# What is the meaning of half window functions?

In fig. 4 on p. 4 of the paper Extracting the frequencies of the pinna spectral notches in measured head related impulse responses (J. Acoust. Soc. Am. 118, 2005 ), a half Hann window is used (Hann window for positive time, zero for negative time). What is its meaning?

When a signal frame is acquired, for instance triggered by a threshold, the useful data may start at time $0$. And some further data might be detrimental to the analysis of the very first instants of measurement. So it is customary to apply a window on the signal, to select the first time samples.
The half-Hann window is a very ad-hoc way to get a simple decreasing function from $1$ to $0$, keeping the left-most values untouched, and smoothly rejecting stuff above some limit (here 1.6 ms).