Most of the preceding discussion has concerned audible sound. Sounds above and below the audible range also influence the living organism. To keep rats and other rodents away from flour mills, bakeries, and restaurants, use is being made of a sound-generating device called the Pied-Piper. This produces an intense ultrasonic sound said to be audible only to rodents, and acts as a repellent.
We know too little about the effect of ultrasound on humans. Even the term is used loosely, and some sounds called ultrasonic are actually audible. The piercing whistle of the "ultrasonic tooth cleaner" and sonic remote-control switches for television sets are cases in point. Public health authorities should be investigating the escalating use of ultrasonics in surgery, in medical diagnostics, and in appliances.
Military research for new weapons has led to the discovery that very low frequency sound can cause a profound disturbance inside the human body. An article in the London Sunday Times of April 16, 1967, reported that French scientists were working on infrasound as an acoustic weapon. These investigators discovered that vibrations of less than 10 cps (human hearing starts at 16-20 cps) create a pendulum reaction within the body that can be built up to intolerable intensities. The sensation of infrasound is similar to that experienced when one is exposed to the low-pitched horn of an ocean liner. During their researches, the investigators suffered internal pain from vibrations induced in the stomach, heart, and lungs. Their subjective reaction was described as a rubbing between the various organs because of a sort of resonance.
Infrasound investigators at the University of Illinois noted that since antiquity there have been reports that changes in barometric pressure and other weather factors accompanied behavioral problems ranging from suicide attempts to forgetfulness and malaise. They studied the sound waves produced by tornadoes, severe storms, winds, earthquakes, and volcanic activity.
Selecting two behavioral items-automobile accident rates and the rate of absenteeism among school children-they sought for a correlation between these two phenomena and naturally-occuring infrasonic waves. They did find a statistically significant relationship between the presence of strong infrasonic waves, generated by natural phenomena, and the behavior being studied. In the laboratory, infrasonic waves produced disturbances that might increase driver-responsible auto accidents, and generalized symptoms that would keep children home from school. Infrasonics has as yet been little explored, and these preliminary findings warrant detailed study.