The claim that plucking a string produces standing wave overtones is a foundational principle of physics and acoustics so widely understood by definition and everyday observation that no citation is needed.
The claim describes the basic physical phenomenon of vibrating strings, harmonic frequencies, and standing wave modes, which is a standard textbook fact in physics and acoustics. Under the instructions, trivially true statements by definition or everyday observation are classified as COMMON_KNOWLEDGE, requiring no citation or paper assignment.