There have been studies and those resulted in the less annoying backup sounds. These sounds are essentially harsh white noise, which has one significant difference to the beeping: it's level drops off differently with distance, meaning you can blast it louder and people who are really in the wrong spot will notice better it means them, while people who are not meant will not be annoyed or fatigued by it. Two noise sources combine different than two tonal sources and the human ear can locate broadband sources better than single tones.
This was developed especially for use in backup heavy environments like harbors where workers started ignoring constant beeps.
There's also another difference: beeps can reflect coherently off of surfaces, causing directionality confusion in a dense environment. White noise is much less likely to have odd interference patterns, maximizing our ability to localize the sound.