Which of the following frequencies of sound waves is audible to human beings?

For a particular sound wave propagating in air, a path difference between two points is $0.54 \, m$ which is equivalent to a phase difference of $(1.8 \pi)$. If the velocity of the sound wave in air is $330 \, m/s$, the frequency of this wave is: (in $Hz$.)

$A$ person in front of a mountain is beating a drum at the rate of $40$ per minute and hears no distinct echo. If the person moves $90 \, m$ closer to the mountain,he has to beat the drum at $60$ per minute to not hear any distinct echo. The speed of sound is .............. $ms^{-1}$.

$A$ steel rod of length $100 \, cm$ is clamped at the middle. The frequency of the fundamental mode for the longitudinal vibrations of the rod is ..... $kHz$ (Speed of sound in steel $= 5 \, km \, s^{-1}$)

$A$ man standing on a cliff claps his hands and hears the echo after $1 \, s$. If the sound is reflected from another mountain and the velocity of sound in air is $340 \, m/s$,then the distance between the man and the reflection point is ..... $m$.

Frequency range of the audible sounds is