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.
(See Survival Options - Kidnapping for dealing with a predator with a gun.) 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.
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.