Why negative decibels
 

Hi, I am simulating noise over NACA0012. I got negative values at some points. I am trying to find out why negative decibels of sound we receive? Can anyone please explain? One more thing I am using Fluent it calculates the overall Sound pressure level and I have acoustic pressure files in .ard. I am trying to calculate sound pressure manually by using this acoustic pressure in files as p and po=1*10^-6 in the formula of SPL=20log(p/po). I am getting positive values, whereas in Fluent I also have negative values automatically in frequency analysis. Anyone please explain this too. Thank you all!