Abstract : A review is made of the statistical theory of signal detection, followed by application of signal detection theory to psychoacoustics. The differences between relatively simple laboratory tests and the complex problems of passive sonar operating in the real world are explored. The concepts of equal detectability curves and figure of merit are covered, relative to prediction of alertment and masking performance. The combined effects of five factors are used to yield a new detection threshold between 5 and 6 db higher than earlier works, and a new system for calculating the effects of broadband noise of any bandwidth on detectability is given.