An ambulance shrieks past, a jackhammer pounds, traffic whirrs by, a horn blasts. Another day dawns in the life of a noisy city.

Some people don't mind. They calmly accept noise as a fact of urban life. Others find it a very real annoyance.

Studies show that elderly people living in African jungles have less hearing loss than people living in an American city, said Dr. David N.F. Fairbanks, a District ear, nose and throat specialist. "City dwellers lose hearing faster, but this is difficult to measure."

Hearing is the body's perception of vibrations and air pressure. These vibrations are measured in frequency of cycles per second, known as the pitch. If air pressure vibrates at 20 cycles per second, that sounds like a low base note on a violin, said Fairbanks. At 20,000 cycles per second, it is the shrill sound of a dog whistle, which most humans can't hear. "In between are all the vibrations we hear, and that is what sound is," he said.

When sound vibrations hit the eardrums, they cause it to move slightly in and out. The vibrations are transmitted to the fluid-filled cochlea, where delicate nerve endings send sound impulses to the brain.

Noise intensity, or loudness, is measured in decibels. For example, a normal conversation between two people sitting across a table from each other is about 60 decibels. Noisy restaurants or busy traffic is 70 decibels. Lawnmowers, motorcycles and chainsaws fall between 90 and 100 decibels. Sitting in front of the loudspeakers at a rock concert is 120 decibels. Aircraft liftoffs (if you are standing 50 feet away) and shotgun blasts are 140 decibels. If you're near the launching pad when a rocket takes off, you will hear 180 decibels of sound.

How much noise is too much?

"It is hard to say that a noise level is dangerous without considering the time you are exposed," said Alice Suter, a Silver Spring consultant in industrial audiology and community noise. "A good rule of thumb is 85 decibels for an eight-hour day, but some people exposed at that level for many years will have some hearing loss."

When noise is too loud, the eardrum and fluid of the inner ear move too much, and this causes a tearing of the nerve endings, which destroys them, Fairbanks said.

Some sounds will damage hearing no matter how short a time the person is exposed. "At 120 decibels, no amount of time is safe," Fairbanks said.

General rules to follow on noise exposure are: :: If it is painful to your ears, it is too loud. :: If you experience a temporary hearing loss or have ringing in the ears, you may have done some damage. It may not be measurable after one exposure, but repeated exposures will cause further damage, Fairbanks said.

"Most kids come away from a concert saying 'I feel o.k.' because you can recover and the guy who is out there breaking up cement may feel better on the weekends," said Fairbanks. "But over time, they will have trouble." :: If you have to shout above the noise at work to be heard, it is probably too loud. Fairbanks recommends that a worker approach the boss about having the sound environment analyzed or contact the Occupational Safety and Hazard Administration. :: Wear ear protection. An earmuff is the best, particularly for people exposed to loud noise all day. It has a hard outer shell with a foam filled cushion inside. They are sold at sporting goods stores and usually cost between $20 and $40. Muffs give 40 decibels of protection.

Ear plugs are also available, but they must be capable of an airtight fit in order to protect the wearer, which is not always comfortable. Ear plugs give about 15 to 30 decibels of noise protection and cost between $2 and $4.

The District has several ordinances to control noise within the city. Construction is allowed between 7 a.m. and 7 p.m., although night work permits are granted, said R. Benjamin Johnson, administrator for the Housing Environmental Regulation Administration. Private contractors for waste removal should not remove garbage before 7 a.m.

In the year ending March 31, 1987, Benjamin's office received 159 noise complaints. Trash trucks, construction and air conditioners were the top three offenders.

If residents have a noise complaint, the department will send out a housing inspector to measure the noise level. This service is available 24 hours a day. Inspectors have the authority to make business establishments control their noise.

Last summer, residents living along the Southwest Freeway complained about traffic noise. At one point, tractor trailors, buses and dump trunks contributed to a noise level of 89 decibels. The city responded by putting up barriers along the highway to contain the sound.Resources

District Housing Inspector: 727-7709. During evening hours, call the Mayor's Command Center: 727-6161. The American Academy of Otolaryngology -- Head and Neck Surgery publishes a pamphlet "Noise, Ears & Hearing Protection." Send a self-addressed envelope with the name of the booklet to them at: 1101 Vermont Ave. NW, Suite 302, Washington, D.C. 20005.