matlab - I have a pressure signal and want to do SPL analysis on it

Signal

I have an acoustic signal from a Ffowcs Williams Hawkings CFD analysis and would like to convert it to the frequency domain and see the SPL and OASPL. I know I need to use fft() but I am unsure why my plot differs so much from the one from FLUENT.

time = data{1};
p = data{2};

Fs = 1/(time(end)-time(end-1));
L = 0.887*Fs+1;
N = 2^9;
Y = fft(p,N);

for k = 1:N/2
   f(k) = (k-1)*Fs/N;

end
spl=20*log10(abs(Y(1:length(Y)/2)));
semilogx(f,spl,'k','Linewidth',2)