With the help of tools like hot-wire anemometers (measure turbulence), pressure sensors, and far-field microphones to capture both the way air moved through the engine and the noise it produced. What they discovered was that the irritating sound wasn’t just about volume. It had to do with how the sound energy was spread out.
They identified two types of noise patterns. First, called duct haystacking, happened during cruising, when the engine was at low thrust. Here, the boundary layer air flows gently into the engine and interacts mostly with the duct, the tube around the fan. This created a fuzzy sound.
The second noise, called fan haystacking, occurred during take-off, when the engine worked at high thrust. Powerful suction pulled in rougher, more chaotic air, which slammed into a large portion of the spinning fan blades. The result was even more scattered and harsh noise.