Need help plotting the function in Matlab.

I recently started using Matlab and I am trying to plot Imaginary part of a function. I can see I am making a mistake somewhere because I have the graph showing what I need to get and I am getting something else. Here is the link of the graph that I need to get, that I am getting and a function that I am plotting: http://imgur.com/LehkAKX

I know that this function has a singularity at frequency around 270 Hz and I don't know if a 'quadgk' can solve integral then. You guys probably know but theta function inside the integral is a heaviside and I don't know if that is causing problem maybe? Is there something wrong I am doing here? Here is the matlab code I wrote:

clear all
clc

ff=1:10:1000;

K1=(2*3)*log(2*cosh(135/6))/pi;
theta1=ones(size(ff));
theta1(ff<270)=0;

I1=zeros(size(ff));

for n = 1:numel(ff)
    f = ff(n);
    I1(n)= K1/f + (f/pi)*quadgk(@(x)(sinh(x/3)/(cosh(135/3)+cosh(x/3))-theta1(n))./((f^2)-4*(x.^2)), 0, inf);
end

plot(ff,I1, 'b');
hold on
axis([0 1000 -0.3 1])