$2.(20$ marks$)$

$($a$)$ Describe the mathematical relationship between the velocity $(\backslash (\backslash$mathrm$\{$m$\}/\backslash$mathrm$\{$s$\}\backslash )),$ frequency $\backslash ((\backslash$mathrm$\{$Hz$\})\backslash )$ and wavelength $\backslash ((\backslash$mathrm$\{$m$\})\backslash )$ of a sound wave with explicit reference to physical examples of sound sources having high frequency soundwaves and low frequency sound waves in the built environment.

$(4$ marks$)$

$($b$)$ Describe the role of each of the structural features circled in red of the retaining wall $($arm or stem, heel, key and toe$)$ depicted in Figure Q$2($b$)$ aimed at resisting the pressures$/$stresses being imposed on the retaining wall by the mass of soil, mass of the retaining wall and any moisture contained in the soil. Make particular references to the actions of normal stresses, shear stresses and bending moments.