in fact, they produce loud sonic booms when flying at supersonic speeds. The perception of quietness associated with supersonic aircraft typically refers to the reduction of noise during subsonic flight conditions and ground operations. Here's an explanation of the factors involved:
Subsonic Flight and Ground Operations: When supersonic aircraft are operating at subsonic speeds or on the ground, their noise levels can be comparable to or even lower than subsonic aircraft. This is achieved through various engineering techniques, such as advanced engine designs, improved aerodynamics, and better insulation and soundproofing materials. These measures help minimize engine noise, airframe vibrations, and other sources of noise.
Sonic Boom: When an aircraft flies faster than the speed of sound (supersonic), it generates shockwaves called sonic booms. These shockwaves are created due to the abrupt change in air pressure as the aircraft moves through the air. Sonic booms are essentially concentrated sound waves that can be perceived as a loud, explosive noise on the ground.
The reason sonic booms are particularly loud is that they are a result of the compression and rarefaction of air molecules occurring at once. As the aircraft moves, it continuously creates and leaves behind a series of these shockwaves. When they reach the ground, they combine and generate a single, intense sound wave.
Efforts have been made to reduce the intensity of sonic booms. For instance, aircraft manufacturers and researchers have explored new designs, such as the "low boom" concept, to minimize the impact of sonic booms and make supersonic travel more viable. These designs aim to shape the aircraft's nose and body in a way that produces a softer, less disruptive sonic signature.
However, even with advancements, it is challenging to eliminate the sonic boom entirely. The intensity of the boom depends on factors like the aircraft's size, shape, and speed, as well as atmospheric conditions.
To summarize, supersonic aircraft can be relatively quieter during subsonic flight and ground operations due to engineering advancements. However, when they exceed the speed of sound, they generate sonic booms that are loud due to the concentrated nature of the shockwaves produced.