2. High-speed hum

In the previous paragraph, it was shown that low-speed hum is a torsional vibration phenomenon caused by the engine's pulsating torque. When the engine is running at low rpm, torsional excitations predominate over translational excitations. As engine speed increases, particularly above 2000 rpm, the torsional excitations of the engine are attenuated when they reach the wheels, due to the dynamic behavior of the drive train. As a result, the torsional hum phenomenon becomes secondary at medium engine speeds, and can even be considered negligible at high engine speeds. In addition, as engine speed increases, the inertia forces generated by the movement of the moving linkage become increasingly important and constitute the bulk of engine excitations. These excitatory forces act mainly in the vertical direction of the engine, which is why they are known as the "engine ram". These vibrations...