Where does noise come from?
In the olden days of analog tape recording, tape hiss was like a lush carpet upon which every instrument and singer stood. But with digital recording, unwanted noise should be a thing of the past. Sometimes it isn't…
Perhaps your microphone is a little noisier than the best a mic can be. For example, the Beyerdynamic MCE 72 is a nice-sounding stereo mic. But it has a certain noise background that must be considered.
Perhaps your preamp isn't up to scratch in respect of noise level. There is no reason why it shouldn't be within a decibel or two of theoretical perfection. But that doesn't always happen, particularly with designs that tend towards a 'warm' sound.
Guitar amplifiers are commonly noisy. So are effects pedals. Compression always increases the noise level of a signal. And some plug-ins even emulate the noise component of the physical hardware they model!
And of course there's the computer. It shouldn't really be in the studio but in a separate machine room, connected by long cables or a KVM extender.
Since noise present in any individual track of a multitrack recording will almost always be masked by the instrument or vocal on that track, noise should not be a problem during the verses and choruses of the song.
But at the end when the instruments play their final notes or chords, the sound will die away leaving the noise clearly audible. This sounds ugly and amateurish, and must be corrected.
One solution to the problem is firstly to fade each individual track in such a way that any noise on the track isn't noticeable. If each track is fixed individually, then the whole mix will fade into absolute silence.
Another is to allow a prominent and noise-free track to dominate at the end, masking the noise on other tracks that should be faded out a little earlier.
One could of course just fade everything out before any noise becomes audible, but this is a rather cheap-sounding solution, as though the mix engineer doesn't really care about their craft.
Sometimes however, one important instrument might pose a real problem and there is no way to fade it out with elegance while at the same time eliminating the noise.
In a case such as this one option worth trying is to insert a low-pass filter into the track, setting the cut-off frequency to 20 kHz, then using automation to bring down the cut-off frequency as the last note fades away. Since noise is more noticeable at high frequencies than at mid and low frequencies, then this, combined with a fade, can lower the perceived level of noise to an extent that few listeners would notice.
One last point – sometimes modern-day digital recordings are just too quiet. Recording a track consisting of nothing but the hiss and hum of a guitar amplifier can provide a useful 'glue' that holds all of the instruments and vocals together. Just be sure to fade it out nicely before the end.