Abbreviation acousticians use for Time-Energy-Frequency. In their work acousticians are concerned with the propagation of sound through a space. In this work one must consider the behavior of the space at various frequencies and energy levels: put a specific amount of energy at a specific frequency into a space and what happens? How much energy comes back (at all frequencies) over what period of time (direct sound versus reflections and reverberations)? It’s more complex than this, but that’s the basic idea. In the late 1970’s Techron, a division of Crown International, made the first practical and widely accepted device for measuring TEF, called the TEF System 10. TEF machines (or TEF analyzers) provide a means to, among other things, measure energy-time curves, which is basically a fancy way of saying they can measure the acoustic energy (sound) in a space at multiple frequencies over time. These devices have been heavily used since their inception for all kinds of related applications including speaker and speaker cabinet design as well as room design or even the design of automobile interiors.