Sound can clearly be used as a deterrent. After all, if something is loud enough, it causes physical damage to the inner workings of someone’s ear. As pointed out by Salon, “guns” that fire narrow beams of deafening sound do exist, and they have been used by vessels in the US Navy and even by city authorities seeking to ward off certain protestors.
The curious thing about these incidents in Havana, though, is that the suspected sonic devices haven’t been accompanied by audible noises. They appear to be operating at a frequency beyond that of human hearing, and yet they’ve reportedly caused hearing loss and headaches in the targets.
There are two ways in which this could be achieved. The first would be to use microwaves. Imperceptible to people lacking detection technology, it has been shown that small beams directed at people’s heads rapidly heat tissue within the brain, generating a small shockwave.
This shockwave is registered as sound by the ears, which then vibrate at an extremely high frequency. All in all, hearing loss and severe headaches are inevitable. Worryingly, such a device would also cause lasting neural damage if the microwaves were energetic enough.
Infrasound – sound that is at a lower frequency than the range that’s picked up by human hearing – can also cause hearing loss in some cases. According to the US National Institutes of Health, being exposed to concentrated infrasound can cause “fatigue, apathy, and depression, pressure in the ears, loss of concentration, drowsiness, and vibration of internal organs.”
At this stage, it’s deeply unclear which is more plausible – but the point is that “silent” acoustic weapons do exist, and they certainly could have been deployed in Havana.