I have 12 piezos strategically placed inside my Explorer, along with a full-sized alarm siren behind the vented rear interior panel in the back, and of course the full-sized siren under the hood, along with horn honk (my horn is pretty loud). It's absolutely ridiculous, but after having all my stuff jacked two years ago, I went crazy with the security measures. I picked up a bunch of the interior piezos at Radio Shack. They were cheap and they get LOUD. Make sure and use relays.