Almost all of the sonic gadgets you see for mosquitoes, birds, rats, mice, and any other vermin is a bunch of junk. Do not waste your money. No scientific evidence exists that such high frequency noises do anything at all. Some noises are beyond the scope of hearing by mammals, reptiles, and insects. Other noises are only mild irritants that are received like any other background noises. If you think they work, then think again. It is probably a change in season, or temperature, or humidity, or the pests are already all dead anyway.  
In summary, stay away from them. The USDA does not recommend them. Neither does the Armed Forces Pest Management Board. No pest controllers with an ounce of education will use them. They are worthless.

The other category of mosquito control devices are the sonic and ultrasonic repellers. Sonic repellers were first marketed in the 1970s. They purported to imitate the flight sound of a male mosquito and worked on the assumption that a mated female mosquito would avoid further contact with males. Other devices were supposed imitate the flight sounds of dragonflies. Although testimonials as to their effectiveness abound, most of these devices have been tested in the laboratory or field and have shown no repellency.
 
More recently ultrasonic repellers have been developed, which purport to mimic the sounds made by bats. These also have been tested in controlled experiments and found ineffective in reducing mosquito biting rates (for example, Table 3, Experiment 1).

The following action will reduce your chances of being bitten by mosquitoes.
 
? Wear light-colored clothing, long sleeved shirts or jackets, and long slacks.
? Use mosquito netting when sleeping outdoors or in an unscreened structure. Protect small children when outdoors.
? Avoid physical exertion, and use colognes and perfumes sparingly as these may attract mosquitoes.  
? Consider the use of mosquito repellant, according to directions, when it is necessary to be outdoors. Chemical repellents are available in aerosol sprays, sticks, lotions, towelettes, and pills. Pills have questionable effect. Sprays and lotions are the commonly available formulations.
? In outdoor areas, aerosol bombs, smoke pots, and citronella candles all have limited use. Mosquito ?Wands? usually contain ?moth balls?, e.g. ?Skeeter Beater?. Electrocution devices attract mosquitoes into the yard. If used, place at the farthest distance from your area of person use. Sonic repellers do not work.
? In indoor areas, citronella candles and mosquito coils (which are actually a pesticide) can be used.
? Put 16 mesh screens on all doors and windows and keep them in good repair.

Products That Do Not Protect You From Mosquitoes:    
Backyard electrocutors, sonic repellers, scented wrist bands, citronella candles, scented geraniums (Citrosa plant), Avon Skin-So-Soft, garlic oil (eaten or sprayed on vegetation).  

Do they really work? Perhaps not. There have been many researchers at many universities who, in the past 20 years, have put many variations of "ultrasonic frequency transmitters" and "electrical wave generators" to unbiased and scientific testing, and their results have not exactly supported the claims of the manufacturers of the magic boxes.  
 
One of those active in this area is Dr. Roger Gold, professor of entomology at Texas A&M University. Some of his comments from reports published in trade journals or personal comments made at educational seminars include these:  
 

• "Based on the research results it appears that sonic and ultrasonic sound is ineffective to control or repel German Cockroaches."
• "There continues to be a lack of confirmed studies to scientifically document the claims of efficacy against insect pests". This comment was in 1993, when the Office of the Attorney General in Texas had just concluded imposing a permanent injunction against the manufacturer of some "electronic vibrating devices", prohibiting future sales in Texas, due to inaccurate claims by the manufacturer.
• "Test protocols and regulations that will protect the public from unscrupulous manufacturers and marketers of these types of devices must be developed."
• "These devices are an outright fraud perpetrated by unscrupulous people."
  

Steve: What about these gadgets that you can buy that emit a high-pitched noise? Do they work at all?  
 
Gord: Absolutely no evidence that sonic repellers work. Too bad.