Birds react to acoustic stimuli. Several acoustic devices make use of that.
Acoustic bird control devices replay attacking calls from predator birds, mixed with frightening calls from prey birds.
Ultrasound systems use ultrasound frequences, which the human ear cannot hear. Professional systems combine ultraouns and infrasound.
The Long Range Acoustic Device directs 150 dB loud acoustic signals in a 30° wide beam up to 1,500 meters far.
Birds are very receptive to air pressure waves. A very effective and harmless way to deter birds from large areas.