Do all frequencies produce standing waves or just the harmonics? My physics textbook stated that standing waves are form when two wave trains with equal amplitude and frequency meet each other in opposite directions. Does the common frequency of the wave trains have to be one of the harmonics? Suppose we have two speakers (two sources) facing each other, both connected to the same signal generator. Will a standing wave form for all frequencies? In another situation where we have a single source, i.e. transverse waves on a string reflected from a fixed point, does the same principle apply?