#### Mean Free Path

A rough guide to the average delay-line length is the ``mean free
path'' in the desired reverberant environment. The *mean free
path* is defined as the average distance a ray of sound travels before
it encounters an obstacle and reflects. An approximate value for the
mean free path, due to Sabine, an early pioneer of statistical room
acoustics, is

*diffuse field*assumption,

*i.e.*, that plane waves are traveling randomly in all directions [349,47] (see §3.2.1 for a simple construction). Normally, late reverberation satisfies this assumption well, away from open doors and windows, provided the room is not too ``dead''. Regarding each delay line as a mean-free-path delay, the average can be set to the mean free path by equating

*diffuse*, especially at high frequencies. In a diffuse reflection, a single incident plane wave reflects in many directions at once.

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