DB AND METERS...WHO ARE THOSE STRANGERS?
In this article, I would go through a complex topic for most producers: dB.
There are many types of dB and meters. Some are related to the audio world, and some others are not. To keep the topic as much useful and straightforward as possible in this article, I'm going to talk about the most common type of dBs we can find in any DAW.
First of all, there are two things we have to define to understand this topic: what a dB is and what scale DAWs use.
From Wikipedia: The decibel (symbol: dB) is a unit of measurement used to express the ratio of one value of a power or field quantity to another, on a logarithmic scale, the logarithmic quantity being called the power level or field level, and respectively. It can be used to express a change in value (e.g., +1 dB or −1 dB) or an absolute value. In the latter case, it expresses the ratio of value to a fixed reference value; when used in this way, a suffix that indicates the reference value is often appended to the decibel symbol. For example, if the reference value is 1 volt, then the suffix is "V" (e.g., "20 dBV"), and if the reference value is one milliwatt, then the suffix is "m" (e.g., "20 dBm"). LINK
Now we have to define which scale our DAW uses. This is quite simple as all the DAWs use the same scale, and it's called FS (Full Scale). To make it clear, it is merely the scale you can find in any channel of your virtual mixer, including the master channel. In this scale, the values go from a very low value (-60 in Logic but can vary by the manufacturer) to 0, where 0 is the maximum possible digital level before the distortion.
So, when we read, for instance -3dB FS (Peak), it means that the peak of that sound arrives at -3dB on a Full-Scale system. That said, on an FS system, we can monitor different characteristics of a sound such as the peak (as mentioned), the True Peak, or the RMS.
dB FS (Peak meter):
A Peak Meter is a very fast meter that reacts to all the transients of an audio signal. It allows us to see if we are hitting our channel (or master channel) too hot in the digital domain. It is very handy when we are recording, mixing, or mastering.
dB FS (True Peak or TP meter):
Due to the Inter-Sample phenomenon making sure that a track doesn't exceed the 0 dB FS is not enough to prevent it from distorting. Some other peaks and consequent audible distortion can be generated by the D/A conversion process. These peaks are not readable by a typical Peak Meter as they don't happen while we are mixing or mastering, but when playbacking the file. A True Peak Meter might become handy as, in some way, it can predict what will happen to the audio when converted.
dB FS (RMS meter):
RMS stands for Root Mean Square. The RMS doesn't react as fast as a Peak Meter, and it represents the average of the total volume of a sound. It's used for the statistical measurement of the overall volume. This kind of dB gives us (as we said) some info about the volume of a track even though nowadays (especially in mastering), we talk more about the perceived loudness and the LUFS. RMS anyways is still a good representation of the volume of a track, and it can be helpful when mixing.
LUFS stands for Loudness Units relative to Full Scale. It has been introduced primarily by the broadcasting industry to level and normalize the perceived loudness of different shows and adverts over time. The music industry followed up, and now many streaming services (such as Spotify, Apple Music, YouTube, etc.) use this method to make the perceived loudness consistent during the playback.
The peculiarity of LUFS is that it reacts not only on the average level of a sound (like the RMS) but also take into account how the human being perceives different range of frequencies, becoming therefore a more accurate scale to measure loudness.
dB VU :
With the dB VU, we have to introduce another kind of scale; in fact, a VU Meter scale goes from -20 to +3 (sometimes to +5 depending on the manufacturer). We can find a VU meter mostly on analog gears or digital emulation of analog gears. In this scale, the 0VU doesn't indicate the maximum level (like in the FS scale) but shows the sweet spot of the gear itself. VU stands for Volume Unit, and it measures the volume level of a sound. Its needle reacts with 300ms of integration time, so it has a slow response ignoring the signal peaks. VU meters work similarly to the human being eras and are very helpful when mixing with analog gears (or analog emulations) to keep a consistent gain staging.