Sirens and strobes are responsible for alarming people that are nearby secured area. Also very important is their preventive function - deterring potential intruders.

Alarm can take form of sound, light or both, because of that we categorize devices on:

- Acoustic sounders,
- Strobes,
- Sounder and strobe units.

Other criteria of categorization is power supply:

- Devices with battery backup,
- Devices without batter backup.

Sirens are supposed to signalize alarm by acoustic signal. Depending on sound generation technology sirens can be divided into dynamic and piezoelectric. Because of lower production costs with little worse loudness more popular are piezoelectric sirens. Depending on models complexity they differ with sounds type, which often is modulated or 2-toned. It is obvious that base feature taken under consideration while choosing siren is loudness and sound modulation.

Strobes are supposed to signalize alarm by light signal. Basic categorization criteria here is light module. Most common light source are LED diodes, light bulbs and xenon. Another parameter is lights color, most common are red, orange blue, yellow and white.

Sirens with optical and acoustic signalization are combination of both groups. Functions and types are same as in case of acoustic and strobe signal units.

Important element that decides about sirens quality is casing - specially in case of outdoor sirens. Temperature variation immunity, leak-tightness, blow endurance are parameters that decide about devices reliability. Very interesting solution is sound module's so called anti-foam protection. It protects sound source from being covered with foam which would silence the siren.