does compression bring about low frequencies to a sound ? sure it does bring about bottom end as the peaks are squashed , but is that bottom end like low frequencies / low end ?
concerned about this because the mixdown sounds muddy after compression on the master ...
It really depends on
what kind of compression and compressor you use. If you are using a multiband compressor and really squashing the top end of the mix then certainly,
the lower end is going to stand out. And as jaysummers75 correctly said,
the 400Hz frequency is the prime offender for mud in a mix.
Usually,
especially with vocals, compressors (non multiband) tend to
accentuate frequencies in the range of 3kHz - 20kHz. This is because, as DarkRed correctly said, the lower frequencies tend to have more power (hence, activating compression when the mix really goes hard on the bass) than higher ones. So you may have issues with sibilance (horrible hissing 'S' sounds) with vocals after compression.
I would recommend as others have said, to start compressing your
individual tracks first then, trying some master buss compression. This is something you need to really practice as compression is a
fickle thing. You need to pay attention to EVERY button and knob on your compressor because it will change the sound in ways you may not even hear...yet still notice.
The rule of thumb for compression is (for me anyway), don't use it unless you need it. I can't explain all the cases where you may need some compression because its something that comes with a little practice. So mess around with it a bit and try EQ-ing first if you are getting a muddy mix. Remove those awful frequencies first then, tame your levels using compression.
More generally speaking...if you don't know if you need an effect...and adding it makes it sound worse...remove it or try tweaking.