Understanding speech in noisy environment

More than half the world's population above the age of 75 years develop age-related hearing loss. They report difficulty understanding speech in noisy environments which is a critical everyday task, like when listening to someone speak in a noisy cafe. But the ability to understand speech amidst background noise varies widely among people.

Question:
Why do some people struggle with speech perception in noise despite having clinically defined normal hearing?

Aim:
To understand the sources of variability in individual subject's speech-in-noise recognition task.

Method:
The ability to hear speech in noise can be explained by people's ability to hear faintest sounds in quiet environment (also known as audiogram) in addition to their ability in detecting patterns of fixed-frequencies (figure) occurring amidst a random tone-cloud (ground) known as auditory segregation.

Outcome:
Such artificial sounds (stochastic figure-ground stimulus) help in this research as these do not have language barriers unlike tasks that use speech and the brain mechanisms underlying their segregation is well established.