Resonance requires round-trip reflections arriving in phase with the source

Evidence

Power is increased and resonance is achieved only if reflections arrive in phase with the source. Consider sinusoidal driving at fixed frequency and therefore fixed wavelength. If a round-trip from source to wall and back to source is 4 1/2 wavelengths, the reflections are arriving out of phase with the source. But it is a simple matter to choose a lower frequency to make this distance a round 4 wavelengths, or choose a higher frequency to make it 5. If there are no other round-trips to worry about, there will be broad resonances at frequencies corresponding to 4 and 5 wavelengths round-trip, with an antiresonance between. — Eric J. Heller, Why You Hear What You Hear- An…, 912

Categorization

• DEFINE: Explains phase-matching mechanism and how frequency can be tuned to hit resonance.