The sound produced by a spinning helicopter rotor is not identical with each rotation, even if all other factors remain constant. The sound signature of a rotating rotor can be influenced by a variety of factors, including interference patterns and fluctuations in the surrounding air.
As the rotor spins, each blade passes through the air and creates a pressure disturbance that propagates as a sound wave. However, due to the complex aerodynamics involved, the interaction of the rotor blades with the surrounding air is not perfectly predictable or uniform.
Several factors can contribute to variations in the sound produced. For example:
Blade passage effects: As each rotor blade passes through the same region of space repeatedly, the exact timing, angle, and velocity of the blades can lead to variations in the pressure disturbances they create. This can result in slight differences in the sound signature from one rotation to the next.
Blade-vortex interactions: Helicopter rotor blades can generate vortex structures in the wake of each blade. These vortices can interact with subsequent blades and cause additional variations in the sound produced.
Turbulence and air disturbances: The surrounding air is rarely completely still and can contain turbulent eddies, wind gusts, or other air disturbances. These fluctuations in the air can interact with the rotor blades and introduce further variations in the sound heard by an observer.
Doppler effect: The relative motion between the helicopter and the observer can cause a Doppler shift in the perceived frequency of the sound. As the helicopter approaches or moves away from the observer, the frequency of the sound waves will be shifted higher or lower, respectively.
Overall, the sound produced by a helicopter rotor is subject to various sources of interference and fluctuations, which can result in a more complex and variable sound signature compared to a simple periodic waveform.