9 original experiments. Something may have been overlooked; at some points speculation is unavoidable; and some questions will remain open, hopefully to be answered by future work. The questions to be answered are the following: C What are the effects of strong, in particular low-frequency, sound on humans? C Is there a danger of permanent damage? C What would be the properties of the sound sources (above all, size, mass, power requirement)? C How, and how far, does strong sound propagate? C Can we draw conclusions on the practical use by police or military? The following subsection (1.5) gives a few general remarks on acoustics. Effects of strong sound on humans are described in section 2. Section 3 deals with production of strong sound. Protective measures and therapy are the subject of section 4. Several allegations made in journalistic articles are analyzed in section 5. Finally, section 6 presents preliminary conclusions. General properties of pressure waves in air are described in the appendix, and details of the analysis of allegations concerning acoustic-weapons effects are given. 1.5 General Remarks on Acoustics In a broad sense, any variation of air pressure in time constitutes sound. For a sinusoidal time course, the number of repetitions per time unit is called the frequency, measured in Hertz = 1/second. Usually, the frequency region below 20 Hz is called infrasound, but this is not an absolute hearing limit—sounds with lower frequencies can be heard and otherwise perceived if the pressure is high enough. To prevent misunderstanding with the term "audible," in this report the range from 20 Hz to 20 kHz will be called "audio." The hearing, pain, and damage thresholds decrease with increasing frequency between a few Hz and 20-250 Hz (see fig. 2 below); thus low-frequency effects will be much stronger at low audio frequencies than with infrasound proper. Therefore, despite the emphasis on infrasound in the journalistic articles, here the range from 1 to 250 Hz is denoted by "low frequency" and treated in common. For frequencies above 20 kHz, the usual term "ultrasound" will be used. Pressure variations mean deviations from the average air pressure toward higher and lower values, denoted by over- and underpressure. Usually these deviations are much smaller than the air pressure; they are called sound pressure. Because sound pressure and intensity vary over many orders of magnitude, and because the human loudness sensation is approximately logarithmic, these physical quantities are often given as levels L in a logarithmic scale, in decibel units, where L p = 20 log( prms / pref ) dB and L I = 10 log( I rms / I ref ) dB
(1)
prms and Irms are the respective root-mean-square values of sound pressure (deviation from static air pressure, measured in Pascal) and sound intensity (acoustic power per area, proportional to sound pressure squared, measured in Watt/square meter). A ten-fold increase in pressure means a hundred-fold increase in intensity and an increment of 20 dB in level. For the reference values, in acoustics usually p ref = 20 ÂľPa and I ref = 10 −12 W / m 2
(2)