A personal alarm is a small hand-held electronic device with the functionality to emit a loud siren

Siren (noisemaker)

A siren is a loud noise making device. Most modern ones are civil defense or air raid sirens, tornado sirens, or the sirens on emergency service vehicles such as ambulances, police cars and fire trucks. There are two general types: pneumatic and electronic....

-like alarming sound. It is activated either by a button, or a tag that, when pulled, sets the siren off.

It is used to attract attention in order to scare off an assailant. The sound emitted can also have the effect of distracting, disorienting, or surprising the assailant.

The volume varies from model to model, with some models having 130 decibel

Decibel

The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...

s. Some personal alarms are also outfitted with an LED light for normal lighting purposes or to help deter an assailant.