The loudness of multitone complexes and bands of white noise was measured as a function of band width at levels between 5 and 35 db above threshold. When account is taken of the changes in the loudness that are due to the nonuniformity of the ear's sensitivity at low levels, the spreading of energy appears to have the same general effect on the loudness of both line spectra and continuous spectra centered at frequencies as low as 400 cps and as high as 5000 cps. At 5 db SL the spreading of energy does not increase the loudness, and may even decrease it. Above about 10 db SL, the spreading of energy in a complex sound over more than a critical band increases the loudness. As the sensation level is raised up to 35 db, the spreading of energy becomes more and more advantageous to loudness. The critical band defines the limits within which the spreading of energy leaves the loudness of a complex sound unchanged. The widths of the critical bands that were measured in these experiments on loudness summation below 35 db are approximately the same as those measured in experiments on loudness summation at higher levels and also in experiments on phase, threshold, and two‐tone masking.