Tupolev Tu-142 —

When a prop or rotor exceeds speed of sound, it induces "rotor tip stall" which reduces the effectiveness of the system. It also produces a 'crack-crack' sound like the snap of a whip. Most engineers are aware of this and typically try to limit the RPM to avoid the effects. Bear has counter-rotating props and I believe that has much more to due with the sound effects.

Hello mentor, Thanks for your comment, I also think the same way about the rear props, they reduced the problem and improved the efficiency.