Yaesu-FT-991 Digital noise filters

I'm new to amateur radio and the Yeasu FT-991. When I try any of the 15 DNF's it sounds like people are talking from under water. Am I doing something wrong?

All digital noise filters produce some alteration to the sound; the underwater effect is a good way describe it.  The trade off is between less white nose in exchange for a more intelligible signal albeit, a little less natural sounding signal.  I suppose it's Nature's way of insuring that we don't get something for nothing!

The trick is to find the DNR setting that works best at a particular time for the particular signal.   I find with my FT-991 that a setting of only 1, 2, or maybe 3, usually works best for voice.  For CW and digital modes I find that higher settings, even up to 15, usually work well.  DNR seems to help CW and digital modes more than it does voice modes.  Sometimes, contrary to intuition perhaps, the DNR works better when the bandwidth of the receiver is made wider.  That is usually the case when there is mostly white noise, but it doesn't particularly help if widening the signal lets in more of other type noise such as splatter.  All in all I probably only employ the DNR to help my reception about 10-20% of the time.

I also have an Icom 7600 and I find that Icom and Yaesu's DNR operations differ somewhat.  Icom's DNR tends to increase in garble and decrease in noise more gradually than does Yaesu's and only helps to about half way to the maximum setting as the DNR level is increased.   Yaesu's DNR tends to increase in garble and decrease in noise more quickly but sometimes helps all the way to the maximum setting.  Both are effective at noise reduction and it just a matter of tweaking the settings to your taste at any given time.  Sometimes the effect isn't all that much and sometimes there's a very dramatic improvement when applying the DNR to a signal, it just depends on the particular signal and noise in a given situation.  Also, besides bandwidth settings, there is definitely interaction between the DNR and other interference rejection controls such as the Contour control.  The Contour control is one in particular for Yaesu radios that can help in conjunction with the DNR.

Personally I find the art of optimizing received signals using the various controls like DNR to be an intriguing aspect of the hobby.  I'm glad it isn't a perfect science!

Yaesu FT-991(A) Users Group (groups.io)
https://groups.io/g/Yaesu-FT-991