A cat repeller is a device or substance used to discourage cat

Cat

The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...

s from entering an area, or to encourage them to leave if they do enter. Such deterrents are most commonly used by gardeners, in order to prevent damage to their gardens, to avoid cat feces

Feces

Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...

, or to protect bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

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Ultrasonic devices

Many retailers sell devices which exploit the discomforting effects of in-air ultrasound

Ultrasound

Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

. These devices are usually combined with a motion sensor which is triggered by movement within the sensors range. This causes the device to emit high frequency

High frequency

High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...

 noise which is uncomfortable to the cats, and inaudible to most humans (although they can still experience unpleasant subjective effects and, potentially, shifts in the hearing threshold). The devices are available in both battery and mains operated forms, the latter generally having a higher output, greater range and requiring less attention.

Some cats are immune to ultrasonic cat deterrents, mainly the ones which are hard of hearing. There are also reports that the devices take a while to become effective, as some cats will stand their ground in a futile attempt to make the deterrent go away. Moving the device to different locations regularly and combining with another form of cat repellent may make these devices more effective.

The Royal Society for the Protection of Birds

Royal Society for the Protection of Birds

Bird Notes and News was first published in April 1903.The title changed to 'Bird Notes' in 1947. In the 1950s, there were four copies per year . Each volume covered two years, spread over three calendar years...

 (RSPB), has endorsed a commercial product called "CatWATCH", for which it receives 2% of the wholesale price of every device sold by the manufacturer. The RSPB tested the original CatWATCH device in a study using 63 and 96 volunteer observers in two long-running (18 and 33 weeks) blind experiments . Results from the study indicated that the device did have a moderate deterrent effect, reducing the probability of a previous cat intrusion into a garden by approximately 32% in the first experiment, but not in the longer running second experiment.

Scatter guns are another form of ultrasonic

Ultrasound

Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

 device. These laser-aiming devices can be targeted at cats and activated by a trigger. They will send out an ultrasonic noise directed where aimed.

Professor Tim Leighton from the Institute of Sound and Vibration Research has expressed concern about the recent growth in commercial products which exploit the discomforting effects of in-air ultrasound. Leighton claims that commercial products are often advertised with cited levels which cannot be critically accepted due to lack of accepted measurement standards for ultrasound in air, and little understanding of the mechanism by which they may represent a hazard.

Electric fences

A variety of commercially produced electric fences are also available, with voltages low enough to deter but not cause harm to cats. Care must be taken with the strength of electric current used; one Cumbria

Cumbria

Cumbria , is a non-metropolitan county in North West England. The county and Cumbria County Council, its local authority, came into existence in 1974 after the passage of the Local Government Act 1972. Cumbria's largest settlement and county town is Carlisle. It consists of six districts, and in...

 pensioner received a fine for setting up a system based on a 12v battery charger, knowingly allowing a dangerously high current to flow through the wiring as he had lied about using a battery charger and had instead had used the 230v mains electricity supply to energize the wiring.
.

Cats Protection

Cats Protection

Cats Protection, formerly The Cats Protection League, is a UK charity dedicated to rescuing and rehoming stray, unwanted or homeless cats and educating people about cats and cat welfare. The organisation was founded as The Cats Protection League on May 16, 1927 at a meeting in Caxton Hall, London...

 describe the use of electric fences as "barbaric" on their website due to the physical harming of a cat or any other creature that comes into contact with an electric fence.

Crystals

A more traditional cat repeller is to use jelly-like crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

s containing methyl nonyl ketone

2-Undecanone

2-Undecanone, also known as methyl nonyl ketone and IBI-246, is an oily organic liquid manufactured synthetically, but which can also be extracted from oil of rue...

, designed to be scattered around the garden, or around the areas the cat likes to foul. These repellents give off a smell that is very unpleasant to the cat, causing it to avoid that place.

Citronella

Citronella oil

Citronella oil

Citronella oil is one of the essential oils obtained from the leaves and stems of different species of Cymbopogon. The oil is used extensively as a source of perfumery chemicals such as citronellal, citronellol and geraniol...

, famous for repelling flying insect

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s and other creepy crawlies, can also be used to get rid of cats. Citronella sticks are a common form, coming in citronella-impregnated plastic "repeller sticks".

Lion dung

Lion dung has been reported to be an effective method of deterring cats, and has received support from the British organisation Cats Protection. One anecdotal experience reported by the BBC found that it was not terribly effective however.