Standing waves qualify as resonant modes and need not require two boundaries

Evidence

Strictly speaking the standing waves we are discussing here should be called resonant modes. This is because it is possible to set up a standing wave without the use of two boundaries. For example, one could use two loudspeakers spaced apart and no boundaries to get a standing wave at discrete frequencies. One can do the same with one loudspeaker, or sound source, and one boundary. — David Howard, Jamie Angus, coustics and Psychoacoustics, 397

Categorization

• DEFINE: Clarifies the technical distinction between ‘standing wave’ and the more precise term ‘resonant mode’, since standing waves can form with a single source and one boundary.