Physics in guitars does actually exist. Since there are nodes at both ends of a string, the length of the string will equal to half of the wavelength of the original fundamental. Strings on a guitar represent standing waves because the string is fixed at both ends of the guitar. When playing a cord, it creates frequency. The more frequency, the louder the pitch or sound will be. To create different frequencies of the strings, you must have different wave speeds. Speed on a guitar is calculated much differently than anything else because not all objects have tension. If a wave travels slower, the frequency will be lower. Many people get confused with the term Hertz but all it represents the measurement of frequency. A twelve string guitar is really cool because it creates such a solid pitch.