Alarms are often required on various vehicles and equipment such as fork lifts, dump trucks, bulldozers, etc. An alarm is disclosed controlled by a microprocessor, which is an integral part of the basic circuit. The alarm has a flat diaphragm, and a novel acoustic lens. The acoustic lens is constructed such that sound emanating from the center of the diaphragm has a direct path to the outside of the alarm. Sound produced around the periphery of the diaphragm must pass through a path containing switchbacks, that serve as an acoustic delay line, so this sound is delayed in exiting the alarm. The result is an alarm that with full sound in the important regions such as behind and to the side rear of the vehicle, but is very quiet for the operator or driver of the vehicle. The microprocessor is relied upon for adjusting alarm output based on source voltage and varying the signal based on ambient noise, as well as specific signals for particular applications.