Greek theater resonators added reverberation-like effects without walls.

Evidence

But more than 2000 years ago, the Greeks (or at least the Roman architect, writer, and engineer Vitruvius, citing the Greeks) were using Helmholtz resonators and apparently claiming much more: increasing the clarity of sound in outdoor theaters, where reflections presumably were not a problem. … The resonators were, in the words of Vitruvius, responsible for “waking a harmonious note in unison with itself,” a poetic and perfect description of resonance. The resonator cannot amplify the sound except inside the cavity, very close to the mouth, and certain locations outside where the incident and reemitted sound are in phase (but also there are other places where they are out of phase). In fact, for a moment the resonator robs transient ambient sound of the resonant frequency while it “loads up,” so to speak. However, after the note has ended, the cavity will release this load, sounding for a time depending on the Q of the cavity. This is another use of Helmholtz resonators: taking advantage of the persistence of the resonance after the resonant driving sound source has been turned off. Optimistically, the effect could be a little like reverberations in a concert hall with walls and ceiling in an outdoor theater with none of these. — Eric J. Heller, Why You Hear What You Hear- An…, 1250

Categorization

• XREF: Vitruvian Man tie-in and the resonator-as-persistence-device echoes the reader’s library on acoustics and human sensory experience.
• QUESTION: Was the Greek theater resonator system actually effective, or a case of classical acoustic myth? Modern replication data would be interesting.