Sound waves above 20 kHz, which are not audible to humans, are known as:

Sound waves above 20 kHz that are not audible to humans are classified as ultrasonic waves. Ultrasound has various applications, such as in medical imaging and industrial cleaning, due to its ability to travel through different mediums and reflect off surfaces without being heard by the human ear. The frequency of these waves falls outside the audible range for humans, thus categorizing them as ultrasonic.

In terms of the context of the other options, high-frequency sound waves is a vague term that could refer to any sound above the audible range but does not specifically point to the category of ultrasonic. Microwave waves refer to a different part of the electromagnetic spectrum rather than sound waves. Short-frequency sound waves is not a recognized scientific term and would typically denote lower frequency sounds, which would not apply to the waves in question. Therefore, the precision and correctness of calling these waves ultrasonic make that option the best choice.