ANNEX A
SOUND
1. Sounds which are sometimes reported accompanying sightings are not unusual in nature (see para 3, below), but the reasons for hearing (or not hearing) an airborne object can be complex. Further, there is wide scope for mis-reporting of those sounds which are heard. (Working Paper No. 8 considers this aspect further). They range from the violent roar or even explosion, to the low hum. The more violent can clearly come from the explosion which sometimes occurs when ball lightning disappears. However, there are reports that objects which seem to emit other sounds (e.g. whistling) or may glow very brightly, can also cause an explosive (i.e. shock-wave effect) as they ‘leave the ground’ or ‘take-off’. These events are apparently sometimes accompanied by the formation of a mist. If an object changes its temperature or surface pressure, condensation can occur if the air is moist.
2. If microwave fields are also involved, the formation of mists may be as the result of water molecule absorption wavelengths forming resonances. (These occur at $\lambda = 0.017\text{m}$ and at four other values when $\lambda < 0.001\text{m}$ ). However, it is not clear how, or if, this causes the sounds which are reported.
3. The most likely reported sounds, described as normal acoustic waves, are humming, buzzing, whistling, beeping or pulsing; and they are either low or high-pitched. Observers use their own perception of sound and a wide range of descriptions are therefore used.
4. Whistling sounds are reported to be equal or louder in intensity than those of jet aircraft, even to the point of being heard inside an aircraft. Changes in pitch have been noted - not as a familiar Doppler effect, but abruptly, suggesting very high sightline (i.e. observer crossing) rates or abrupt changes of direction.
5. Animals can be 20-30% more sensitive than humans. Frequent reports suggest extreme animal reaction to unexplained sightings. Human audio upper cut-off frequency is at 20kHz. The cut off, for example, for cats, is 60kHz and the microwave density for sound to be induced in cats is about one quarter of that for humans.
6. It is well established that microwaves can be sensed, i.e. ‘heard’, as well as normal acoustics. Experiments show that this is likely to occur at RFs between 200 and 3000MHz. ( $\lambda = 1.5\text{m}$ to $\lambda = 0.10\text{m}$ ), with peak power densities of $300\text{m.W. cm}^{-2}$ . Hence PRFs of 50-100Hz can be heard as humming.
7. An entirely different audio mechanism can occur where human biological tissue absorbs energy, which, in turn, increases temperature, causes expansion and pressure - which then propagates as vibration and cause sounds to be heard by the auditory nerve.
8. Sound effects caused by microwaves are dependent on head size (and clearly vary from person to person and animal type). The microwave density levels for humans to receive sound effects is several hundred times the current microwave safety limits. Subjects receiving these extreme levels often describe them as producing very highly pitched whistles.
SUMMARY
9. In summary, there are two mechanisms which can produce sounds which humans have described when in close proximity to unexplained phenomena. For weak intensities and low prfs (or CW) the low audio acoustic range is the normal human reception mechanism. However, when microwaves are present, if the mechanism of tissue vibration is caused, then the noise heard is described as having a much higher pitch. About 6% (of over 300 humans tested experimentally) could hear’ S(E/F Band) EM emissions as ‘buzzing’.
A-1
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