Passage Three

　　The ear is indeed a remarkable mechanism; it is so complicated that its operation is not well-understood. Certainly it is extremely sensitive. At the threshold of audibility, the power requirement is inconceivably tiny. If all the people in the United States were listening simultaneously to a whisper (20 decibels (分贝)), the power received by all their collective eardrums would total only a few millionths of a watt (瓦特) — far less than the power generated by a single flying mosquito.

　　This aural organ is also remarkable for its ability to distinguish various pitches and other qualities of sound. In the range of frequencies where the ear is most sensitive (between 500 and 4,000 vibrations per second), changes in pitch of only 0.3 percent can be detected. Thus, if a singer trying to reach the octave above middle C (512 vibrations per second) is off-key by only 1.5 vibrations per second, the fault can be detected.

　　The normal ear can respond to frequencies ranging from 20 to 20,000 vibrations per second. In this range, it is estimated that the ear can distinguish more than half a million separate pure tones; that is, 500,000 differences in frequency or loudness. The range varies somewhat from ear to ear and becomes somewhat shorter for low-intensity sounds. Above the audible range, air vibrations similar to sound are called supersonic vibrations. These may be generated and detected by electrical devices and are useful particularly for depth sounding at sea. The time for the waves to travel from the generator to the bottom of the ocean and back again is measure of the depth of that particular spot. Supersonic vibrations apparently can be heard by some animals — notably bats. It is believed that bats are guided during flight by supersonic sounds (supersonic only to humans) which they emit and which are reflected back to their ears in a kind of natural radar. Humans can tell approximately where a sound comes from because we have two ears, not one. The sound arriving at one ear gives the brain information, which the latter organ interprets to note the direction from which the sound originally came.

　　Questions 21-25 are based on Passage Three.

　　21. The amount of wattage received by the normal eardrum ____.

　　A. indicates the amount of electrical energy

　　B. is extremely sensitive

　　C. is remarkably long

　　D. is extraordinarily small

　　22. A Sound coming from a person’s left side would ____.

　　A. hit the left ear first

　　B. hit both ears at the same time

　　C. pass on to the right side

　　D. generate electrical devices to the brain

　　23. All of the following is true EXCEPT that ____.

　　A. the ear is so complicated that its operation is not fully understood yet

　　B. the ear is so sensitive that it can distinguish full range of frequencies

　　C. the ear is remarkable for its ability to tell the differences of various pitches

　　D. the ear can distinguish more than 500,000 separate pure tones when the frequencies range from 20 to 20,000 vibrations per second

　　24. The direction of sound can be detected ____.

　　A. because sound arrives in each ear at different time intervals

　　B. by the frequency and duration of the sound

　　C. by the inner ear only

　　D. by combining kinesthetic and tactile data

　　25. The best title for this passage is _____.

　　A. How Ears Tell Directions

　　B. How Sounds Come to Ears

　　C. The Ear — An Amazing Part of the Body

　　D. How Ears Distinguish Pitches