Sound moves form its source to the ear by wavelike fluctuations in air pressure, something like the crests and troughs of ocean waves. Once way to keep from hearing sound is to use ear plugs. Another way is to cancel out the sound with anti-sound. Using a noisemarker controlled by a microprocessor, engineers have produced sound waves that are half a wavelength out of phase with those of the noise to be quieted-each crest is matched to a trough, and vice versa. Once the researchers have recorded the offending sound, a microprocessor calculates the amplitude and wavelength of sound that will cancel out the crests and troughs of noise. It then produces an electric current that is amplified and fed to a loudspeaker, which produces antisound and wipes out the noise. If the anti-sound goes out of synchronization, a microphone picks up the leftover sound and sends it back to the microprocessor, which changes the phase of the anti- sound just enough to cause complete silence.

   The research team has concentrated on eliminating low-frequency noise from ship engines, which causes fatigue that can impair the efficiency and alertness of the crew, and may mask the warning sounds of alarm and fog signals.

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions

What is the main purpose of the passage?

Câu hỏi liên quan

• Which of the following is NOT mentioned as the denomination of an American coin?

• The first to support the equality of the sexes was .................

• Which of the following pairs is INCORRECT?

ZUNIA12

• Which of the following statements does the author support most?

• The word “laundry” in paragraph 2 is closest in meaning to ___________.

• The word "discrete" in line 16 is closest in meaning to ......

• What is the main topic of the passage?

• Pollution in water is noticed .............

• The word “basis” in paragraph 1 is closest in meaning to ............

• The word "contacts" in paragraph 3 is closest in meaning to ..............

• According to the passage, why is the shape of sea cucumbers important?

• How did Juliette Davidson benefit from the experience of her first job?

• The word -plausible‖ in the passage is closest in meaning to .............

• According to the passage, approximately how many non-native users of English are there in the world today?

• What kind of film is each program?

• The word “hazard” in the passage means ................

• According to the passage, which of the following factors might spoil the tourists’ fun at Mediterranean resorts and beaches?

• The word "informally" in this context mostly refers to an education occurring _______.

• What can the word adorning be best replaced by?

• All of the following are true about the 1970 survey of Antarctica EXCEPT that it______.