Passage 4

The controversial Anglo-French supersonic transport plane, the Concorde, had attracted much attention because it is unusually loud. A late-model jumbo 747, for example, creates 100 or more decibels of sound over an eight-square-kilometer area. The Concorde spreads a 100-decibel-plus blanket of noise over some 140 square kilometers. On takeoff, the Concorde sounds like four F-4 fighter jets taking off at once. Its penetrating, low-frequency rumble makes the Concorde "completely distinctive" from other jets. What bothers noise experts is that plenty of commonplace machines are just as loud as, or even louder than, the Concorde. A sanitation truck can be noisier, and so can a heavy diesel truck pulling away from a stop sign. One expert who has gone out of his way to dramatize the prevalent offensiveness of the city sound-scale is Dr. Thomas H. Fay, director of speech and hearing at Columbia-Presbyterian Medical Center in New York. Dr. Fay took a sound-meter into a New York City subway and proved that an approaching train can be twice as loud as the Concorde. Since World War Ⅱ, the number of the high-powered noise-makers, from trucks and motorcycles to air conditioners and sirens has increased almost geometrically. It’ s no wonder that in many areas of the country, especially in the suburbs, the average sound level has doubled in 20 years. Noise experts estimate that city noises are increasing a half-decibel a year. They found that in 1971 the quietest parts of Los Angeles--thought to be a relatively quiet city--were louder than the loudest districts of New York in 1937.

The word "geometrically" in the first sentence of the last paragraph means ______ .