Ultrasonic waves or vibrations are defined as sound waves of frequency greater than approximately how many Hertz?

Ultrasonic waves or vibrations are indeed defined as sound waves that have a frequency greater than approximately 20,000 Hertz. This distinguishes ultrasonic sounds from audible sounds, which typically fall within the frequency range of 20 Hz to 20,000 Hz that the average human ear can perceive. This threshold of 20,000 Hertz is important in various applications, including medical imaging, industrial testing, and cleaning processes, where ultrasonic frequencies are utilized for their ability to penetrate materials and produce detailed information about their internal structures, while remaining inaudible to human perception.

The other frequency choices mentioned do not align with the commonly accepted definition of ultrasonic sounds. Frequencies such as 10,000 Hz and 15,000 Hz, while higher than normal speech, still fall within the audible range for humans and are considered high-frequency sounds rather than ultrasonic. Frequencies above 25,000 Hz are also considered ultrasonic, but 20,000 Hz is recognized as the lower limit to qualify as ultrasonic, making it the most appropriate answer for this question.