Metering is a very useful way to visually see what the audio is doing; checking the frequency range of a multi-track is a great way to see if your mix has an overabundance of some frequency or other, and where to make corrections. Using a Spectrum analizer is very important in the mixing phase in order to "assign" to each instrument a "core" range of frequencies they should tend to, in order to clean each channel of the useless frequencies (for example using high pass or low pass filters), avoiding to put too many sounds layered in the same area.
Making your track loud enough without squashing the dynamics, is also a priority in these days of over-compressed masters, so metering is very important in the Mastering Phase too, in order to check the whole track and eventually apply some last-minute corrections.
When mixing or Mastering, it is crucial to use your ears, but it is also important nowadays to have a visual reference of what you are doing, to see how narrow your equalization cuts are, or to point out in an easier way the changes you are making (for example the Compression you're applying) is going to impact to the whole song or to the single instrument's wave.
Sometimes metering is very useful also to find where the muddyness of a song comes from, for example from a reverb applied to a single instrument, so that we can correct the problem by Eqing the effect.
Especially in the Mastering Phase, Metering comes handy to check out the Dynamic Range: using a Limiter, in facts, there is a strong risk of overcompressing the sound, ruining all the dynamics. Therefore when limiting is a good rule to check the analyzer, in order to keep a Dynamic Range (DR) of 10 to 14 db (Dynamic Range is the difference between the quieter and the louder parts). Squashing the sound too much will result in ruining the pleasure of listening.
As I often say on these articles, the best and most expensive DAWs often features some metering tool too, but if you need some free Vst frequency analyzer, the best around are two made by Voxengo: SPAN and AnSpec. Make sure to check them out!
In the Mastering Phase, another metering tool that can prove to be very useful is the Goniometer.
This type of spectrum analyzer basically works as a Phase meter, which means that it tells you if your track has any phase problems (any sound that has some frequency that "cancels" other frequencies, making them disappear). If the meter stays somewhat in the middle, with no parts creating any weirdness you should be fine, or you can check the Correlation Meter: if the meter is close to all the way to the "+1 side", you shouldn’t be having any phase problems. If there are phase problems, instead, you should go checking your mix for any instrument and/or effect (e.g. the tail of the reverb) that may contain frequencies that may cancel some other. To discover which parts of your mix has Phase problems, you must SWITCH YOUR MIX TO MONO, this way you'll be able to point out more clearly which frequencies may cause the phase erasement problems.
Goniometers are plugins that not often are found bundled in DAWs, so, among the many commercial ones, here is a free one: Flux Stereo Tool.
