Sabine defined total room absorption as equivalent open-window area.

Evidence

In the Sabine equation, this is easily done by multiplying the absorption coefficient of the material by its total area and then adding up the contributions from all the surfaces in the room. These resulted in a figure that Sabine called the “equivalent open window area,” as he assumed, and experimentally verified, that the absorption coefficient of an open window was equal to 1. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1890

Categorization

• DEFINE: Introduces the ‘equivalent open window area’ — the conceptual unit of total absorption.
• XREF: An open window as the reference absorber is an intuitive anchor for understanding a dB or a metre.
• CANDY: Sound leaving through an open window is the calibration standard for measuring absorption — genuinely tweetable.