Question

what is meant by frequency of oscillation ? which sound are audible and which ones are inaudible to human ear

Answer 1

the no os vibrations made in one second is called frequency.
sound above 20 hz and below 2000 hz is audible to human ear

Answer 2

Frequency is the number of oscillations occurring in one second. The unit of frequency is Hertz (Hz). The frequency of sound which is audible to human ear is between 20Hz to 20000Hz. The frequency of sound less than 20Hz and the frequency of sound more than 20000Hz is inaudible to human ear