I have a average or slightly above average clifford/viper alarm fitted on my car which does the job, but then i also have a seperate screamer alarm which is installed and set to trigger once the viper alarm is disabled and also if the car is moved (movement sensor of some sort).. ie.. being towed on a flatbed. The screamer i got was an old stock alpine siren rated at 182db .. so if u want a good deterent that ppl can hear a block away... thats one economical way to go that works well.