Jacopo Prisco , CNN Written by

If you were to walk at night along the waterfront in West Palm Beach, Florida, you might hear something strange: A playlist of annoyingly catchy children's songs -- including "Baby Shark" and "Raining Tacos" -- blared on loop all night to deter homeless people from sleeping near an event center.

Kathleen Walter, a spokeswoman for the city, said in a statement that the music is played overnight to discourage "congregating at the building" and to "encourage people to seek safer, more appropriate shelter."

When sound becomes hostile

Hostile architecture -- such as slanted or segmented benches, uneven pavements or metal spikes -- has traditionally been employed by municipalities around the world to deter rough sleeping and anti-social behavior, leading to discussions about whether these tactics are discriminatory, unethical or even effective. Although music has long been used to alter or affect public behavior, its use as a deterrent in urban design appears to be more recent.

































1 / 17 – London, United Kingdom Hostile architecture is a form of urban design that aims to prevent people from lingering in public spaces. The anti-homeless spikes here, for example, were installed to deter beggars and those sleeping rough. Credit: amer ghazzal/Alamy Live News

their parking lots. In 2005, the London Underground dozens of stations after a trial revealed that instances of physical and verbal abuse decreased by 33% when the music was played. Classical music is a popular choice when it comes to discouraging antisocial behavior. A group of 7-Eleven stores in Canada is credited with being the first to use it to that effect in 1985, when they played Mozart and Beethoven to disperse crowds of teenagers intheirparking lots. In 2005, the London Underground started piping classical music intodozens of stations after a trial revealed that instances of physical and verbal abuse decreased by 33% when the music was played.

It is easy to see how music can affect our mood, though it is trickier to ascertain why classical music might have these specific effects. In general, people find it soothing -- and that is linked to the production of dopamine, a feel-good neurotransmitter that resides in the brain's so-called pleasure centers. Teenagers, on the other hand, tend to dislike classical music, so it follows that it would have the opposite effect. The power of music to affect our perception of a public space is the reason why there's a big business around muzak, the kind of background music commonly heard in malls, airports and hotels.

And the British Navy has weaponized music by using Britney Spears hits to repel Other musical genres have been employed to similar effect. Rite-Aid stores in California, for example, have recently blasted Barry Manilow songs outside their premises to deter loiterers.And the British Navy has weaponized music by using Britney Spears hits to repel pirate ships along the east coast of Africa.

In 2018, Germany's national train operator Deutsche Bahn announced plans to use atonal music in the city's railway stations to drive away drug users and the homeless. Atonal music is an experimental type of music created in the early 20th century that doesn't rely on conventional harmonies and rhythms. It can be unsettling and discomforting to some listeners. The plan was abandoned after complaints from passengers and musicians, who organized a concert to promote the virtues of contemporary music and protest against its use as a deterrent.

The Mosquito

A device called the Mosquito takes sound deterrents one step further. It emits a high-pitched noise similar to the buzz of a mosquito, but because of its very high frequency -- 17.4 kHz -- it is inaudible for most people over 25.

"It's a very simple idea," said Tim Leighton, a professor of ultrasonics at University of Southampton, in a phone interview. "People lose sensitivity to high frequencies as they get older. So a frequency was chosen for the device that young people would hear as a high-pitched whine and old people would not hear at all," he said.

"A proportion of young people don't like it and leave the area. I wouldn't say it sounds like nails on a blackboard, but that's the kind of effect it would have on you."

The company that launched the Mosquito in 2005, Compound Security, says that around 40,000 have been sold to date, with the US, the UK and Sweden as the primary markets. Sales have seen an uptick in recent years, as more shops and businesses use the devices to deter teenagers from loitering around their premises.

The Mosquito device. Credit: Compound Security

Some municipalities in the US use the Mosquito in parks and other public spaces to keep people away at night. But because it only targets young people, including innocent bystanders, the device has been branded discriminatory by critics.

In 2008, the UN Committee on the Rights of the Child asked the United Kingdom to reconsider its use, as it may violate children's right to free movement and peaceful assembly. In 2010, the UK government rejected calls from the Council of Europe to ban the device, because there was "little likelihood" it would cause long-term damage.

Simon Morris, Director of Compound Security, maintains that the Mosquito is legal and harmless. "We're all parents as well," he said in a phone interview. "The Mosquito was born out of our children suffering because they were attacked or assaulted at local shops by older kids. We've never had the intention of curbing anybody's rights. People have the freedom to walk away from the sound and the sound is highly targeted into a very small area," he said.

Morris added that the Mosquito is sold at an artificially high price -- £600 (about $700) -- to deter unscrupulous buyers. "We priced it at a point where individuals and businesses really need to think about it before they buy. We took a conscious decision not to have it made in China for £5.50 and then sell it for £99.99, because every idiot in the country would get one every time they were annoyed by the next door neighbor's kids. It's something designed to be used when there's a problem, not 24 hours a day, nor to create a no-go area for kids."

Can sound attack?

Beyond the frequency of the Mosquito, and above the limits of human hearing, lies ultrasound, which can have potentially harmful effects on humans.

"In the 1940s people started complaining about ultrasonic sickness, with symptoms like headache, excessive fatigue (and) irritability," said Leighton, from the University of Southampton. "The first legal case was in 1948."

Sound waves with a frequency higher than 20 kHz are considered ultrasound, although young people can hear frequencies up to 28 kHz. "A high frequency above 20 kilohertz, if you can hear it, will affect you for sure," Leighton said. "It can affect your concentration, your ability to do tasks, give you headaches (and) make you uncomfortable.

"We haven't been allowed, under our ethical guidelines, to look for stronger effects. It's almost likely that stronger signals will lead to a temporary, perhaps even permanent loss of hearing. But we can't test for that because it would be unethical for us to expose people to that."

Ultrasound has long been rumored to have been behind illnesses reported by personnel at the US Embassy in Havana. Credit: ADALBERTO ROQUE/AFP/AFP/Getty Images

Ultrasound has long been rumored to have been behind the sickness reported by personnel at the US Embassy in Havana, Cuba, starting in late 2016, and by a US government employee stationed in the city of Guangzhou in southern China, in 2018. But Leighton doesn't believe ultrasound is the culprit.

"The evidence does not warrant the hype. Physically, it's certainly very hard to use at range, because ultrasound is absorbed by air and is blocked by walls," he said. "Also, the people who are most resistant to it would be older men. So the idea of using it to affect embassy staff, which do tend to be male and older, makes it a foolish choice of weapon."

The cause of the sickness remains unknown, with micr owaves and even crickets brought up as possible explanations.

Rumors of a "sonic attack" persisted after a brain study performed on the Havana diplomats in 2018 revealed results similar to symptoms of a "mild traumatic brain injury following blast exposure or blunt trauma."

Whatever the cause, "Baby Shark" suddenly doesn't sound too bad.