Most of the gear in our studios uses the dBu reference, and will be labeled “+4dBu”, meaning that it’s expected unity line level voltage is 1.228 volts. Just know that while all the decibel scales we use daily are 20Log scales, the dBm is a 10Log scale, making for messy cross referencing, and the 0.775 volt dBu reference level is evidence of that. We aren’t going to do more than mention them here, since we rarely have to deal with them in the studio. By the way, dBm is a measurement of milliwatts, and used mainly for electrical power. You see, a 0.775 volt reference made a 0dBu measurement the same as a 0dBm measurement.

There’s a story as to why this number was chosen over an integer, or even a cleaner decimal. The reference point for the dBu measurement was chosen so that 0dBu = 0.775 volts. We like to call that “signal”, as in “Do you see any signal on that channel?” We typically measure this signal in volts. When we pass signal through our studio gear it’s all done by bothering electrons. That’s 1,000,000 times more pressure, represented as a change of 120 decibels. In comparison, the threshold of pain is 120dBspl, and that is equivalent to 20 pascals of pressure. This is what dBspl is measuring sound pressure level referenced to pascals, where 0dBspl = 0.00002 pascals. The force that a specific sound enacts on said air molecules is called Sound Pressure Level, and we measure that force in Pascals (named after Blaise Pascal) Our ears pick up sound through the vibration of the air molecules bouncing into our tympanic membranes (that’s a fancy name for ear drums). Now that we have a better understanding of how decibels work, let’s look at the different things we measure with decibels. The change between any one decibel and the next is never the same as the change between another, different, two decibels. But what if your ruler’s marking got closer together as you moved to the higher numbers? That’s what a logarithmic scale is doing.Įven just understanding this much will help you realize how much variation there is within the decibel scale. On a ruler the distance between markings is consistent all the way up and down the ruler. It’s a bit confusing, but if you think of it like a ruler it might help. The logarithmic scales also change as you move away from your reference point. Decibels are measured on a logarithmic scale, which is a way to express large data ranges in a compact way. That is to say that the difference from 1dB to 2dB is not equal to the difference between 10dB to 11dB, even though they are both represented by a 1 decibel change. So the first point to make is that decibels are not measured on a linear scale. Before we do, we need to understand something about scaling, and how it relates to decibel measurements. Fortunately for us, those that have come before us have done all the hard work of figuring out those references and how they all relate. So the reality is, when talking about decibels, we need to know what reference we’re dealing with, and what we’re actually measuring. Since the decibel is one-tenth of the Bel, it’s a simple way to measure smaller increments without all those nasty decimal places. It was named after Alexander Graham Bell (scientists and engineers like to name things after their heroes). A Bel is a unit of measurement that measures the ratio of a source against a known reference level. You see, the word decibel simply means one-tenth of a Bel. We’ve all said things like, “Hey, can we bring that down a dB or 2?” or, “Bring that up about 3dB,” but what do we really mean? Or, even more to the point, what does that really mean? I mean what is a decibel, really?

Decibel scale explained