An ultrasonic communication system includes an omnidirectional ultrasonic transducer including a housing having a closed side and an open side defining an aperture in the housing. A diaphragm is mounted into the housing to form a sealed cavity between the diaphragm and the closed side of the housing. A second side of the diaphragm is free to transmit ultrasonic signals through the aperture. The diaphragm and aperture both have a largest dimension that is less than or equal to one-quarter wavelength of an ultrasonic signal to be transmitted by the transducer, such that the transducer behaves as a monopole and radiates substantially omnidirectionally.