  We propose and experimentally achieve a directional dipole field radiated by
an omnidirectional monopole source enclosed in a subwavelength structure of
acoustically hybrid resonances. The whole structure has its every dimension at
an order smaller than the sound wavelength. The significance is that the
radiation efficiency is up to 2.3 of the radiation by traditional dipole
consisting of two out-of-phase monopoles in the same space. This study
eventually takes an essential step towards solving the long-existing barrier of
inefficient radiation of directional sound waves in low frequencies, and
inspires the ultimate radiation of arbitrary multipoles or even a highly
directional beam by a monopole in limited spaces.
