Originally Posted by
helispotter
If there are Rotorheads that have experience in acoustic analysis, would welcome their feedback.
Just to note, while both can use frequency (Hz) as a unit of measurement, the vibration values used with helicopters (1 per, 2 per, etc.) are based on a physical oscillation felt in the structure and not what is heard acoustically as a sound. And in practice the force of those vibration oscillations are measured with specialized balance equipment in units of inches per second (IPS) and not with a sound meter or similar equipment. So the concern in the VH letter applies to the 5Hz vibration oscillation force in the tailboom structure and not any 5Hz sound heard on the video. If that makes sense.