i need help with audacity

[cns00]

i am using audacity to edit audio and then i am exporting the audio as an aac by clicking on export audio in the main menu and then clicking on custom ffmpeg format options and then i am doing https://forum.videohelp.com/images/imgfiles/UI3Ib4d.jpg. as you can see i set 128 kb/sec bitrate

the problem is that when i check the properties of the aac file by using mediainfo it doesn't say the audio bitrate and it says variable for the bitrate. i need to add the aac file to mkvtoolnix and i mux an mkv file that has the new aac file that i made in audacity so that the audio bitrate shows. that's the first problem

the second problem is that i am not getting the bitrate that i am setting. for example assume that i set the bitrate to 128 kb/sec in audacity. when i check the audio bitrate in mediainfo after i mux the mkv file it is not saying 128 kb/sec and it is saying 127 kb/sec. i need to set it to 129 kb/sec in audacity so it says 128 kb/sec in mediainfo. that doesn't always work and i have to keep on trying different values in audacity. why is that happening? if i say 128 kb/sec in audacity then it should say 128 kb/sec in mediainfo and also i want the bitrate to show in the aac file and i don't want it to say variable

[davexnet]

I don't think aac audio uses constant bitrate, only variable. The number given must be the average