A device to produce high amplitude impulsive pressure waves may be based on several different schemes. Electrical energy may be utilized to produce sound waves through loudspeakers or piezoelectric devices, but high power requirements may result in energy storage difficulties as well as problems with the large physical dimensions necessary to produce high acoustic intensities (low power densities). Mechanical devices may be used to produce repetitive loud sounds, but would be inefficient and unwieldy. Methods which convert chemical energy to acoustical energy are ideal because of the high power densities which may be achieved. Solid explosives have very high energy densities and are capable of producing extremely high peak pressure levels (i.e., blast waves from bombs), but are dangerous to work with and are not practical to use if a repetitive impulse is required. Gaseous and liquid chemicals can be easily stored, are typically quite safe when fuels and oxidizers are separately stored, and can be mixed and combusted in a very rapid manner. Although not as high in energy density as solid explosives, gaseous or liquid combustible mixtures provide reasonable energy densities which may be quickly converted to pressure or acoustical energy. Repetitive release of stored chemical energy (via an energetic chemical reaction) to produce high amplitude pressure/acoustic waves can be achieved through pulsed combustion technology. Pulse combustion includes two different modes of burning: detonation and deflagration. Detonative combustion is characterized by an extremely fast flame speed (2,000 to 4,000 m/s) and very high amplitude pressure waves, while deflagrative combustion typically exhibits a much slower flame speed (generally less than about 200 m/s) and significantly lower amplitude pressure waves.
Repetitive, high amplitude pressure or acoustic waves can be utilized as a non-lethal effects device. The detrimental effects on humans of continuous exposure to high levels of "noise" (broad band and discrete frequency) are well studied and have been known for many years. These detrimental effects are usually long term in nature and consist of symptoms such as permanent hearing loss, general fatigue, elevated stress levels, and other physiological effects. The sound pressure and corresponding sound pressure levels (SPLs) of continuous exposure with which the average person is familiar are shown in Table 1.
TABLE 1 ______________________________________ Examples of typical sound pressure levels (SPLs) and sound pressures for common environments. Sound Sound Pressure Pressure Level dB Pa(N/m.sup.2) (2 .times. 10.sup.-5 Pa ref.) Typical Environment ______________________________________ 0.000020 0 Threshold of Hearing 0.000063 10 Rustle of Leaves 0.00020 20 Broadcast Studio 0.00063 30 Bedroom at Night 0.0020 40 Library 0.0063 50 Quiet Office 0.02 60 Conversational Speech 0.063 70 Average Radio 0.1 74 Light Traffic Noise 0.2 80 Typical Factory 0.63 90 Subway Train 2.0 100 Symphony Orchestra 6.3 110 Rock Band 20. 120 Aircraft Takeoff 200 140 Threshold of pain ______________________________________
Sensations of feeling or tickle commence at approximately 130 dB (0.009 psi rms) while significant discomfort occurs at approximately 120 dB (0.003 psi rms). Thus a pressure rise as small as 0.003 psi may cause considerable discomfort.
Non-continuous tone (impulsive noises) may have different effects on an individual, especially if the impulses are unexpected. An impulsive noise is one which has a high peak pressure acting over a short duration. The form of the impulses can be high amplitude sound waves suddenly switched on which then rapidly decay in amplitude or discrete pressure pulses which may contain many frequencies.
The physiological effects of low amplitude impulsive noise consists mainly of the startle response if the peak amplitude is not excessive. At higher peak amplitudes, in addition to the startle response, temporary threshold shift (TTS) occurs. TTS is the temporary increase in the threshold of hearing (the minimum sound level which evokes an auditory response) as a result of exposure to noise. TTS generally occurs at a minimum sound pressure level of 140 dB for gunfire and 130 dB for impact noise in an enclosed space (TTS is reported to increase when exposure occurs in an enclosed space). In general the amount of TTS increases with peak sound pressure level, but as the duration of the impulse decreases below 5 milliseconds, the effect is lessened for a given peak amplitude. In addition, the amount of TTS increases approximately linearly with exposure time, resulting in an increase in TTS with the total number of repetitive pulses one is exposed to (not the total exposure time). Upon cessation of exposure to repetitive impulsive noise. the threshold shift immediately begins a rapid recovery and reaches a minimum after approximately 1 minute, but then rebounds to a maximum at approximately 2 min. This is known as the bounce effect and may be useful in attempts at incapacitation/impairment using repetitive impulsive noise.
The threshold of pain normally associated with continuous exposure (non-impulsive noise) cannot be used to predict the risk of damage due to non-continuous sounds (impulsive noise). In fact intermittent noise has been observed to be less hazardous than steady-state noise for an equivalent amount of sound energy delivered to the ear.
Eye and hand coordination are particularly affected by impulsive noise, with significant impairment lasting from a typical 2 to 3 seconds to as much as 30 seconds in some individuals.
At still higher peak pressures, the physiological effects are centered mainly on damage to the structures of the ear. Peak impulse pressures of a few pounds per square inch can rupture the eardrum with smaller pressures capable of permanently damaging the conducting mechanisms of the inner ear. The ear's greatest mechanical sensitivity lies in the 1,500 to 3,000 Hz range, and thus is particularly vulnerable to short-duration blast waves which may contain many such frequencies at significant amplitudes.
Additional non-lethal effects of high level impulsive pressure waves include the potential ability to physically move or knock down an individual at close range due to the over-pressure associated with an impulse of sufficient strength. Non-auditory damage occurs at impulse peak pressures of approximately 1 atm (14.7 psi) with little physical damage occurring for peak pressures less than 1 atm which last for very short periods of time (milliseconds).
Infrasound (sound frequencies below approximately 16 Hz) may also have a non-lethal effect on the human body. Pulse jets may cause nausea and difficulty breathing due to the large amplitude impulsive waves generated by the devices, which pulse at up to 45 times per second.