The present invention provides a unique wave-trigon transformation (WTT) method for performing transformation process over a wave signal. The present invention also provides a pitch detecting method and apparatus for detecting pitch based on the WTT process as well as a sentence detecting method and apparatus for detecting a sentence in a sound signal based on the WTT process. The pitch detecting method and apparatus can effectively detect pitch in a sound signal. In the WTT process, an inputted wave signal (such as a sound signal) is transformed into a series of trigons, and an energy-width spectrum is formed using these trigons. For a sound signal containing voice, the distribution of trigons transformed from the sound signal has a certain pattern. By analyzing the pattern, whether a pitch is contained in the sound signal can be determined. In particular, existence of a pitch can be determined by determining and evaluating the periodicity of trigons in a candidate chained peak in the energy-width spectrum.