Recent data related to internal noise [D. R. Soderquist and J. W. Lindsey, J. Acoust. Soc. Amer. 52, 1216–1220 (1972)] have shown that the normal heart function produces a physiological noise capable of masking low-frequency signals. In addition, other investigators have shown that noise within the ear canal (aural noise) may also be a low-frequency masker. Furthermore, it has been suggested that the intensity of the aural noise varies as a function of the volume enclosed by the earphones. This investigation was subsequently designed to reveal the relative effect of each type of internal noise (physiological or aural) on the detectability of a 100-Hz tone. Three well-trained listeners participated in a single-interval signal detection paradigm. Detectability (d′) was calculated for each subject under each of the three variables of interest: type of earphone (insert, supra-aural, circum-aural), listening condition (binaural, monaural—contralateral ear occluded, monaural—contralateral ear unoccluded), and temporal location of signal relative to the subject's EKG R-wave (0.0-, 150-, 300-, 450-msec delay). Results show an increase in the effect of aural noise as the volume under the earphones increase. In contrast, the effect of physiological noise does not diminish across the different conditions. That is, a decrease in sensitivity occurs at the 0.0 and 300-msec delays across all conditions regardless of earphone or listening condition.