What characterizes a prepared instrument?

A prepared instrument is defined by alterations made to its standard setup, specifically through the addition of foreign objects that modify its sound. This technique, often associated with avant-garde and experimental music, can dramatically change the timbre and overall sonic qualities of the instrument. For instance, adding objects such as paper, rubber, or metal to the strings or body of a piano can create unique and unconventional sounds, leading to innovative musical expressions that differ from the instrument's original acoustic capabilities.

The other options do not accurately describe the characteristics of a prepared instrument. For example, while some prepared instruments may involve electronic elements, not all prepared instruments are electronic; many classic examples are purely acoustic. Similarly, while mastery of prepared instruments can indeed require specialized techniques, extensive training is not a defining feature, as some prepared sounds may be accessible to musicians with varied skill levels. Lastly, prepared instruments can often produce a mix of acoustic and electronic sounds, making the statement that they are always played acoustically misleading.