  The present study addresses the following questions: How representative of
the actual velocities in the solar atmosphere are the Doppler shifts of
spectral lines? How reliable is the velocity signal derived from narrowband
filtergrams? How well defined is the height of the measured Doppler signal? Why
do phase difference spectra always pull to 0 degrees phase lag at high
frequencies? Can we actually observe high frequency waves (P<= 70s)? What is
the atmospheric MTF of high frequency waves? How reliably can we determine the
energy flux of high frequency waves? We address these questions by comparing
observations obtained with Hinode/NFI with results from two 3D numerical
simulations (Oslo Stagger and CO5BOLD). Our results suggest that the observed
high frequency Doppler velocity signal is caused by rapid height variations of
the velocity response function in an atmosphere with strong velocity gradients
and cannot be interpreted as evidence of propagating high frequency acoustic
waves. Estimates of the energy flux of high frequency waves should be treated
with caution, in particular those that apply atmospheric MTF corrections.
