Audio in the region of 20Hz-20Khz can create psychological disturbance in individuals at levels substantially below those required for bodily discomfort or trauma.
From the thunderous hypnotic drumming of Zulu warriors to riot police beating their batons on shields whilst marching towards confrontation, the psychological effects of sound have been used extensively throughout history as a warfare device. Noise has always been experienced as ‘destruction, disorder, dirt, pollution, and aggression’. All cultures associate noise with the idea of the weapon, blasphemy and plague. “Behold, I will bring evil upon this place, the which whosoever heareth, his ears shall tingle” (Jeremiah 19.3)”. “When the drums of the resurrection sounded, they filled the ears with fear” (Al-Din Runir, Diavani, Shansi Tabriz).
Recently, psycho-acoustic warfare was allegedly used in the Waco siege at the Davidian compound in Texas, where it is said that the FBI used sounds of babies crying, dentist drills and a variety of other unpleasant sounds to mentally influence their opponents. The Waco compound was allegedly bombarded for long durations by these sounds via large public address systems.
Although this type of sonic assault can have a profound emotive effect on individuals, it relies heavily on the individuals particular experiences. This is where the actual physiological effects of sound are unique. Physiological changes in the body only start to occur at greater sound pressure levels. At about 120 dB discomfort begins in the ear and pain occurs when levels reach approximately 140dB.The eardrum subsequently ruptures at levels of about 160 dB. Pain becomes evident when the middle ear system is mechanically displaced beyond its normal operational limits
These acoustic effects are only apparent on the ear mechanism. The ear is a very easy structure to attack. Due to evolutionary processes the ear is particularly sensitive to midrange frequencies inherent in the human voice. Subsequently, all that is needed is an increased sound intensity at these frequencies for the threshold of pain to be readily reached. This is also related to the properties of the acoustic reflex in which a small muscle in the middle ear pulls the stirrup back from the oval window and subsequently reduces the amount of acoustic energy transmitted to the middle ear. This however only has a significant impact at frequencies lower than about 1000 Hz so that frequencies between 500 to 4000 Hz, the range at which the auditory center is most sensitive, are largely unaffected.
The acoustic effects on the body are more complex. Research has concluded that with low frequency sound in the region of 50 — 100Hz at levels of 150dB or more, intolerable sensations in the chest and thoracic region can be produced–even with the ears protected.