I spent some time in a house in Auckland with neighbours who were also heavy metal fans. They worked at constructing ferrocement yacht hulls. They combined work and business by making an exponential horn out of ferrocement, about 12 feet long and 8 feet across at the bell, and mounted on a trailer at the bottom of their garden pointing at the house. The idea was they could drive it from an amp kept indoors.

Fortunately they blew the driver speaker before I moved in.

The previous occupants of our house had left gouges along the hallway from where they'd driven their motorbikes up a ramp of planks through the front door, through the house and flying off the back porch. So with neighbours like us I guess the heavy metal yacht builders could easily get the idea that anything went.