One of the most challenging listening situations a person with hearing loss encounters is following speech in the presence of background noise.
Noise reduction in a hearing aid attempts to distinguish and separate speech from noise, before reducing the sound level of noise so speech can be heard with less listening effort.
Hearing aids use different algorithms to differentiate between speech and noise. Most focus on fluctuations in amplitude (sound level) given speech tends to fluctuate more than noise. Several too focus on fluctuations in frequency given speech includes a wider range of frequencies. A few algorithms also use probability scales to calculate how much noise and speech is present in a particular hearing aid channel, before providing less amplifications in channels where it is likely that there is more noise present than speech.