Sirens are loud noise-making devices. Civil defence sirens are mounted in fixed locations and used to warn of natural disasters or attacks. Sirens are used on emergency service vehicles such as ambulances, police cars, and fire engines. Depending on their working principle sirens are classified into two groups: mechanical and electronic.

A PA system (or ‘’public address system’’) is an amplified sound system. A typical PA system includes microphones, amplifiers and loudspeakers (like subwoofers). PA systems are used like sirens on emergency vehicles such as police cars, ambulances and fire engines.