A vehicle presence notification apparatus uses several ultrasonic transducer devices to generate a notice sound by emitting an ultrasonic wave in mid air based on an ultrasonic signal modulated according to an audible sound. The vacuum pump serves to draw liquid to the pump for priming and the float valve shut of flow to the vacuum pump when liquid reaches a predetermined level to prevent entry of liquid into the vacuum pump. The magnitude of the output value could be an indication of the delay, or there could be several output values each for a respective differently-delayed version of the second signal, in which case these could be evaluated to select which corresponds to the actual delay. The tension nut has a threaded interior surface complementary to the threaded exterior surface of the tension cylinder, and is rotatable about the tension cylinder. In other aspects, the optical engine can be implemented in a wall-mounted projection system, a multimedia system, a compact integrated monitor system, and a portable projection unit. Torsion oscillators are arranged symmetrically on both sides.