Question

an \idiophone\ refers to an instrument that produces sound waves when:

1. a membrane is struck or hit (the membrane is what is vibrating, not the body of the instrument itself).
2. it is struck or hit by a mallet or against itself (its own materials cause the vibrations/sound waves).
3. a column or air is produced inside of it.
4. a string is plucked or bowed.

Explanation:

Idiophones are musical instruments that create sound through the vibration of their own solid material, typically when struck, shaken, or rubbed. The other options describe different instrument classes: membrane vibration is a membranophone, air column vibration is an aerophone, and string vibration is a chordophone.

Answer:

It is struck or hit by a mallet or against itself (its own materials cause the vibrations/sound waves).