There are many physical ways to deter pigeons from roosting on your home or business, yet noise prevention is growing in popularity, as it doesn't deface the property itself. There are three different types of sound systems that deter pigeons, like ultrasonic sound systems, sonic systems and bio-acoustic systems.
Ultrasonic Sound Systems
Ultrasonic sound systems emit a high pitched frequency (above 120 dB) that can bother or frighten animals, but can't be heard by most humans, though women and small children are occasionally an exception. These systems are usually installed around where pigeons roost or hang out. They look like small speakers and are rather easy to conceal from human observers. It is integral to make sure the areas where these ultrasonic systems are not installed in areas inhabited by bats, as this will seriously affect the bats and their hunting abilities.
Sonic Sound Systems
Sonic sound systems emit sounds that can be heard by humans and other animals, though only in a higher decibel range (below 120 dB, as above that qualifies as ultrasonic). These systems work on deterring pigeons but are often obnoxious to anyone in the area while they are in use. Sonic sound systems are best at deterring pigeons from areas where humans aren't usually around or shouldn't be around but are also necessary to keep free of pigeons. The best example of this are on runways at airports.
Bio-acoustic systems are natural alarm and distress call systems that make sounds that sound like predators, creating a hostile environment for pigeons. These are usually the sounds of predatory birds or a sound made by another bird that is in distress, keeping pigeons away from the area. However, these systems are audible to the human ear and can be distracting or obnoxious in areas where a large number of people are regularly gathered.