At higher frequencies, the sound power weighting factors
derived with the cross-spectra are converging to 6 dB value. As
can be noted from Fig. 4, a similar trend was achieved experimentally in [15]. The sound pressure at the duct wall is related to the
sound pressure averaged over the cross-section. According to
Joseph et al. [26], theoretically this relationship tends to two, that
is 3 dB. In the semi-diffuse field, the acoustic energy at a point in
the duct arrives equally from all angles over a hemi-sphere. This
also leads to a power weighting of 3 dB. In that sense, the convergence to the value of 6 dB is reasonable.