As per the formula of Doppler's effect we know that
[tex]f = f_o\frac{v}{v - v_s}[/tex]
now we know that
v = 340 m/s
[tex]v_s = 100 m/s[/tex]
[tex]f_o = 1100 Hz[/tex]
now we will have
[tex]f = 1100(\frac{340}{340 - 100})[/tex]
[tex]f = 1560 Hz[/tex]
so the frequency heard by the crew will be approx 1560 Hz