Nearby reflective surfaces can load a source and boost its output

Evidence

We first recall the simpler case of a small baffled speaker inserted directly in the throat of a horn. Proximity resonance (owing to very early reflections from the nearby walls) loads the source and makes it put out much more energy. If the horn releases all this sound energy without reflections, then other types of resonances do not develop and the speaker’s frequency spectrum is reinforced without coloration. — Eric J. Heller, Why You Hear What You Hear- An…, 1706