I wonder if any of these SPL apps are any good. Has anyone compared the results to an actual SPL meter? Also, do the apps measure flat SPL or A or C weighted? I have a SPL meter, but it only measures A or C weighting. When I listen to classical music with my home stereo system, I try to set the volume so that the A weighted SPL doesn't go over 90dB (A weighted) at the peaks, using the hold function and fast response. When listening to Bach organ music, jazz or pop then I play the peaks lower, normally about 80dB. Otherwise, everything is too loud. I use A weighting because that's what counts for hearing damage and only expensive meters have a flat function.
Measuring the SPL with closed headphones like the HD 25-1 is more difficult than with open headphones, as it somewhat depends on the seal that you get. I have a cardboard cutout with a hole in it the size of the SPL meter probe. When my X1 arrives on Monday, I'll check what levels I get with some different music.
Before I posted I decided to try it using my Rockboxed Clip+ and HD 25-1. I played the Telarc recording of Michael Murray playing Bach's Toccata and Fugue in D Minor. For those who aren't familiar, that's the one used for the old haunted house movies. With the volume set at -6dB, the meter peaked at 89.0 left and 89.9 right (A weighted) on the loud beginning part. I don't really have any rock music, but I can try the X1 with something like Bond, Mariah Carey, Sheryl Crow or Norah Jones with my HD 25-1 or DT880-250.