What it shows:
As a passive amplification device, the exponential horn is amazing. Using a "talking" greeting card as a feeble source of music, the intensity of the sound gets amplified by about 18 dB when the greeting card is coupled to the horn ... a dramatic effect.
How it works:
The multicellular horn is a cluster of eight smaller exponential horns, each with a small mouth to avoid beaming in a large frequency range, but together they form a sector of a sphere large enough to control directivity at low frequencies — the cluster acts as one big horn at low frequencies. At high frequecies the individual horns start to beam, but because they are distributed in an arc, coverage is still quite uniform. The entrance, or throat, of the horn is a 1½" diameter hole and the mouth of the horn measures 16"×31"
It turns out that the best shape for a horn is the exponential horn. This can be understood in terms of impedance matching. See, for example, chapter 7 "Sources of Sound" of Eric J. Heller's book, Why You Hear What You Hear, (Princeton University Press, 2013).
Setting it up:
The horn simply sits on an AV cart and can be wheeled around to best serve your purposes. Open up a greeting card and let the audience hear it without amplification. Then position the speaker of the greeting card (which happens to be 1½" in diameter — a perfect match) in the throat of the horn.
This particular horn was designed for the Altec model 288C driver, which we still have (in case you want to use that as a more controlable source of sound).