It is my understanding that the Viet Cong employed a similar system. They would hollow out a mound of earth in the shape of a RADAR dish. A VC would sit at the focal point of the reflected sound and they could hear helicopters and aircraft long before they normally would. Of course for a Bell helicopter the normal hearing range was ten or so miles. The listening device added several miles to that giving them the advantage of knowing that the helicopters or aircraft were coming.
