The police on Sunday confirmed that they had used a long-range acoustic device on an armoured vehicle during an operation to clear protesters near Polytechnic University, saying its purpose was to issue warnings.
Police said the device was a broadcasting system and not a weapon. They also dismissed suggestions that it could generate an ultra-low frequency which could lead to dizziness, nausea or an impairment to a person's sense of direction, saying this was inaccurate speculation.
Throughout the afternoon, police said the system was used to intermittently to broadcast 'alert tones' similar to sirens used by the police, and issue warnings to 'rioters'.
The force said that there are strict guidelines for its use.
In 2012, the then acting Secretary for Security, John Lee, told lawmakers at a Legco question-and-answer session that the government had bought two long-range acoustic devices in 2009.
Lee said they could convey important messages over a long range in a noisy environment and they would only be used during serious disasters, counter-terrorism operations or severe security incidents.
Lee also said the devices did not generate ultra-low frequency, but may cause hearing impairment if used improperly.