Earthquakes produce which kind of sound before the main shock wave begins?

For hearing the loudest ticking sound of the clock,find the angle $x$ in the figure. (in $^o$)

Given that sound travels in air at $340 \, m s^{-1}$,find the wavelength of the waves in air produced by a $20 \, Hz$ sound source. If the same source is put in a water tank,what would be the wavelength of the sound waves in water? (Given,speed of the sound in water $= 1480 \, m s^{-1}$)

$A$ key of a mechanical piano is struck gently and then struck again but much harder this time. In the second case,

State whether the following statement is true or false:
The audible range of frequencies for human beings is $20 \, Hz - 20 \, kHz$.

Explain with the help of a diagram,how defects in a metal block can be detected using ultrasound.