At any moment, the city air can be split by the insistent whine of a car alarm going off. Such sounds have become familiar - at least to many city dwellers.
While car thefts in medium-sized cities declined by about 4 percent in 1981, theft is still an escalating problem in most large cities. More than a million cars disappear from the street each year. This translates into the fact that in 1980, one of every 43 registered motor vehicles in the United States was either stolen or had its contents and accessories removed.
A lot of people are responding by purchasing anti-auto-theft devices. These can range in cost from about $20 to $400 or more. Obviously, the more you spend, the more protection you will get for your car.
No device can, of itself, prevent a car from being stolen. What the alarms, ignition or fuel interrupters, and ignition locks do, one hopes, is deter a thief from making off with the protected vehicle. According to Paul Graham of Louisville Lock and Key in Louisville, Ky., the most sophisticated systems ''make it just about impossible to drive the car away.''
A professional car thief, who can be expected to know how to disconnect most antitheft devices, can have a car rolling down the street in just a few minutes. The benefit of the antitheft systems is to make the break-in slow or obvious enough so that the thief will pass up a protected car.
Mr. Graham says his company recommends that customers with good cars go with the best system and then take such obvious precautions as parking in a well-lit, busy area. Parallel parking and turning the steering wheel into a full-lock position are also helpful to deter thieves who try to tow cars.
In choosing a system, check to make sure it is compatible with your car model. Some devices don't work on certain makes.
The most expensive system that Louisville Lock and Key markets is called the Mantis, produced by E. G. Technology in Los Angeles. At $450, it has a keyboard into which the driver enters a code within 10 seconds of entering the car. If he doesn't punch in the code, an ignition-kill system and alarm are activated. There is also a hood lock, which keeps the thief away from the ignition wiring and siren.
A low-end lock is the Secret Switch, made by Taylor Lock Company of Philadelphia. Costing $26.95, it allows the thief to break into the car and drive for a short distance, at which point the engine dies and the horn starts up. The theory is that by the time this happens, the car will be in traffic - and the thief will bail out rather than stay to silence the alarm.
In between these two devices is the $229 model 2002 of Crime Stopper Security Products in Chatsworth, California. It offers a keyboard arm-and-disarm system with an ignition kill. It also has an electronic motion detector, which means that, if someone tries to remove the hubcaps, mirrors, or tires, for example, the alarm will start. Some of the more sensitive systems can go off if a car is just jostled, which explains why a car-theft alarm sometimes breaks into song almost spontaneously.
Despite the expense of some antitheft systems, Mr. Graham says that much of his business is with ''normal folks with normal cars,'' although he sells antitheft devices to many car enthusiasts. He does think some people get carried away at times, citing one owner of a 1972 Chevrolet Vega who bought a $250 security system. As far as Graham could tell, that was about the full value of the car itself.
Security systems, besides protecting a car, can also cut the cost of auto-theft insurance. New York, Massachusetts, and Illinois give discounts of 5 to 15 percent for the devices, depending on their quality.
A Chapman lock, a popular though expensive (more than $300) choice, marketed by Chapman Industries Corporation, Elk Grove Village, Ill., might qualify for the 15 percent discount. It interrupts the ignition system, sounds an alarm, locks the hood, and has electric current and vibration sensors.
Insurance companies do not disclose which systems get which discounts, however, as they feel it might constitute an endorsement. The consumer must judge for himself, or get information from salesmen or manufacturers.
Dick Ames of the Automobile Association of America, Washington, D.C., suggests some simple measures owners can take to protect their cars, regardless of whether they choose to have an anti-theft device installed.
Parking in a busy area is very helpful, he says. A motorist should lock his car at all times and close the windows, even if he is running just a short errand. Almost one in five car thefts occur when the car is unlocked. Locking a car can also keep the driver from leaving the keys in the ignition by mistake. If anything is left inside a car, it should be placed out of sight. Stereo equipment should be disguised as much as possible.
The National Auto Theft Bureau adds such recommendations as keeping the license and registration in your wallet or purse. Thieves can use these documents to sell your car. When parking in a public lot or garage, leave only the ignition key with the attendant and take your key ring with you. The Bureau also recommends etching your vehicle identification number in several obscure spots, using an electric pencil. This can help to identify a stolen vehicle.
A few common-sense measures can often do as much as an expensive alarm system to protect a car.
(A catalogue is available for $3 from Louisville Lock and Key, 3926 Shelbyville Road, Louisville, Ky. 40207. Phone: (502) 896-0456. Antitheft devices are also listed in the catalogues of J.C. Whitney & Co., Parts and Accessories, 1917-19 Archer Avenue., P.O. Box 8410, Chicago, Ill., 60680.)