Easy to InstallEliminates DraftsTheft-Proof Locking DeviceCRL No-Draft Speak-Thrus are designed to allow audible transmission of voice requests at ticket windows, box offices and more. They prevent tampering from the outside, and stop drafts to the interior. The 834 Series is made of aluminum. Each unit is individually boxed with gaskets and mounting screws for installation on glass from 1/4 inch (6 millimeters) to 1/2 inch (12 millimeters) thick. For thicker glass up to 3-3/4 inch (95 millimeters), you can use the optional four inch (102 millimeter) screws Cat. No. 8324. CRL No-Draft Speak-Thrus require a minimum hole diameter of 2-1/4 inch (57 millimeter), and will cover holes up to 4-1/4 inch (108 millimeter). Recommended hole diameter is 3-1/2 inch (89 millimeter).