Use And Effectiveness Of An Ultrasonic Deterrent
The basic principle of an ultrasonic deterrent is that it emits sound at frequencies beyond our normal range of hearing, which other creatures can hear and find very disturbing. When we hear a high frequency sound, especially a loud one, that is nevertheless within our range of hearing, it can be very irritating or uncomfortable, often uncomfortable enough to make us wish we, or the sound, were somewhere else.
The purpose behind an ultrasonic deterrent this is to make a dog, cat, bird, insect, rodent, or even a teenager, want to be somewhere else. Ultrasound is more of a deterrent to some creatures that to others. Some want to leave simply because they can't stand the sound, while others, like bats and some insects who rely on higher frequency sounds, aren't necessarily bothered by them, but can become disoriented.
Teenagers? - Teenagers, and younger children as well, can usually hear much higher frequency sounds than do adults, whose ability to hear high frequency sounds generally deteriorates with age. There are products on the market designed to be placed in areas where one wouldn't want teenagers to congregate, such as in a parking lot, or a vacant lot next to your home. Presumably the sounds the teenagers could hear, in the neighborhood of 16kHz, wouldn't bother adults, and would keep the area clear of those troublesome kids. How effective or even desirable these products are has not been well documented.
Dogs - Dogs seem to be a better target for an ultrasonic deterrent device than teenagers, or and humans for that matter, since dogs can hear much higher frequencies, and are generally bothered by them. Ultrasound devices have been used as an electronic fence to keep dogs in or out (usually more effective at keeping dogs away), and for protection against a dog attack, but are probably the most effective, or at least have a good track record, as a bark deterrent or a training device.
Cats - The tomcat that plays a visit to your yard in the early morning hours, and makes quite a bit of noise in the process, can be kept at a distance with ultrasound. Ultrasound won't hurt a cat, or a dog for that matter, but they do find it unpleasant, and will want to go somewhere else. Having said that, you don't want ultrasonic deterrents working in areas where your pets would normally be, as those devices could make life unbearable for them, so where they are placed and when and how they are activated needs to be carefully considered.
Birds - Ultrasound can keep cats away from a bird feeder, but unless highly directional, can also keep the birds away, as the high frequency disturbs birds as well. An ultrasonic deterrent can however be very effective in keeping pigeons away from statues, and starlings away from you lawn.
Rodents, Bats, and Insects- Ultrasound can keep rabbits out of your garden, and seem to be somewhat effective in driving mice out of a house, though if you have a cat or dog in the house, mousetraps may be a better approach. Bats are useful animals, and we generally don't want to bother or harm them. But if they become a nuisance, an ultrasonic deterrent will usually drive them away, though not necessarily far away. The effect of bats is usually one of disorienting them, since they rely on ultrasound to navigate and catch their prey.
As far as insects are concerned, we know that not all insects have ears, and to be affected by sound, you have to be able to hear the sound. Insects that cannot hear often feel sounds as vibrations through their feet or antennae. Some insects will be driven away by ultrasound, others thrown into a state of confusion, while still others will not show any noticeable reaction. Ultrasonic deterrents are sometimes advertised as mosquito repellents, but a number of studies have indicated the effectiveness of such products is questionable. Still, the uses of high frequency ultrasound as a repellent are many.