It's a shame that car alarms vary so much. They switch from sound to sound, from steady to pulsing to wailing. So many different sounds, that coming up with something unique becomes virtually impossible. I agree that personal alarms should not sound like anything else, but what else is there?
Car alarms do serve a purpose most of the time, they scare off the thief. They do not, nor were they ever intended to bring help (or the police). Personal alarms (IMHO) do the exact same thing. The are designed to scare the perpetrator. Not to summon help. Depending on the environment, it may or may not even have that effect. Car alarms are ignored in most parts of the country (not just New York), as will most personal alarms. Your best hope is to simply scare the attacker with the noise, but don't fool yourself in thinking that people will come running to your aide. As long as this is kept in mind, personal alarms do have their place..