But tests showed that the four devices emitted sound waves at between 6,000 hertz and 12,000Hz, while ultrasound waves are defined as being above 16,000Hz by international organisations such as the United Nations Environment Programme.

Connie Lau Yin-hing, the council's chief executive, said the manufacturers were in breach of trade descriptions laws.

The Customs and Excise Department says it is investigating the case. Trade descriptions violations carry a maximum penalty of a HK$500,000 fine and five years in jail.

Even if the devices had true ultrasound capacity, the strength of the signal would probably be diluted by walls and furniture in the home.

The United States Centres for Disease Control and Prevention and the American Mosquito Control Association say ultrasound repellents will not be effective.

Manufacturers of sonic mosquito repellents claim their products emit sound waves at between 5,000Hz and 9,000Hz, mimicking the flapping wings of male mosquitoes and keeping females - which suck blood and lay eggs - away. But the council says a male mosquito flaps its wings at a much lower frequency.

The Food and Environmental Hygiene Department says more than 70 species of mosquito have been identified in the city, but the devices can only operate in one mode at any one time, making it unlikely that they will be effective against various types.

Meanwhile, even those who keep mosquitoes away may struggle to get a good night's sleep, the council says.

The number of complaints about mattresses in the first six months of this year rose to 68, compared with 38 in the same period last year.

'People usually complain about coil damage and mould,' Lau said.

The council tested 25 double mattresses to see how well they supported the body. The lowest ranked was a polyurethane foam mattress by Nice Dream, while top-scoring mattresses were pocket spring designs, such as by Slumberland and Arthur Martin.