CRL aluminum 5-5/16" no-draft speak-thru is 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 549 Series is made of brass, the 834 Series is made of aluminum. Each unit is individually boxed with gaskets and mounting screws for installation on glass from 1/4" to 1/2" thick. For glass up to 3-3/4" thick you can use the optional 4" screws (8324). No-Draft Speak-Thrus require a minimum hole diameter of 2-1/4", and will cover holes up to 4-1/4". Recommended hole diameter is 3-1/2".