The elderly in particular use a cat-deterrent in their garden. It is a small apparatus with a speaker attached to it. It produces a high-pitched bleep of more than a 100 decibel. The sound is mostly only audible for children, youngsters and people with ears that are still good. Many cat-deterrents use the same noise as the Mosquito. The sound causes pain in the ears, headache, and ringing in the ears. If you have any complaints consult your GP and make mention of it on social media! Working Mosquito: 17KHz-20KHz 90dB Working Cat Deterrent/Animal Chaser: 16KHz-23KHz 130dB  call us +31649137777

More info?

http://www.icben.org/2014/papers/Team4/4_16%20MariUeda_1.pdf
http://www.mosquito.pieptonen.nl

It seems that there is a very real medical phenomenon known as presbycusis or age related hearing loss which, according to The Merck Manual of Diagnosis and Therapy, "begins after the age of 20 but is usually significant only in persons over 65". It first affects the highest frequencies (18 to 20 kHz) notably in those who have turned 20 years of age". It is possible to generate a high frequency sound that is audible only to teenagers.