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

• What is the most important purpose of the ozone layer?

• According to the writer, many people from Jamaica at that time ________.

• The word “fledgling” in the third paragraph could be best replaced by which of the following?

ZUNIA12

• The passage implies that Melville stayed in Tahiti because____________.

• What do the Hindu people think about marriage?

• The adaptation process described in the article was completed……

• Where did Marianne Moore grow up?

• According to the passage, an exuberant tone of voice may be an indication of a person’s ............

• The word “prior” is CLOSET meaning to ...............

• These changes in the Amercan home may ______________

• In paragraph 1, “streaming pupils"________.

• What does the passage mainly discuss?

• Which of the following could be considered a cause of the distress signals of trees?

• It can be inferred from paragraph 3 that _______.

• The word “it” refers to __________.

• The word "symptoms" is closest in meaning to ___________.

• What does the writer’s mother look like?

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

• What is the author’s main purpose in this passage?

• The word “odd” refers to which of the following?