沿变幅杆侧面或端部、垂直于螺柱轴线加工的半圆形沟槽。凹槽通常位于变幅杆靠近前端的位置，用于改善端面振幅的均匀性。
A semicircular groove machined perpendicular to the stud axis along the sides or ends of a horn. Flutes are normally found toward the front of a horn and are used to improve the face amplitude uniformity.