LARSA works by doing a reverse-deconvolution with the inverse of the sweep, to obtain a very clean impulse response. This is then windowed, depending on the setting of the ETC Window option on the Setup page. 

A 2ms Triangle window is applied before the direct sound, and a Hann window of varying size is applied starting at the direct sound. The window size is defined as below:

Type     1.5s / 0.5s sweep

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None =  none / none

S = 300ms / 150ms

M = 600ms / 300ms

L = 900ms / 450ms