Acoustical repellents rely on sound to scare birds away. Birds have a hearing range similar to humans, so if people can hear it, birds can hear it. Unlike rodents, birds cannot hear ultrasonic sounds.

  Propane-Fired Cannons (Bird-Bangers)

The most familiar acoustical bird repellent equipment is the propane-fired cannon. These units cause birds to flee by producing loud, unexpected blasts. Also known as bird-bangers, these are available in a wide range of configurations, from mechanical single-shot units, to fully electronic, randomized, rotating multi-shot units. These latter units are the most effective over the long term, since the timing and direction of the blast is truly random and the birds do not know when or where the next blast will occur. Although the loudness of the blast is important, it is the unexpected nature of the blast that keeps birds nervous. Units should never blast at intervals of less than 3 minutes. Birds quickly get accustomed to units that are stationary, shoot at regular intervals, or fire very rapidly. More blasts are not better.
Unfortunately, neighbours never get used to propane-fired cannons because of the loud sounds produced (measurements taken beside the units have shown up to 115 dBA), the frequency of the blasts (up to 20 times/hour), and the time of day they must operate (starting at dawn when people are still sleeping). Many people work shifts and must sleep during the day, so propane-fired cannons are not very popular with them.
The newer fully electronic propane-fired cannons are equipped with automatic electronic timers which allow growers to program the operating hours of the unit. These timers, unlike older light-activated photocell timers, provide the flexibility of allowing the unit to be turned on and off during the day when birds may not be feeding as heavily.

Guidelines on the use of propane-fired cannons are:
· Set at intervals greater than 3 minutes.
· Use between sunrise and sunset when birds feed.
· Operate no more than one unit per 2 ha (5 acres), unless it is absolutely necessary.
· Avoid operating near neighbours' houses.
· Ensure that propane tank valves do not leak, as this can cause units to blast unintentionally, even when they are shut off.
· Move the units around to keep the birds off-guard.
· Use electronic clock timers that automatically shut off the units. ；

  Electronic Sound Devices

Electronic sound devices are considered less irritating to neighbours than propane-fired cannons. There are two types currently used that broadcast electronic synthetic sounds to repel birds:

· random noises, irritating to birds (traditional)
· reproduced distress calls that mimic individual bird species.

The traditional electronic sound device is often referred to as the AV Alarm?, a trade name that has been familiar in the industry for 30 years. Their electronic, warbling sound interferes with the bird's sensory system, producing an environment that makes birds uncomfortable and insecure. In some cases, the birds are unable to communicate with each other freely and will leave the area. These units work well when used in a combination with propane-fired cannons. The electronic sound device keeps the birds irritated and edgy, while the cannon provides the stimulus to drive them away.
A new generation of electronic sound devices uses digital technology to produce distress calls of specific birds. They are only effective against the bird species whose distress calls are encoded on the microchip. However, some residual repellent effect may be noticed on species that normally travel together with the target birds. Some farmers report that digital units also attract birds of prey, such as hawks, who misinterpret the electronic distress calls for the real thing. Hawks circling the farm will also scare away birds. Even though these units produce electronic bird distress calls, they sound to us like real birds, and are usually not as objectionable to neighbours. These devices also produce a lower decibel sound level than the more traditional units.
Both types of electronic sound devices can be used to discourage birds from nesting in nearby trees in the spring.

  Whistling and/or Pyrotechnic Pistol Cartridges

These are cartridges that are launched from a hand-held pistol and produce a loud, whistling sound throughout their flight. There is also a delayed reaction banger cartridge that travels through the air in silence, then explodes high in the air near flocks of birds as they are flying with the same sound level as a propane-fired cannon. Shooting these ear-piercing sounds into a flock of birds works well as a repellent. These units can quickly clear a field, woodlot or hydro line before the flock lands on the crop. Airports often use this type of equipment, but they are especially popular with farmers, and may be the most effective manual scaring device available.

  Shotguns

Shotguns are often used to repel birds, but they are not as effective as launching the sound directly into the flock like the pyrotechnic units. Although shotguns might provide a frustrated grower with a form of stress relief, even an excellent marksman will never seriously reduce the bird population. Shotguns should only be used by trained, responsible people to ensure no one is hurt. Firearm Acquisition Certificates are required for shotguns, but not for pyrotechnic pistol cartridges.

  Other Sound-Producing Devices

Many other sound producing devices or strategies have had only limited success. These can include air horns, clanging aluminum pie plates, shiny flashing metal objects, firecrackers and Mylar humming lines. These devices generally work only for a few days before the birds start to ignore them again. They might be most effectively used in the days just before harvest when bird pressure is the greatest.