For example, of the two words lion and lioness, lion is unmarked for gender (it can denote either a male or female) whereas lioness is marked, since it denotes only females. b. Explicitly characterized by or having a particular linguistic feature.
20 Hertz to 20,000 Hertz is the frequency response range that a human can hear. So in answer to your question 20 Hertz would be the lowest pitch a human can hear.
For a high-frequency dog whistle (50+ kHz), hardly any human would detect the high-frequency components; For a low-frequency whistle (22-25kHz), most children, many teenagers, and some young adults will hear them.