The sounds of frequencies less than about _____ cannot be heard by the human ear.

Humans can hear sounds from 20Hz to 20kHz. Hence human ears can't detect frequencies less than 20Hz.

Sounds less than 20 Hz are called infrasonic sounds. The sound produced by earthquakes, and volcanoes are in the infrasonic range. These sounds are inaudible to humans however, some animals like elephants, and whales can hear these sounds.