C-sub

HI i have built a C-sub using a kilomax pro 18 for the driver, the frequency response graph is shown below with the box design, the only problem is at around 38-40hz theirs a huge increase in SPL and does does not represent what is shown in the graph?? any advice on this would be great.

That graph wasn't meant to represent the response of the Kilomax in the C-Sub in the first place. The Kilomax has a high Qes which often causes it to have a peak somewhere in the response (for variable cabinet types), where lower and medium Qes drivers offer flatter response like in the graph.

Best regards

Cubo