Which frequency of sound waves is least likely to be heard by humans?

The frequency of 20 Hz is considered least likely to be heard by humans because it falls at the very low end of the audible spectrum. Humans generally hear sounds in the frequency range of approximately 20 Hz to 20 kHz. While 20 Hz is technically within this range, it is at the threshold of human hearing. Sounds at this frequency are often felt as vibrations rather than heard as distinct sounds.

In contrast, frequencies such as 500 Hz, 5000 Hz, and 20,000 Hz are well within the range of human hearing and are more easily discernible. Frequencies between 500 Hz and 2000 Hz are typically where the human ear is most sensitive, making it easier to perceive sounds in these ranges. As the frequency approaches the upper limit of human hearing, 20,000 Hz, it becomes more challenging for older individuals or those with hearing impairments, but the sound can still be perceived by many young, healthy individuals.