Decibel : What is dB and its importance for audio professionals : Magroove Blog

Decibel : What is dB and its importance for audio professionals

July 23, 2019 • 10 min read

If you have ever discussed any audio theory, this certainly was involved – either you acknowledge it or not. Even then, very few people understand effectively it importance. And what not many people know is that a decibel is, in fact, the tenth part of a Bel (that’s why “deci”), a unit of measurement created by Bell Labs, telephony company owned by Graham Bell, to measure loss in transmission lines. But because in audio we measure in smaller scales, the “deci” was adopted for practical reasons. Today we are going to understand its different types and the practical use of it. That way, more than just explaining the subject, you’ll end up with more skills to record and mix with better results. Shall we?

Decibel from A to Z

This is a brief list of things you should keep in mind about decibel:

  1. There is more than one kind of decibel. In fact, there’s a bunch.
  2. The dB has no embedded unit. Meaning that this unit alone doesn’t represent anything. It has to be applied to things like tension, voltage, sound pressure, etc. and what defines that is what’s written after dB. Example: dBFS, dBSPL, dBv, dBu.
  3. Decibels are relatives. The dB will always be comparing something to a unit of reference. That being said:
  4. All kinds of decibels have their own unit of reference.
  5. Decibels are a comparative unit. When we talk about dB, we are talking in comparison. If we say something has X decibels, we are already comparing two things – in this case the actual number with the unit of reference of the scale we are using.
  6. Any value of decibel is equivalent to a REAL value in the unit that the dB is comparing it with.

If you have no idea what’s going on above, stay calm! Let’s move on to a deeper explanation about what those affirmations mean exactly. Beginning accordingly.

Most common types of decibel

The different types, what is used to compare and the unit used to measure that:

  • dBSPL: Sound pressure (Pascal)
  • dBu: Electric tension (Volts)
  • dBv: Electric tension (Volts)
  • dBVU: Electric tension (Volts)
  • dBFS: Bits
  • dBw: Potency (Watts)

Where can we find each type of decibel

We will explain further on what each type of dB really is. But here is a list of where decibels are easy to find:

  • dBSPL: Charts comparing volume in different situations of our daily life; decibel meters, recomendation of mixing volume levels; car audio championships.
  • dBu: Line level of professional equipment, fader level in a mixing board, potentiometer of some power amplifiers.
  • dBv: Line level of semi-professional or vintage equipment. Highly used in calculation, due to the 1V reference.
  • dBVU: Pointer display of your analog or external peripheral equipment.
  • dBFS: Recording level of the audio interface; DAW channel volume.
  • dBw: Power amplifiers (dBw or dBu), output attenuators on power amplifiers.

All those types of decibel are different from each other, because they are representing different units. The only thing they have in common is that they are all comparing values with their respective units of reference.

What’s the unity of each one of the decibels?

A value in decibel is always translatable into a real value in the unit you are comparing it with. Remember we said that each decibel has a point of comparing reference? Here we are going to see what’s that point to each one of the decibels. But first, a very important information: the reference is always the 0 dB point! Meaning that if in that said decibel scale we have 0 dB, we will have exactly the value of reference. That way, we can say that 0 dB is the calibration point, or the starting point of the scale.

Here they are:

  • dBSPL: 0dBSPL = 20μPa, or 20 Micro Pascal. Pascal is the international unit of measurement for sound pressure.
  • dBu: 0dBu = 0,775V, or 0,775 Volts. In the form of +4 dBU (1,23V), it represents the line level on professional equipment.
  • dBv: 0dBv = 1V. In the form of -10 dBV (0,316V), represents line level in semi-professional equipment. Besides, it is used to facilitate calculation, since it has reference in 1V, instead of the 0,775 of dBu.
  • dBVU: It varies because it is not standardized. Read more about it on the paragraph below!
  • dBFS: 0dBFS means that all bits are being used. How many bits will depend on the audio bit depth of your project. That means that anything over 0 dB is clipping/distorting.
  • dBW: 0dB = 1W, or 1 Watt.
Decibel scale on a console's fader

In the mixing board, 0dB refers to line level. Meaning that if you enter 1,23V and keep the faders on 0dB, the output will be 1,23V!

Why so many types of decibel?

That is a similar question to “why have we created “Liter” if the already had “Meter”?”

Decibel is only the method of comparison, but we still need to identify the subject and the unit used. It’s as if we said that a person is “ten times more” than other person. But ten times what? Faster, smarter, lazier, taller, skinnier? Without the indication of the decibel type. we cannot know what are we comparing it with.

Why do we use decibel?

If every dB value can be translated into a real value in the unit we are comparing it with, well… why don’t we use the actual value instead?

Great question! And the answer for it comes in two parts:

  • The unit we are comparing it with is not as easy to work with because it’s too small or two big.
  • That unity is not intuitive if we compare its gross values. That happens because the unit we are comparing with scales logarithmically and not linearly.

Want proof? Here’s an example using the actual Pascal (unit in dBSPL) values

  • The sound pressure level of two people talking is around 50dB (and now you know that that dB is SPL!). 50dBSPL is equivalent, approximately, to 0.006324555 Pascal.
  • A busy street with intense traffic is around 85dB. The equivalent of 0.355655882 Pa.
  • A siren is around 120db. The equivalent of 20 Pa.

If we calculate, we’ll see that the differences are around:

  • The siren and the busy street: 19,644344118 Pa difference. Rounding up to 19,6 Pa.
  • Busy street and two people talking: 0,349331327 Pa difference. Rounding up to 0,3 Pa.
  • Siren and two people talking: 19,993675445 Pa difference. Rounding up to 19,9 Pa.

Analisis

Doing that, the comparisons and their values become high really quickly (near 20), or too low (less than 1). But we know intuitively that these sound differences are much more linear. That the difference between two people talking and the intense traffic on a busy street exists, but it will be more or less the same thing between traffic and siren. However, the numbers tell that the difference between siren and intense traffic is 65 times bigger that the difference between two people talking and the intense traffic. Seems like there is something wrong, doesn’t it? That is because for us humans, the perception between some sizes and intensities of phenomenas we see in the world, don’t work in a linear fashion, but in an exponential, logarithmic way.

We measure two people’s heights in centimeters. Taking one person’s height and subtracting by the other’s, we will have the difference between those two people. Subtracting centimeters linearly when we are talking about people’s height is pretty practical, because height is linear and humans perceive height as linear too. Now, when the matter is sound pressure, using linear subtraction is not practical at all. The example we had above, using Pascal, would be the equivalent to “subtracting one person’s height by the other’s” but in a sound pressure sense. We have seen, however, that it doesn’t seem right at all. And is here that the decibel comes to the rescue. The dB is precisely that: instead of making a linear comparison, we will be doing it logarithmically. Instead of comparing units, we compare the decibel of the units.

Same example, now using dBSPL (logarithmic)

Let’s see now how loud the sounds are, in decibels:

  • Two people talking: 50dB SPL.
  • Busy street with noisy traffic: 85dB SPL.
  • Siren: 120dB SPL.

Differences:

  • Between the busy street and two people talking: 35dB SPL.
  • The siren and the busy street: 35dB SPL.
  • The siren and two people talking: 60dB SPL.

A lot better, no? Those representative numbers and their differences make a lot more sense with our perception of the phenomenas, doesn’t it?

The magic is that linear differences in the decibel scale, which result in logarithmic differences in the actual values, allow us to work linearly once again. This makes naturally logarithmic phenomena linear, and thus, intuitive again.

Curiosity: That sensorial perception happens because human hearing is also logarithmic, so we perceive logarithmical changes as if they were linear.

dBVU – Decibels on your audio peripherals

VU means “volume units”. If your compressor, equalizer or other peripheral has a pointer scale, to measure output or input, it is possible that that meter is in VU. The VU is more than a pointer meter, it has to follow some basic calibration and “decay” rules (mainly for the pointer to fall). Those rules allow the VU to be a good translation of the intensity of the signal.

The dBVU is the scale used in this kind of pointer display. It normally indicates the beginning of the “red zone” of the pointer.

Zero dB VU in simple terms

There is no standard as to the dBVU comparison to a real unit, as in volts. Each manufacturer defines their own “0dB VU” as they desire. Check out what it might mean:

  • It indicates a little below the saturation point of the equipment, that enables the pointer and the signal to advance the red zone without distorting.
  • The ideal working point of the equipment, the limit where you should allow the signal to enter without distorting the peaks and keeping the equipment working dynamically.
  • The nominal input or output tension on the equipment. For example, if is a device a standard 1,23 volts line level input. Those 1,23 volts can be equivalent to 0dB on the VU, so you know that when it reaches 0 on the display, that you have exactly 1,23 volts. The equipment can have headroom (its circuit won’t distort when receiving more than 1,23 volts), in which case, it allows to go over 0dB without distortion.
  • A little lower of the input or output level on the equipment. That is the case in devices that 1,23 volts is near the maximum it can take without distorting (meaning, almost no headroom). Manufacturers do it so you can work safely, but below 1,23v, only reaching this value in the peaks that get to the end of the red zone of the VU.

 

VU Scale

Zero dBVU in technical terms

In technical terms, the 0dBVU will normally be one of the following values:

  • +4dBu (1,23v)
  • 0dBU (0,775v)
  • -10dBV (0,316v)

In practice, it is easy. Are you using a peripheral? Adjust the mid signal of input and output to 0VU and let the peaks go a little over the red zone, but never getting to the end of the scale by excess. With this proper gain staging, you should be safe and working within the equipment’s optimal zone.

Decibels in DAW’s: dBFS

The decibel inside the DAW. That is the only one going from -∞ (minus infinite) until 0. All the others normally start at 0 and only work with positive values or start at 0 and work around 0, in either positive or negative numbers.

When you reach 0dB on your meter, all your bits are filled and cannot go any higher. As your computer cannot create more bits, anything above 0dB is distortion. This was so aggressive that the DAW’s, after a while, started to “fake” a 0dB that in reality isn’t 0, giving a little breathing space. So even if you reach 0. the DAW can still handle it without distorting it. But don’t count on that or work on that basis, never let reach zero decibel!

 

Logic 9 fader, with the decibel full scale (DBFS) meter

Logic Pro 9 and the scale going to 0dBFS

Pro tools channel, with the decibel full scale (DBFS) meter

Pro Tools. Notice how the fader scale is intentionally (and mistakenly) shown with 12dB on top. That is done to induce you to do the right thing while mixing (imitating a mixing console).