As everybody knows, helicopters need a tail rotor to offset the torque of the main rotor, otherwise the body would spin beneath the blades (which would look hilarious) instead of vice-versa. Tail rotors are a major source of a helicopter's distinctive- read loud- aural signature, and research into silent helicopters often involves eliminating the rotor as a matter of course.
One method of doing this is to go coaxial, with one rotor atop another, contra-rotating. My question is, could the upper & lower blades effectively be tuned to cancel each other out acoustically? Like the Infinitis with speakers that fire counter waves in response to intrusive sounds from outside?