With the aid of a vacuum tube “impulsator” the limits for which the integral law of apparent duration holds true, were determined; they are expressed by the value∫t1t2logIdt, for the extent of the apparent duration. Experiments with different impulses of tones of 1000 vib./sec., showed that the law holds true for limits of from 2 to 50 units of apparent duration. As a unit of apparent duration we take the apparent duration of a pure, constant tone which has sounded for one second at a loudness level of 1 db. Further experiments with tones of 50, 100, 500, 1000, 2000 and 4000 vib./sec. showed that the integral of apparent duration holds true for these tones and does not depend on the frequency of the tone. The musical optimum of reverberation corresponds to 37 units of apparent duration. As a result of the limits determined as indicated above, we can consider that the integral law of apparent duration in its unvaried form is applicable for the determination of the musical optimum of reverberation for halls up to 80,000 cu. m in volume.