I noticed it earlier this fall: Instead of rising and falling in a wail, some ambulance sirens were instead alternating between two tones.

I swear NYC ambulances didn’t used to sound like this. We’re becoming so European. pic.twitter.com/5RKxwg5m6K

— Kate Hinds (@katehinds) October 7, 2017

The distinctive sirens came from Mount Sinai ambulances. I got the backstory from Bob Levy, a supervisor in the Mt. Sinai EMS department.

"We really agonized over this," he said. 

Sirens have to be loud enough to penetrate vehicles and get the attention of drivers. But car companies prize minimizing outside noise (yes, you can find rankings of cars with library-quality quiet). Which means sirens have gotten progressively more aggressive. Next stop:  noise complaints. So Mount Sinai -- which responds to 65,000 emergency calls a year -- began tweaking the siren.

"We looked for sounds that were a little bit gentler," Levy said.

After some experimentation, they hit upon the two-tone siren known as the 'hi-lo,' and EMS workers began beta testing it.

"We asked people that had been complaining about the siren to listen to it," he said. "And everyone agreed that was a much better alternative."

Not only is the hi-low unique, Levy said, but it also has "the penetration so that it gets through the distraction, but not enough so that it's really annoying."

Speaking of which, Levy has a lot of sympathy for people who hate sirens.

"No one wants to hear the siren," he said. "I live in the city too, and I don't want to hear the siren."