  We show that there are two types of acoustic monopoles, namely the pressure
monopoles (PMs) and the velocity monopoles (VMs). Under one wave incidence,
only the pressure field near a PM at resonance possesses monopolar symmetry,
while for a VM at resonance only the velocity field possesses monopolar
symmetry. It is the dipolar response function of the monopoles that dictates
whether they are PMs or VMs. A hybrid monopolar device with neither velocity
nor pressure field symmetry is also demonstrated by combining a VM and a PM,
which exhibits two monopole resonances with neither PM nor VM symmetry. The
breaking of symmetry could lead to higher absorption of the subwavelength scale
devices.
