Final answer:
The human ear might hear subharmonic noise during ultrasonic operation, as ultrasonic frequencies are above human perception, but their byproducts could fall within audible range.
Step-by-step explanation:
During ultrasonic operation, the human ear cannot hear the ultrasonic noise itself because it has a frequency above 20 kHz, which is higher than the highest frequency humans can perceive. Instead, the ear might hear subharmonic noise, which is a byproduct of ultrasound and consists of frequencies that are within the range of human hearing.
Sounds below 20 Hz are known as infrasound, and while they are also generally inaudible to humans, they are not a typical byproduct of ultrasonic operations. Therefore, when ultrasonic equipment is in use, one might actually hear subharmonic noise or might think they hear something, but not the ultrasonic sound itself.