Amateur radio receivers often have several types of audio filters, and a common one is simply called "Noise Reduction." Compared to notch filters, impulse filters, and adjustable bandpass filters, where the operation is fairly obvious, I have little idea how "Noise Reduction" works.
Some googling implied maybe it was simple smoothing, i.e. some kind of low pass filter, and it does sound like the high end is attenuated. But I can already do that with the adjustable bandpass filter. So I wonder if it is a little more sophisticated than that, perhaps with some non-linear or adaptive characteristics designed for voice on radio.
Any thoughts, and is there a term for this kind of filter that I could use for searching?