The judge in the George Zimmerman trial did something unusual - she ruled that the State could not use an audio expert who was going to testify that a scream heard on a 911 call was from the victim. The ruling is an unsual victory for the defense. In the past, judges have normally let in evidence from experts offered without much scrutiny. That has been one of the criticisms of the use of experts in criminal cases - and is the reason why experts have often gone unchallenged. Experts have often been admitted without challege, or admitted simply because they had been recognized as an expert before.
The role of a judge is to be the gatekeeper - determine what evidence should be admitted, and what evidence should be excluded. The Judge here appears to take that role seriously - and shows the country how the system is really supposed to work.