Bioacoustics researchers can use a computer as a powerful tool to measure, classify, compare and synthesize sounds. Vocalisations on tape are commonly converted to a digital format suitable for a computer by using an analogue to digital converter and then a Fourier transformation. Alternatively, sonograms can be measured, for example by using a digitizing pad or an image analysis system. Correlations and indices of similarity have been used to compare sounds, but variations in both the time and frequency dimensions of a noise are a problem. A solution may be the use of pattern recognition methods such as elastic matching and time warping. These methods are briefly described and assessed.