This has been done before- The "New York Experience" Theatre in New York City had a 26 foot horn Sub-woofer built beneath the floor of the audience.

It no longer exists, but was on 5th avenue in the basement of the office building for a book publisher. In the 70's I was given a tour by the operators. The theatre was housed in space that had been built to be used as a small planetarium, but had not been completed. A seating floor was built at the base of the dome, and the speaker was built in the space below. The show was a multi-screen multi-media production giving a virtual tour of NY, with physical props included.

The speaker was an exponential horn, 26 foot in length, and used twice during the show (once was during a subway station scene, I forget the other). The cones of the drivers would only last for about 4 hours of operation, so would have to be reconed every few so many shows. The was built of wood, and curved so that the opening pointed up towards the feet of the attendees.

Having attended the show many times, (early geek destination in NY), I can attest that it was an intense experience, sitting in the opening of such a speaker.