Guest post: Bill Boothby on “Are sonic weapons legal?”

You may have seen reports about the alleged use of a sonic weapon by Serbian authorities on demonstrators in Belgrade.  I asked Lawfire^® contributor Bill Boothby to give a use a short primer on the legality of such weapons, and he kindly obliged.  Bill’s certainly the right expert to do this as he has, literally, ‘written the book’ on weapons’ law (see here).

I’ll have a few additional comments after Bill’s essay.

Are sonic weapons lawful?

Bill Boothby

During November 2024 a concrete canopy at a railway station in the north of Serbia collapsed killing 15 people. This and concerns relating to corruption led to protests, and hundreds of thousands of protesters were on the streets in central Belgrade on Saturday, 15 March 2025.

Reports of what happened are mixed, but during a 15-minute period of silence being observed as part of the protest, large numbers of protesters were seen to be dispersing frantically.

President Aleksandar Vučić has reportedly denied that a sonic weapon was being used. Witnesses variously report a strange, unnatural noise resembling a jet engine, a low howling sound and a rush of wind, all resulting in a stampede.

Reports indicate that the intensity of the sound varied depending on where the affected individual was located, and another witness described the sound as like the roar of a Formula One race, or like an aeroplane flying low overhead. ‘It was a horrible sound. It felt like something was going to crash on to our heads.’

What are sonic weapons?

Also known as the Sound Cannon, the Long Range Acoustic Device’s (LRAD) reported functions are to broadcast sound messages over long distances and to emit pain-inducing deterrent tones.

Different versions produce different degrees of sound. The sound is emitted in a beam spanning up to 30 degrees. Reports indicate that the military grade LRAD2000X ‘can transmit voice commands at up to 162dB up to 5.5 miles away.’ The devices can be used as bird scarers at airfields and, in the public order context, was apparently first used in Pittsburgh during G20 protests in 2009.

It would appear that LRAD’s potential effects include discomfort, permanent hearing loss at or above 130dB and loss of balance coupled with inability to move out of the path of the sound at or above 140 dB.

The author concludes that much will depend, inter alia, on the power rating of the device, on any setting that is applied to the device by the operator, on the direction in which the device is being pointed and on the distance from the device to targeted individuals or groups.

Physicians for Human Rights have expressed ‘significant concerns about the high potential of acoustic weapons to cause serious and permanent injury’ and, unsurprisingly, drew attention to human rights law implications.

The Law

We should start by assessing the lawfulness of such systems when used in connection with an armed conflict. It will be assumed that the device is being used as a weapon, not to transmit voice instructions over long distances. The criteria that are usually considered when determining whether a new weapon, in the intended circumstances and manner of its use, is lawful are the following:

• • Is the weapon of a nature to cause superfluous injury or unnecessary suffering, i.e. injury or suffering for which there is no corresponding military purpose?
  • Is the weapon indiscriminate by nature, i.e., will it strike military objectives (lawful targets) and civilians and civilian objects (protected persons and objects) without distinguishing between them?
  • Does the weapon have an impermissible effect on the natural environment?
  • Are there legal provisions that specifically prohibit the weapon, or that restrict the lawful circumstances in which it can be used?

It is evident from the explanations given earlier in this piece that the weapon is designed to emit a beam spanning up to 30 degrees. This suggests that the weapon is capable of being directed at specific locations, so it cannot be described as inherently indiscriminate.

It seems unlikely that the weapon has any, or a sufficient, effect on the environment, so that criterion is also unlikely to be breached. There is no legal provision in the law of armed conflict dealing specifically with sonic weapons.

Superfluous injury or unnecessary suffering?

The remaining issue, therefore, is whether the injury or suffering caused are, respectively, superfluous or unnecessary. Sonic weapons as such are most unlikely to breach this principle, because it will be possible to select a range from the target at which permanent injury, and perhaps any injury, can be avoided.

If the weapon is equipped with power settings that result in differing volumes of sound being emitted depending on the setting that is chosen, this will make it even less likely that the very nature of the weapon would breach the superfluous injury/unnecessary suffering principle.

While a legal review of a less lethal weapon such as this should apply the same criteria as are employed when considering lethal weaponry, a less lethal weapon when used in its designed, intended manner will generally expose targeted persons to a lower risk of death or permanent harm than is to be expected, say, from live high velocity rounds.

However, any evaluation of the superfluous injury/unnecessary suffering aspect of any weapon must take into account the intended circumstances in which the weapon would be used and the specific purposes it is intended to fulfill.

Consideration would also need to be given to any alternative methods of achieving those purposes. If, for example, an alternative weapon such as tear gas or a malodorant would achieve the desired purpose(s) without causing the injuries that are being attributed to the sonic weapon, the superfluous injury/unnecessary suffering principle might well imply that the less injurious option should be chosen.

It will be appreciated from the discussion in this paragraph that it is not sensible to say simply that sonic weapons in general are either lawful or unlawful.

What is required is a careful assessment of the particular weapon and of the way and situations in which it is designed to be used, information as to the medical effects as identified during controlled tests, and a comparison of the resulting data with that associated with the weapons currently used to fulfill the relevant tasks. Only then can one reach an informed conclusion.

Law enforcement use

If the sonic weapon is intended for law enforcement use, it will be the domestic law of the relevant country that will determine the acceptability of the weapon.

In the United States that means that the US law dealing with the control of demonstrations, protests, public assemblies and riots will apply.

While readers are likely to be more familiar with relevant US legislation than I am, I do note that the First Amendment of the US Constitution provides as follows: ‘Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.’ Care is therefore required when a law enforcement agency is policing a peaceable assembly.

It would seem to the author that if, for example, an assembly of people was in the process of holding a 15 minute silence as a mark of protest, the use during such a period of peaceable silence of a sonic weapon to cause panic among, and thus to disperse, the crowd would likely to be hard to reconcile with that First Amendment right. If, by contrast, a riot is underway, the domestic US law will determine the nature and extent of the force that may lawfully be used by police agencies.

In the UK, the law as to the use of force by law enforcement agencies centres on Section 3 of the Criminal Law Act 1967 which states: ‘A person may use such force as is reasonable in the circumstances in the prevention of crime, or in effecting or assisting in the lawful arrest of offenders or suspected offenders or of persons unlawfully at large.’

The European Convention on Human Rights provides for a right to life under Article 2 and a right to ‘freedom of peaceful assembly’ under Article 11(1).  The right to peaceful assembly may only be restricted in accordance with law and to the extent necessary in a democratic society in the interests of national security or public safety, for the prevention of disorder or crime or for the protection of the rights and freedoms of others.

Clearly, the proper training of users of such devices will be of greatest importance, but the details of that aspect lie outside the scope of this piece.

About the Author:

Air Commodore Bill Boothby retired as Deputy Director of Royal Air Force Legal Services in July 2011. He took a Doctorate at the Europa Universität Viadrina, Frankfurt (Oder) in Germany and has published books, inter alia, on Weapons Law, Targeting Law, the law relating to conflict and on new and emerging technologies.

More recently with Prof Wolff Heintschel von Heinegg, he published a Commentary on the US DoD Law of War Manual and ‘Nuclear Weapons Law’, both with CUP. His book on AI Warfare and the Law was published by USNWC in January 2024 and a revised book on Nuclear Weapons Law will be published in May 2025.

He has been a member of Groups of Experts that addressed Direct Participation in Hostilities and that produced the HPCR Manual of the Law of Air and Missile Warfare, the 2013 Tallinn Manual on the Law of Cyber Warfare and the Leuven Manual on Peace Operations Law.  He is Visiting Professor at the University of Johannesburg and an Associate Fellow at the Geneva Centre for Security Policy.

The views expressed by guest authors do not necessarily reflect my views or those of the Center on Law, Ethics and National Security, or Duke University.  See also here.

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A few additional observations: The rise of acoustic aggression?

We may see these weapons used by militaries more frequently.  In an article written just last year, “International Law and Acoustic Antagonism in East Asian Waters,” authors Matt D. Montazzoli  and John C. Tramazzo, discussed a 2023 incident where a Chinese warship allegedly used a sonar device in away that injured an Australian diver.

Montazzoli and Terrazzo noted that a number of militaries have sonic (or acoustic) weapons.  They said:

“[S]ince the attack on the USS Cole in Yemen in 2000, U.S. Navy ships have relied on the long-range acoustic device (LRAD), a massively powerful loudspeaker, to warn and deter encroaching ships.  American ground forces used the LRAD to disperse crowds in Iraq.”

They also wrote about the “offensive use” of acoustic devices, observing:

[T]hey are evidently appealing tools for coercion and intimidation during strategic competition in the so-called “gray zone” between armed conflict and peace.  Acoustic devices can be employed as non-lethal weapons, “intended to have relatively reversible effects and minimize risk of fatalities, permanent injuries,or permanent damage to materiel.” As Professor David Fidler surmised,

The development and use of “non-lethal” weapons may also give rise to new kinds of low-intensity conflict between States and within States. “Non-lethal” weapons may be attractive tools for the conduct diplomacy [sic] by other means in connection with, for example, border disputes or disputes over the control of economic or natural resources.

Montazzoli and Terrazzo did, however, conclude that the use of a sonic weapon in the Chinese warship (the Ningbo) case amounted to a “use of force” as that term is used in Article 2 (4) of the UN Charter.  They conclude:

As States jockey for advantage in the congested and disputed global commons while the rules-based international order is subject to increasing challenges, the Ningbo–Toowoomba incident will not be the last acoustic use of force. On balance, the PLAN activities represent a violation of due regard, an unlawful interference with Australia’s freedom of navigation, and an unlawful use of force. The narrative malleability of incidents at sea and the legal ambiguity surrounding whether acoustic aggression animates a State’s right to self-defense means we have almost certainly not heard the last of this issue.

I agree!

Remember what we like to say on Lawfire^®: gather the facts, examine the law, evaluate the arguments – and then decide for yourself!