This work reports on the use of bandgaps to increase the efficiency of sound radiation employing defect modes on a phononic crystal (PnC). A PnC consisting of a 2D array of studs on an aluminum plate is considered, and a defect is created by removing four studs. Numerical simulations predict 8 dB higher radiation efficiency and significantly more uniform directivity of sound due to the piston-like defect modes that suppress interference between acoustic waves. An experimental study of the vibrational response is carried out in order to validate the numerical result. Comparisons of the radiation efficiency and the directivity index between the numerical and experimental results show good agreement. These findings may pave the way to use bandgap structures as effective acoustic radiators.