During his pre-trial testimony, Nakasone testified that there wasn't enough clear sound to determine whether Zimmerman or Martin was screaming on the best 911 sample, an assertion he repeated Monday.
"It's not fit for the purpose of voice comparison," Nakasone said.
Nakasone also said guessing a person's age by voice is "complicated" in general, and it was impossible to determine with the 911 sample he heard.
The FBI expert said that it's easier for a person with a familiarity of a voice to identify it than someone who has never heard it previously. "It's better," he said.
But under cross-examination by defense attorney Don West, Nakasone said there was a risk of increased listener bias if people trying to identify a voice are listening to a sample in a group, as Martin's parents did, rather than individually.
"There might be a risk of bias included in the end results," Nakasone said.