Horn loudspeakers amplify sound through proximity resonance in a narrow throat

Evidence

As we have said, most sound sources are inefficient (wasting energy) and may be amplified without any electronics by making them more efficient at making sound. We have just seen this in the case of proximity of a source to a wall or corner in a room. The horn loudspeaker takes this principle to the extreme. Although it cannot be treated by the method of images, it does use proximity resonance by launching sound in the confines of a narrow throat. From there, it leads the sound out by gradually enlarging, ending in a large mouth that passes all but the lowest frequencies. — Eric J. Heller, Why You Hear What You Hear- An…, 525