Final answer:
Humans can generally hear frequencies from 20 Hz to 20,000 Hz. Sounds above 200 Hz can arrive at the ears at similar phase, making phase differences harder to detect. A frequency difference of at least 1 Hz is needed for the average person to distinguish two separate sounds if not simultaneous.
Step-by-step explanation:
The range of frequencies that humans are capable of hearing typically extends from 20 Hz to 20,000 Hz. Therefore, it is possible that sounds above 200 Hz may arrive at the ears at a similar phase, making it challenging for the auditory system to distinguish phase differences for source localization. The average person has difficulty discerning between two frequencies that are very close together, such as 2002 Hz and 1999 Hz, if they are not played simultaneously.
However, humans can typically distinguish sounds that vary in frequency by about 3-5 Hz or more when they are far from the threshold of hearing. For individuals with normal hearing sensitivity, frequencies should ideally differ by at least 1 Hz for distinct recognition if the sounds are not overlapping. Additionally, a person's hearing threshold can affect their ability to perceive sound intensity and frequency, as seen in cases where higher intensities are necessary for certain frequencies to be barely audible.