Not being quite comfortable with the Radio Shack SPL meter I obtained another with connections for an external microphone. I have a Behringer 8000 microphone which I plan to use with it to measure room responses. My question now becomes how do I know the readings this microphone is giving me are flat? I have seen references to microphone calibration services. Is this important to have done, and if so who does it, and roughly what would be the cost???