Standing waves can form without any physical boundaries via interfering sources.

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 M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 401

Categorization

• DEFINE: Clarifies the precise distinction between standing wave and resonant mode terminology.
• QUESTION: How does a single source plus one boundary actually produce a full half-wavelength standing wave?
• corroborated: 2 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 401