Tuning fork of 512 FPS is used to test the hearing because it is -
Tuning fork tests can be done with tuning forks of different frequencies like 128, 256, 512, 1024, 2018 and 4096 Hz but most commonly used is 512 Hz because

“Tests are done with various tuning forks, but 512 Hz is the most commonly used as it has longer tone decay and sound is quite distinct from ambient noise.”
Forks of lower frequencies produce a sense of bone vibration while those of higher frequency have a shorter decay time and therefore not commonly used

Ref. Tuli 1/e, p 28
Better heard
Better felt
Produces over tones
Not heard