Learning a second language can boost brain power, scientists believe.
The US researchers from Northwestern University say bilingualism is a form of brain training - a mental "work out" that fine-tunes the mind.
Speaking two languages profoundly affects the brain and changes how the nervous system responds to sound, lab tests revealed.
Experts say the work in Proceedings of the National Academy of Sciences provides "biological" evidence of this.
For the study, the team monitored the brain responses of 48 healthy student volunteers - which included 23 who were bilingual - to different sounds.
They used scalp electrodes to trace the pattern of brainwaves.
Under quiet, laboratory conditions, both groups - the bilingual and the English-only-speaking students - responded similarly.
But against a backdrop of noisy chatter, the bilingual group were far superior at processing sounds.
They were better able to tune in to the important information - the speaker's voice - and block out other distracting noises - the background chatter.
And these differences were visible in the brain. The bilingualists' brainstem responses were heightened.
Prof Nina Kraus, who led the research, said: "The bilingual's enhanced experience with sound results in an auditory system that is highly efficient, flexible and focused in its automatic sound processing, especially in challenging or novel listening conditions."
Co-author Viorica Marian said: "People do crossword puzzles and other activities to keep their minds sharp. But the advantages we've discovered in dual language speakers come automatically simply from knowing and using two languages.
"It seems that the benefits of bilingualism are particularly powerful and broad, and include attention, inhibition and encoding of sound."
Musicians appear to gain a similar benefit when rehearsing, say the researchers.
Past research has also suggested that being bilingual might help ward off dementia.