Usually one source eg one loudspeaker at the end of a long glass tube . Large amplitude and low amplitude recorded. As in N05P3Q3 page 122
OR It can be one source eg one loudspeaker with reflector at the other end.
OR If 2 loudspeaker , it has to be facing each other and have the same frequency.
Basically stationary wave must be produced by two waves of the same frequency and speed traveling in the opposite direction.
