Biomed. Eng. Prog., Virginia Commonwealth Univ., Richmond, VA 23298-0168
Raven, Inc., Alexandria, VA 22312
Wright-Patterson AFB, OH 45433
Bird strikes have become an increasing problem at commercial and military airports. With the demand for reduced aircraft noise and increased speed, birds are presented with reduced auditory cues and less reaction time when confronted with planes. The major drawback to the use of acoustic deterrents is the rapid habituation. Caged sparrowhawks readily detected signals between 5 and 40 Hz with thresholds of detectability ranging from 76 to 64 dB SPL. No flying induced startles were noted, although the birds did try to localize the source. Habituation to low-frequency stimuli occurred with as little as three presentations. Free-ranging geese were presented with the same frequencies as substrate vibration along with sound pressure stimulation. In a limited sample (three geese) avoidance was noted. The application of low-frequency sound as substrate vibration on runways is promising. Alternatively, microwaves produce an auditory stimulus to the ear that can be altered by changes in frequency, pulse width, and overall power. Pulsed microwaves or microwaves modulated by low-frequency sound are potential sources alerting birds from a flying plane already carrying navigation radar.