Last time, I explained what a decibel was, and how it is a dimensionless relationship between two sources
But wait! There’s more. Decibels can have dimensions, as long as they are referenced
Now we’re going to look at the common decibel types you are likely to encounter in the audio world
A referenced decibel unit will usually be written as “dB” with some characters after it to tell you what exactly its referenced to
First off, dBu or dBv (do NOT confuse this with dBV) – 0dBu = 1 miliwatt (mW) coming out of a 600 ohm impedance, or in a more math fun way, 0dBu = √0.6Volts or roughly 0.775Volts. You will usually see this reference in what we used to call “professional” equipment. Often an XLR jack or balanced 1/4″ jack on a piece of gear would be specified as having an operating level of +4dBu
Next, and frightfully confusingly, because of the shared “V” character with the above mentioned dBv, is dBV. For dBV, 0dBV=1 Volt RMS. You will most often see this on what we used to call “prosumer” equiment. Often the RCA jacks or 1/4″ jacks of a device would be specified as having an operating level of -10dBV
Next is a biggie in the modern world: dBFS or Decibels Full Scale. This one will be very important when dealing with digital audio. in dBFS, 0dBFS is as loud as you can get, everything is referenced downward from there, like 0VU = +4dBu = -15dBFS (as a for instance). Get this one straight early on, in dBFS, 0 is the upper limit. Theoretically, anything above there would be just digital hash, though in practice, internally in a DAW (Digital Audio Workstation), you can go above 0dBFS, just dont expect it to make it out of the Digital to Analog Converter (DAC) nicely
Finally you have dBSPL, the one you see the “ghost hunters” and cops running around with in the Sound Pressure meters most don’t know how to use. The “SPL” means Sound pressure level. 150dBSPL is pain, and just about surely hearing damage. 65dBSPL would be about normal conversation level. Measuring SPL is actually pretty tricky, and leads to all sorts of problems regarding noise laws. dBSPL is actually referenced to micropascals. You can look that one up yourself. For our purposes, 80-85dBSPL is about where music “sounds right” most of the time. Just underneath a snare drum can be 125dBSPL – don’t EVER put your ears there if the drummer has a stick in his hands.