A conical flame, in the presence of high-frequency (≈1000 Hz) and high-amplitude acousticmodulation of the cold gases, deforms to a shape which is approximately hemispherical. It isshown that the acoustic level required to produce a hemispherical flame is such that theratio of acoustic velocity to laminar combustion velocity is about 3. This flame flatteningis equivalent to the phenomenon of acoustic restabilization observed for cellular flamespropagating in tubes. The transition between the conical flame and a hemispherical flame isdescribed. The surface area of the reaction zone of the flame is found to be unmodified whenthe flame flattens. The velocity field at the burner outlet is examined with and without aflame. The mean flow lines are strongly deflected when the hemispherical flame is present.We show that the presence of the flame creates an unusual situation where the oscillatingflow controls the geometry of the mean flow.