It is not mandatory that backup alarms be 97 to 112 db. They only need to be "above the surrounding noise level". The loud beeping alarms were installed on most vehicles because most of them operated at loud constructions sites, so needed to be louder than that. it was easier to just buy the loud model to CYA, even if it was a delivery truck. They also don't need to 1000 Hz or to actually beep. White noise backup alarms are allowed, and in use in many delivery trucks now, and make a sound attenuated above 4000 Hz, which is much more localized and dissipates over much shorter distances. Waymo could absolutely have installed 85dB white noise alarms but chose to install 112dB beeping alarms. This is not a regulation problem.

From the link:

These backup warning systems operate at approximately 1,000 Hz, producing sound levels between 97 and 112 decibels.

Santa Monica’s municipal code adds another layer of complexity, prescribing exterior noise limits of approximately 50 decibels during the day and 40 decibels at night.

The continuous operation—with vehicles reversing dozens of times hourly, including during late-night hours—continues to challenge community peace.

So, constant car screaming BACKUP BACKUP BACKUP I'M BACKING UP HERE right outside your residential window. Kinda sucks. A whole lot.

Ironically the car is virtually purpose built to not run over people, and likewise has an extensive sensor suite to detect people around it.

I suppose regulations don't care if you can see no one is behind you.