The noise from a device used to deter mosquitoes from Christchurch's Papanui Library is having an effect on teens.

A mysterious high-pitched sound is being used to deter youths from fighting at a Christchurch library.

The sound, only detectable by young ears, can be heard by the doors and in the foyer of Christchurch's Papanui library.

Christchurch City Council acting head of libraries Erica Rankin said the sound was beamed from a $1350 "mosquito device".

STACY SQUIRES/FAIRFAX NZ The noise can be uncomfortable to youth if they spend any length of time nearby, the city council says.

"The mosquito device emits a noise that is undetectable by adults and proves uncomfortable to youth if they spend any length of time in close proximity. The alarm is on all the time."

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Papanui High School student Lexie Narbey, 16, said that while the sound was annoying, it had no effect on her.

STACY SQUIRES/FAIRFAX NZ Students from Papanui High School walk past the library on the way to and from school, each day.

Narbey, whose school is across the road from the library, said the library was a popular spot for students who had "literally nothing to do".

On the way to and from school, Narbey said conversations were often interrupted by the piercing sound, but some of her friends could not hear it.

Fights were a weekly affair inside and outside the library, and the sound did nothing to stop them, she said. If people talked loud enough, the sound was drowned out.

SUPPLIED The Christchurch bus interchange is an example of "youth relevant design," says youth worker Penny Prescott.

Youth from "other hoods" would look for fights outside the library, she said. Some of the fighters, who Narbey thought were in their teens and early 20s, had been banned from the library and nearby mall.

Penny Prescott, of the Canterbury Youth Workers Collective, said areas such as the New Brighton library and old bus exchange faced similar problems, but these issues were allayed through new activities to engage youth.

Prescott suggested the Papanui library hire youth workers or add activities and areas of interest to young people.

The New Brighton library hired a security guard from April to June. Prescott said their role became unnecessary when the library opted to hire a social worker.

The new bus interchange was an example of "youth friendly design", with its openness and proximity to affordable food outlets making young people happier and less likely to cause havoc, she said.