+1 for Tony's advice. But if you mean "work well with SPL," then it depends on your phone. What kind do you have? This thread recommends Decibel Meter V3.0 by Ashlogic.com for an LG g3 phone. You should still plan on a separate room mic.Are there any iPhone apps which are known to work well with REW? I've seen a bunch of SPL apps, but its not clear which could export data or do line out.