The frequency of ultrasound waves is
- A.less than 20 Hz
- B.between 20 Hz and 2 kHz
- C.between 2 kHz and 20 kHz
- D.greater than 20 kHz
Correct answer
Explanation
The correct answer is D, greater than 20 kHz. Ultrasound is sound with a frequency above 20,000 Hz, the upper limit of human hearing.
Humans hear sound between about 20 Hz and 20 kHz. Below 20 Hz lies infrasound, made by earthquakes, elephants and whales. Above 20 kHz lies ultrasound, which bats, dolphins and porpoises use for echolocation and dogs can hear. Because its wavelength is short, ultrasound travels in a narrow beam and reflects well from small objects, so it is used in SONAR to find depth and submarines, in ultrasonography to see unborn babies and internal organs, in cleaning delicate parts and in detecting cracks in metal.
A is wrong because frequencies below 20 Hz are infrasound.
B is wrong because 20 Hz to 2 kHz is part of the audible range, which includes much of the human voice.
C is wrong because 2 kHz to 20 kHz is still audible, the higher part of the range.
Exam tip: infrasound below 20 Hz, audible 20 Hz to 20 kHz, ultrasound above 20 kHz.
Practice Questions
View allWhich one of the following frequency ranges is sensitive to human ears ?
- A.0-200 Hz
- B.20-20,000 Hz
- C.200-20,000 Hz only
- D.2,000-20,000 Hz only
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