You can find sirens either for inside or outside locations. Because of some noise ordinances introduced in some towns, it’s not such a good idea to install a siren outside unless you live on a ranch. It can be dangerous for neighbors who would want to give a helping hand but instead they can confront with a dangerous situation.
Using an inside siren can be much better because it just simulates a doorbell and in most of the times is placed in the air vent. This can scare off a burglar, letting him know that this is a protected house and it would be better for him to get lost. It will also be sign for you to take action, whatever that is.
Tip for users
The sirens are measured in decibels and wattage. A large one can have up to 30 watts and over 100 decibels. If in the end you will settle for a non-descriptive commitment, there is the possibility to permit the installation of a smaller and cheap siren. Even this one can be loud but if the siren is louder, there is a bigger chance for the burglar to leave.