Evans Genera HD Dry drumheads feature precision-drilled dry vents around the perimeter of the head reduce overtones and control sustain. These drumheads contain two plies of film, an outer ply of 5mil film, an inner ply of 7.5mil film, and a 2mil overtone control ring on the underside that floats with the head, eliminating excessive overtones and controlling sustain. The HD Dry model incorporates Evans Level 360 Technology for ease of tuning, extended pitch range, and optimum quality of sound.