All the wave (no matter what kind of shape) can be decompositive into series of wave with the same base frequency(or period). For example: You can look at sound wave of your voice with oscilloscope or recored with computer. You know everyone sound differently. But it not easy to find out the difference from the waveform. If we make a Fourier transform of the sound wave, i.e., calculate the weighting of each frequency component. Then, the ratio of those components are characteristic of each person. So it can be used as voice recognition. You should be able to find detail information from physics textbook.