Which of the following statements is true of vibrations? a. the frequency of infrasonic vibrations is much too high to be heard by humans. b. infrasonic vibrations are used in sonar equipment and to detect flaws in steel castings. c. ultrasonic vibrations have a frequency lower than the range for normal hearing. d. neither ultrasonic nor infrasonic vibrations can be heard by humans

D. ( Neither ultrasonic nor infrasonic vibrations can be heard by humans. )

The correct statement is option D, that is, neither ultrasonic nor infrasonic vibrations can be heard by humans.

Explanation:

The ultrasonic vibrations are the sound waves exhibiting the frequency of more than 20000 Hz, whereas the upper-frequency limit in human beings is about 20 Hz. Humans cannot hear the ultrasound as it is more than 20 Hz.  

On the other hand, the infrasonic vibrations refer to a low-frequency sound, it is lower in frequency in comparison to 20 Hz, that is, the usual limit of human hearing. Thus, for humans to perceive infrasound, the pressure of the sound must be adequately high.  

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