Nti makes portable measurement tools as well as measurement microphones and such.
You may be able to learn some useful stuff from watching this series of video’s even if you don’t have an NTi measurement system.
NTi Audio – webinars on demand
Here is a good explanation of “frequency weighting for sound level measurements”
NTi Audio – Frequency-Weightings for Sound Level Measurements
More NTi FAQ can be found here: