39 CIRCUMSTANCES OF POTENTIALLY UNLAWFUL USE 7.7.4 Water cannon shall not target a jet of water at an individual or group of persons at short range owing to the risk of causing permanent blindness or secondary injuries if persons are propelled energetically by the water jet. Water cannon shall not be used against restrained persons or persons otherwise unable to move.114 7.8 ACOUSTIC WEAPONS AND EQUIPMENT UTILITY AND DESIGN 7.8.1 Certain acoustic warning devices are sometimes used as sonic weapons.115 CIRCUMSTANCES OF POTENTIALLY LAWFUL USE 7.8.2 In the maritime environment, acoustic weapons may be used to dissuade potential pirates from continuing an attack; in such situations, the risk of indiscriminate effects caused by acoustic weapons is substantially lower. The warning mode of an acoustic weapon may be useful during an assembly, subject to appropriate testing and avoiding the risks set out below. SPECIFIC RISKS 7.8.3 A number of serious health risks are associated with the use of acoustic weapons, particularly at close range, at loud volume and/or for excessive lengths of time. Such risks range from temporary pain, eardrum rupture and loss of balance to deafness.116 In order to mitigate risks and prevent injuries, an appropriate decibel limit and a minimum range should be set in advance Ibid. See INCLO and PHR, Lethal in Disguise: The Health Consequences of Crowd-Control Weapons, United States, 2015, pp. 70–77; and J. Altmann, “Acoustic Weapons – A Prospective Assessment”, Science & Global Security, vol. 9 (2001), pp. 165–234. 116 See for example “Acoustics – Estimation of noise-induced hearing loss”, International Organization for Standardization (ISO) 1999:2013, Standard reviewed in 2018 (available at http://bit.ly/2MlNrNi). 114 115