  Observations show the increase of high-frequency wave power near magnetic
network cores and active regions in the solar lower atmosphere. This phenomenon
can be explained by the interaction of acoustic waves with a magnetic field. We
consider small-scale, bipolar, magnetic field canopy structure near the network
cores and active regions overlying field-free cylindrical cavities of the
photosphere. Solving the plasma equations we get the analytical dispersion
relation of acoustic oscillations in the field-free cavity area. We found that
the m = 1 mode, where m is azimuthal wave number, cannot be trapped under the
canopy due to energy leakage upwards. However, higher ($m \geq 2$) harmonics
can be easily trapped leading to the observed acoustic power halos under the
canopy.
