All musical tone is produced by vibration of objects and the transfer of those vibrations through air (or water). For harps, the foundation of these sounds comes from the strings. The strings of the harp are strung across the instrument's sturdy frame in such a manner as to pull them tight and maximize the vibrations of the strings. The frame is also shaped to allow the strings to be positioned from short to long in a triangular shape, letting the player pluck both short strings and long strings at the same level. The shorter strings play higher pitched notes, while the longer strings play lower pitched notes. This is because the larger an object is, the more slowly it will vibrate, creating a deeper sound.
The body of a modern harp serves as a frame to stretch the harp's strings, but also to enhance the sound made by the strings' vibrations. The lower part of the harp body is a large, hollow chamber. This chamber enhances the sound made by the string in both volume and resonance, just as an empty cave will produce a loud, reverberating echo of a human voice.
There are many varieties of harps, ranging from small handheld instruments to the large orchestral harps that require wheeled dollies in order to be moved. The smaller harps can only play as many pitches as there are strings--or perhaps a few extra if there are sharping levers in place to pull each string a half step tighter (these levers are located at the top of the strings and must be manually positioned). Orchestral harps, on the other hand, are equipped with seven pedals to alter the pitch. These pedals control mechanisms that operate on a similar principle as sharping levers, but with more sophistication. Discs inside the neck of the harp are turned, pulling the strings tighter. This creates a wider range of alterable pitches. These mechanisms can also be operated during performances.