Media Center's Analyze Audio tool writes two different measures of dynamic range to your audio file's tags : Dynamic Range (DR) (also called "crest factor DR") and Dynamic Range (R128).
What is Being Measured
Crest factor DR measures how many decibels there are between the average loudness of the track and the loudest sound on the track (the peak). The dynamic range database uses crest factor DR measurements.
By contrast, R128 Dynamic range measures how many decibels there are between a track's 95 percentile for loudness and the track's 10th percentile for loudness. This sounds technical, but in laymen's terms the measure screens out the very loudest and very softest sounds in a track out of is analysis to avoid allowing brief sudden loud sounds and momentary silences to affect the measure of dynamic range.
What the Measurements Mean
If you're comparing different versions of the same recording, you would use the R128 readings the same way you use the crest factor DR readings. Look at each version's R128 DR and compare them across (apples to apples). A recording with a higher R128 DR is less compressed than a recording with a lower R128 DR. You can't necessarily compare a crest factor DR to an R128 DR straight across because they are measuring different things (as described above). For example, see below three CD versions of Yes's Close to the Edge (an album with three songs):
|Version||Song 1||Crest||R128||Song 2||Crest||R128||Song 3||Crest||R128|
As you can see Crest Factor DR and R128 DR do not agree, especially on the third track, and that's to be expected because they're measuring different things. Crest factor compares average volume level to peak volume level, R128 compares the 95th percentile for loudness to the 10th percentile for loudness.
In the second version of the third track, the R128 peak level for that track is +0.6 (which means clipping), and there are a handful of narrow spots where the track clips. But the spots where the volume is at or near peak make up a fairly small portion of the track's running time (they are a handful of fairly narrow dynamic peaks). For crest factor DR, all that matters is the actual peak, so it compares the actual peak to the average volume level of the music and gets a "9" db result. R128 dynamic range ignores the peak and looks at the 95th percentile for volume (the volume level that 95% of the song is less loud than) on the theory that a handful of very small peaks should not set the dynamic range, and so it discards that information and arrives at a smaller number "5.5" dB. The 10th percentile for loudness forms the bottom of the R128 DR range (rather than the average level for crest factor), and that will often lead to R128 DR ranges that are wider than crest factor ranges, but, in this case, there aren't many quiet moments in this song, so the difference here is mostly at the top.
Which one is more accurate? It depends on what your criteria are. If you consider extremely narrow dynamic peaks to be an important part of the picture, then R128 may sometimes give you a misleadingly narrow impression. If you instead want to get a sense of how dynamic "most of the song" is, crest factor DR may give you a misleadingly good impression (in the example above because of a handful of clipped peaks). It's best to look at both and try to understand why when they disagree.
Dynamic Range (either one) is not the be all end all of musical quality. You may find you prefer some tracks that are more compressed than others. Dynamic Range isn't a perfect test, it's just more information.