A hawk flying at $\backslash (31\backslash$mathrm$\{$~m$\}/\backslash$mathrm$\{$s$\}\backslash )$ emits a cry whose frequency is $400$ Hz $.$ A wren is moving in the same direction as the hawk at $\backslash (16\backslash$mathrm$\{$~m$\}/\backslash$mathrm$\{$s$\}\backslash ).($Assume the speed of sound is $\backslash (343\backslash$mathrm$\{$~m$\}/\backslash$mathrm$\{$s$\}\backslash ).)$

$($a$)$ What frequency does the wren hear $($in Hz $)$ as the hawk approaches the wren?

$\backslash [$

$\backslash$mathrm$\{$Hz$\}$

$\backslash ]$

$($b$)$ What frequency does the wren hear $($in Hz $)$ after the hawk passes the wren?

$\backslash [$

$\backslash$mathrm$\{$Hz$\}$

$\backslash ]$