According to the principle of sound different points are:

1.Electric alarm: The electrical energy into mechanical energy, and then by the mechanical energy conversion to sound energy. 

2.Electro-acoustic alarm: The audio small signal by low-frequency power amplification after conversion to sound energy.That is, the electron energy can be converted to sound energy. 

3.Pneumatic alarm: The electrical energy into mechanical energy, and then by mechanical energy conversion to gas energy.Converted from gas energy to acoustic energy.

There are different points according to the setting conditions:

1. Fixed alarm: An alarm used to install at a fixed location. 

2. Motorized alarms; alarms installed in motor vehicles or taken in other forms and used in motion.