Percussion instruments initiate sound through striking or hitting. There are two types of percussion instruments those that are pitched and those that are not pitched. Pitched instruments have definite pitches that can be determined. In most cases the instrument can produce a variety of pitches. Many of these instruments are set up like a keyboard, such as the piano, glockenspiel, marimba, and xylophone. Another pitched percussion instrument is the tympani. It changes pitch through the use of a pedal. Non-pitched instruments include: drums, cymbals, shakers, castanets, etcetera. Note that even though the piano has strings, it is a percussion instrument because those strings are struck with hammers inside the piano. Percussion timbres are the most heterogeneous. Because there are so many different instruments there are equally many different timbres possible. These instruments also tend to play at a louder dynamic level, as compared to all the other timbres.