Hearing thresholds for low-frequency complex tones were measured to investigate effects of intensity and frequency differences between components and effects of the number of components on the detection of complexes. Complex tones comprised two tones, geometrically centered at 60 Hz, with frequency differences of 30, 60, 90, or 120 Hz. Sound pressure levels of the two tones were set to equal intensity or to equal sensation level. Additional threshold measurements were conducted for complex tones comprising 2-6 components at 25-145 Hz. A complex signal with multiple tones was detectable even if the levels of individual components were below the threshold. The improvement of complex signal detection varied greatly with the level difference and number of tones. Threshold levels for complex tones (in terms of level per component) decreased as the number of tones increased. Complex tones whose components have mutually similar frequencies were more detectable than those with separated components.