I'm using Welch's method (pwelch, in octave) to get the PSD of a recorded sound signal (i.e. pressure level vs. time). I then took the PSD data and took the sum of the amplitude in each octave band (1/3 octave). My question is: how can I convert the PSD data to a sound pressure level (per band, or per frequency if I do it before 1/3 octave sort/sum). For an overall SPL, I can take the pre-PSD estimation data and calculate via the equation Lp = 20 log10(Prms/Pref).