Frequency Response Curve

Hey there, 
 

 I'm currently trying to obtain the sound pressure level for different frequencies on a plate. It's embedded within a region which has acoustic medium elements. I am able to get the POR pressureat a point within the acoustic medium at different frequencies but can't get the SPL. ABAQUS will not allow me to put a reference pressure in as an initial condition, and only offers me the option to "Specify Acoustic Wave  Formulation" when i try to edit the model attributes. Any ideas what might be wrong here? How do I add the SPL Reference Pressure?  
 

 Thanks, 
 

 Josh