Final answer:
The correct answer to the student's question is 'infrasound', which refers to sound waves with frequencies below 20 Hz that are too low for humans to hear.
Step-by-step explanation:
The low-frequency sound waves that David Fee studies, and that are too deep and low for humans to hear, are called infrasound. Normal human hearing ranges from 20 Hz to 20 kHz. Any sound that has a frequency lower than 20 Hz falls into the category of infrasound, which can sometimes be felt as vibrations rather than being heard as sound. These low-frequency sounds are different from ultrasound, which are waves with frequencies higher than the human hearing range, above 20 kHz. Ultrasound is commonly utilized in noninvasive medical procedures, such as fetal imaging. In contrast, infrasound is often imperceptible to humans but can be detected and utilized by various animal species.