This is the wireless Godzilla roar car horn available from Boom Blasters ($60). You just hook up the included horn to any 12-volt power source, and start jamming on the wireless button to let Godzilla do his thing. In this case, roar at 120 decibels. For reference, most standard car horns are 100 - 110dB, so it's pretty loud. Maybe not as loud as my roommate coming home drunk at night, but I'm fairly certain at least one of his parents was either a gunshot or a rocket taking off.
Keep going for the video demo while I speculate just how effective a Godzilla car horn is going to be.
Thanks to Tank, who is also loud, especially when he fires that cannon of his.