dB in Relation to Logic’s Faders

[Gtrmatic]

Greetings,

Despite having scoured the internet and several books, I still grapple with basic concepts regarding Logic’s faders.

One issue involves the term “dB” used in relation to the fader. For example, someone may say I increased the fader “by 3 dB”. To me this means that such changes to the fader increase the amplitude of the signal. Also, dB in this context is not an absolute measure. However, when the fader is increased by 3 dB, what exactly does the 3 dB increase on the fader signify? Is it just a measure of the digital headroom that will be used up (and reflected in the meter)? What do the numbers on the fader signify? Normally, when I see a meter with numbers I associate these with some representation of an underlying physical or digital quantity.
 

My understanding is that the term dB, by itself, is meaningless unless associated with some point of reference, e.g. dBu, dBV, dBfs, etc. In Logic, is it correct to simply say “dB” with no point of reference? On the faders, is dBfs implied? I’ve read that all the measurements pertaining to dB inside the DAW are in dBfs since it’s a digital environment. Assuming this is the case again what is the significance of the numbers on the fader? Are they just for convenience?

I apologize for the numerous questions but I really want to resolve these fundamental issues.

Thanks in advance for your guidance. It is greatly appreciated.

Gtrmatic

[David Nahmani]

You could compare levels with elevation, where the unit (feet) can be used with or without a reference depending of whether you want to measure an absolute elevation or a gain in elevation. 

There are two types of measurements here: 

1. Absolute elevation: that is your altitude referenced to the sea level, which is considered the 0 elevation. So if you're 300ft high, that means you're 300 ft higher than the sea level. This is comparable to a level in digital audio. If your signal peaks at -18 dBFS, that means that it's peaking -18 dB below the Full Scale (defined as the max level in digital audio). 
2. Relative elevation: that is the measurement of a gain in elevation. You're at a certain altitude and you're going up by another 35 feet. You've gained 35 feet in elevation. The new altitude depends on the original altitude. This is comparable to a gain in digital audio. You can apply 3 dB of gain to an audio signal, and its new level will be 3 dB higher than its original level. 

Hope that made sense? 

So in Logic's mixer, the fader applies a gain (measured in dB) and the meters measure a level (measured in dBFS). 

[des99]

Remember - the meters show you the actual level of the signal, in dBFS. The levels are absolute.

The faders show you (and let you change) how much the signal level on that channel is *changed* from the original source - so if the fader is at the 0dB point, it means "don't change the signal level that's coming into this channel - at whatever level it is at - at all", or if your fader is at -1dB it means "turn down the signal on this channel - at whatever level it is at - *by* 1dB".

So the fader indicates how to change the current signal level, which is relative (hence, calibrated in dB's of change).

And therefore the fader says absolutely nothing about the actual absolute level of that channel (well, unless it's all the way down, where we can be sure there is no signal at all.)

Edited June 20, 2023 by des99

[Gtrmatic]

Thanks for your quick responses! I think I need to rephrase my question. I totally get the gain vs absolute level measurements as shown by, respectively, the fader and the meter.

When you Google the term “decibel” an array of measurements come up: dBu, dBV, dBSPL, dBfs, to name a few. The term “decibel” by itself seems meaningless in many respects. In Logic, when we refer to “decibels” on a fader is dBfs implied? I know it’s dBfs on the meter. Also, I know that the fader can adjust the gain of a signal and that the adjustment is shown in “decibels”. Are these decibels considered a “dimensionless quantity” because they don’t correlate to a physical occurrence such as voltage, sound pressure level,etc. https://en.m.wikipedia.org/wiki/Dimensionless_quantity#:~:text=A dimensionless quantity (also known,which is not explicitly shown.

Thanks again! Much appreciated!
Gtrmatic

[des99]

The fader is only a relative gain change. It has no reference, you are *only* adjusting the level up or down by a relative amount - I explained that in my previous post. Therefore, the relative dB, with no absolute reference, is perfectly applicable. It's simply a ratio between the original signal, and the signal after gain change - no absolutes involved, therefore no scale to an absolute reference is needed.

Many of those references you mention are related to *electrical* references (eg, how many volts a certain signal induces down a wire), and aren't applicable in software.

A +3dB gain change is irrespective of the actual signal level you are changing - it could be small, or large, you're simply turning it up by 3dB. If you want to actual dimensions of the resultant signal after you've turned it up, you consult the meter, which gives you the absolute level, related to dBFS.

Edited June 20, 2023 by des99

[David Nahmani]

4 minutes ago, Gtrmatic said:

When you Google the term “decibel” an array of measurements come up: dBu, dBV, dBSPL, dBfs, to name a few.

Yes, they're measuring levels compared to different references. 

5 minutes ago, Gtrmatic said:

The term “decibel” by itself seems meaningless in many respects.

It's just as meaningful as the term "feet" is for measuring a difference in elevations. dB is a unit to compare two levels. It measures the difference between two levels, one of which can be a reference level. 

5 minutes ago, Gtrmatic said:

n Logic, when we refer to “decibels” on a fader is dBfs implied?

No. A fader applies a gain, and a gain is measured in dB. 

6 minutes ago, Gtrmatic said:

Are these decibels considered a “dimensionless quantity” because they don’t correlate to a physical occurrence such as voltage, sound pressure

I'm not sure what you mean by dimensionless. dB can correlate to sound pressure or voltage etc... only they measure the difference between two levels. So for example 1 dBV + 2 dB = 3 dBV but 1 dBu + 2 dB = 3 dBu. In both cases you've applied a gain of 2 dB to an absolute measurement. 

[David Nahmani]

15 minutes ago, Gtrmatic said:

I totally get the gain vs absolute level measurements as shown by, respectively, the fader and the meter.

Then it's just like with elevation-gain vs absolute elevation: the former is measure in feet, the latter in feet-above-sea-level. Think of "feet" as dB and "feet-above-sea-level" as dB-with-a-reference such as dBFS or dBV or dB SPL etc. 

But yes, if that's what you meant by "dimensionless", dBs can be used to measure different things such as sound pressure levels, voltages, digital audio levels etc. 

[des99]

Yep - it's like the difference between:

"Add 500 feet to the current altitude my plane is flying at", versus "Raise the plane's altitude to 5000 feet".

The first is relative - it doesn't matter what altitude the plane is flying at, we're just going to add 500 feet to whatever it is. It gives a change in altitude, but doesn't tell you anything about the actual altitude of the plane. It just says the new altitude will be +500 feet over what the previous altitude was.

The second is absolute, and needs a reference to be meaningful (in this case, to ground or sea level).

Edited June 20, 2023 by des99

[Gtrmatic]

Thank you David and des99. I really appreciate the assistance and your patience. I will ponder these concepts based on your responses.

Thanks again.

 

gtrmatic