A personal security alarm (noisemaker or screamer) has a piercing, 120 decibel shriek that alerts witnesses nearby to your emergency - and might spur someone to come to your rescue.
At the same time, anyone attacking you fears witnesses calling the police as well as identifying him and/or having to deal with a possible rescuer. Thus an attacker usually will flee.
A personal alarm will probably keep you safer or even save your life whenever you're outdoors where other people are nearby - such as on the street, in a parking lot, or in a public park.
A safety whistle (a kid's, coach's, or referee's whistle) does the same thing.
Which one is best for you?
The upside - a whistle costs less and has no battery to replace.
The downside - a whistle stops sounding when you stop blowing (while you're maneuvering for escape). And, it has a cord for hanging it around your neck (which can be used to strangle you).
[But if you do rely on a safety whistle, rather than using a cord (lanyard) to hang it around your neck, instead use a common beaded-metal chain – it breaks too easily for strangling. Or wear a whistle on a wrist bracelet.]
The upside - an alarm continues sounding while you maneuver for escape, and frees both your hands for defense. And as you run away, an attacker will likely NOT chase after a noisy target.
The downside - an alarm costs more and has a battery that needs replacing occasionally.
All things considered, an alarm is your best choice – especially if you also have Pepper Spray as a backup safety device.
The best alarms continue shrieking until deactivated by
reinserting a pin in a hole.
Always walk on the street or in parking lots prepared for action. On your keychain, keep pepper spray and an alarm quickly at-hand, ready for predators looking for easy prey.
(In your car, with your keys now in the ignition, keep another unit of spray as a back-up within easy reach in your console.)
[See Survival Options - Kidnapping for dealing with a predator with a gun.]
Though NO personal security device can be guaranteed to prevent crimes, they'll likely serve you well if used with the safety tips in Outdoor Safety Overview.