Follow the link narrow_to_one_third_octave.m to download a Matlab/GNU Octave script which converts narrow frequency band results to third-octave band results:
% function [one_third_freq,band] = one_third_octave(frequencies,measurements)
% Narrow bands to one-third octave bands representation.
% Example: [one_third_freq_preferred,band] = narrow_to_one_third_octave(frequencies,alpha_diffuse)
% The input parameters are:
% o frequencies: frequency values
% (with a fixed of variable frequency step),
% o measurements: acoustic absorption coefficent values
% (corresponding to the frequency vector defined above).
% The output parameters are:
% o one_third_freq: center frequencies of 1/3 octave bands,
% o bands: values of the acoustic absorption coefficent in 1/3 bands
% Copyleft 2007-2011 email@example.com
% cf. APMR on the web,
% for more information