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How to convert different scales to another?
Old 2nd February 2009
  #1
Gear Maniac
 
jujumies's Avatar
 
🎧 10 years
How to convert different scales to another?

I was wondering how could I convert say Dbfs (used by logic 8 etc.) to SPL scale. I would like to know how loud I monitor when I mix and I need this info in Db (SPL).

Simo Orpana
Studio Bad Mama
Old 2nd February 2009
  #2
Moderator
 
narcoman's Avatar
 
🎧 10 years
they have nothing to do with each other since any dB scale is is with respect to a reference level. dB(FS) is with respect to a full scale digital representation and dB(SPL) is with respect to physical sound pressure levels. They have no exact conversion since they are entirely different things. deciBels (or rather Bels) are a ratio and not an absolute.

SPL levels are set with amplifier gain..... so you could quite happily play a -20dB(FS) level at any dB(SPL) level you like, dependent upon monitor gain. It is up to you to set an equivalence if you want one. There are some guides out there - I set up -18dB(FS) to equate to 83dB(SPL) in the studio...... but there is no conversion or formula to follow.
Old 2nd February 2009 | Show parent
  #3
Gear Maniac
 
jujumies's Avatar
 
🎧 10 years
Oh... this so confusing for me. I guess I have to take a closer look into decibels etc. I cannot understand why these both scales don't interact anyway because both are using decibels as a measure...:( Am I stupid or what?

Simo Orpana
Studio Bad Mama
Old 2nd February 2009 | Show parent
  #4
Moderator
 
narcoman's Avatar
 
🎧 10 years
no not stupid !!!

Look at it this way. If you have your monitor gain set up so that, at 2 metres from the speakers you have a -20dB(FS) reference equating to 83dB(SPL) then if you turn up the DAW sounds by 3 dB the same will happen in the room (well just about the same !!!)....

It's up to you were you set the reference level. The important thing is for it to be repeatable.... then you get consistency.

The problem you're having is that you think because they both have "deciBel" in their name that they are the same thing. The Bel is a unit of ratio - it isn't a direct unit itself. If it makes it any easier you could think in terms of Pascals - the REAL (heh) sound level measure. It's a litle bit like asking "how much does one dollar buy?".... well how much of what? Defining WHAT the dollar buys is our equivalent of defining the monitor gain and reference set up. Then the two dB scales have a correlation. Withouth that.....:(
Old 2nd February 2009 | Show parent
  #5
Gear Maniac
 
jujumies's Avatar
 
🎧 10 years
Thanks a lot for giving a really good and comprehensive answer. This thing has troubled me for a while and now I somehow got an idea what is going on. I have to study more but now I see what is the problem in comparing these two scales. Thanks!!!

Simo Orpana
Studio Bad Mama
Old 2nd February 2009 | Show parent
  #6
Moderator
 
narcoman's Avatar
 
🎧 10 years
no problem...
Old 2nd February 2009 | Show parent
  #7
Lives for gear
 
🎧 10 years
Hope I don't add confusion here but I'll give a try..

The dBFS reading tells you the strength of the signal out from the DAC. The DAC outputs a certain voltage that controls the power amp (or control amp).

The dB SPL thingy going on in the room is about the sound pressure in the listening position (or any other choosed position as 1m distance i front of speakers) from the output of the speakers.

As Narcoman mentioned, we could skip dB and call it for what it is.. max level out from the DAC may be 2V rms or 8V rms or what ever and the speaker output produces a pressure that in the physical world is measured in Pascal. The hearing limit, 0db usualy is refered to 20uPa.

If you hae a DAC that puts out 2V rms at max (0dBFS) then you will see 1V if you reduce the level in your DAW to -6dBFS.


The gain of power amps and the speakers sensitivty to voltage (which the power amp controls the speaker with, amps are called voltage sources) differ a lot and therefore there are no other relation between dBFS and dB SPL then a relative one. Change the DAW setting from -12dBFS to -15dBFS and the speaker output will follow from let's say 92dB SPL to 89dB SPL.

It's a calibration thing and you can choose freely what kind of reference level in the DAW that matches a certain output from the speakers.


/Peter
Old 2nd February 2009 | Show parent
  #8
Gear Maniac
 
jujumies's Avatar
 
🎧 10 years
I thought I got it but then I tried to measure my monitor output with a measurement mic. So I wanted to figure out how loud I'm monitoring when I mix. -20Dbfs is 83SPL and that's clear but how to figure out how loud I am monitoring (in Dbfs) when the strength of incoming signal is dependent on the mic amp gain (if you use mic)???

Simo Orpana
Studio Bad Mama
Old 2nd February 2009 | Show parent
  #9
Lives for gear
 
🎧 10 years
Measuring absolute SPL you need a mic of known sensitivity (usually expressed in mV/millivolts output at 94dB SPL) and you need to measure that voltage with a scope, DVM or with a calibrated micpre and AD.

Easier is to find out the sensitivity for the speakers and use a DVM at the speaker inputs, which can only be done easily with passive monitors.


/Peter
Old 2nd February 2009 | Show parent
  #10
Moderator
 
narcoman's Avatar
 
🎧 10 years
Quote:
Originally Posted by jujumies ➑️
I thought I got it but then I tried to measure my monitor output with a measurement mic. So I wanted to figure out how loud I'm monitoring when I mix. -20Dbfs is 83SPL and that's clear but how to figure out how loud I am monitoring (in Dbfs) when the strength of incoming signal is dependent on the mic amp gain (if you use mic)???

Simo Orpana
Studio Bad Mama
You don't monitor anything in dB(FS). Your speaker produce sound measurable in dB(SPL). Get a measurement device and set playback at -20dB(FS) on your DAW and set your monitors so that this plays at 83 dB(SPL). -20dB(FS) is only equivalent to 83dB(SPL) when you define it and set it up as so. It isn't automatic!! You could quite easily set -20dB(FS) to be 116.2 dB(SPL) - but I wouldn't advise it !!
Old 2nd February 2009 | Show parent
  #11
Gear Guru
 
theblue1's Avatar
 
🎧 15 years
Quote:
Originally Posted by jujumies ➑️
I thought I got it but then I tried to measure my monitor output with a measurement mic. So I wanted to figure out how loud I'm monitoring when I mix. -20Dbfs is 83SPL and that's clear but how to figure out how loud I am monitoring (in Dbfs) when the strength of incoming signal is dependent on the mic amp gain (if you use mic)???

Simo Orpana
Studio Bad Mama
SPL is an attempt at an absolute measurement. A sound source of a given loudness, when measured by an SPL meter from x feet will produce y dB SPL. If you have the same sound source and loudness and the same SPL measuring position, you'll get the same loudness.


dB Full Scale, on the other hand, is measured relative to the maximum level of a digital signal. If that signal is at the maximum loudness, it is by definition 0 dBFS -- the so-called digital brick wall. If the signal is 6 dB below the maximum digital level, that level can be said to be -6 dBFS. (Well, duh, hunh? heh )

So let's say you have a -6 dBFS scale coming out of your DAW and going to your DA converters and then to a power amp and speakers (powered speakers, whatever). The SPL level that you would measure in the room would be dependent on the amount of amplification, the efficiency of the speakers, and the room itself. But if you were, say, to turn the signal in your DAW down by 12 dB more -- the signal in your room would drop by a ratio roughly 12 dB. (Give or take, speakers and room responses are not always strictly linear, but you get the drift.) A change of z dB in the DAW's signal should produce a change in room SPL level roughly the same.

Now, let's say your SPL meter says that the sound in the room at your listening spot is 80 dB SPL. Because dB SPL is intended as something of an absolute measure, you can have some assurance that the loudness will be the same in some other listening environment when the SPL meter reads 80 dB SPL --or in the same room, tomorrow, which is really the focus of using an SPL meter in your studio to help you maintain a personal 'standard' monitoring level.


I fear my attempt at illustrating the differences is superfluous but maybe it'll help.
Old 2nd February 2009 | Show parent
  #12
Gear Maniac
 
jujumies's Avatar
 
🎧 10 years
theblue1, it actually did help. I have studied the whole day this issue and I think I am getting the picture now. I somehow simplified these different scales way too much. And previously I haven't been in a situation where I have needed the knowledge...

Simo Orpana
Studio Bad Mama
Old 2nd February 2009 | Show parent
  #13
Gear Addict
 
RockManDan's Avatar
 
🎧 10 years
i like to look at it like measurements of distance. a db is like an inch. just saying x number of Db is like saying 10 inches....10inches from where? you can say 'turn up the snare by 5 db' but you cant say 'turn the snare up to 5db' because theres no frame of reference. DBFS is like saying 'x db from the top' or like saying 20 inches from teh ceiling. expressed in figures like -9 or -18. thats where you have figures that meaures peaks and RMS levels. then you have DB(SPL) which is the equivalent of saying x number of inches high your pencil is, but taking into account how high your desk is, just like you have to factor in how loud your monitors are set. You cant just set your monitors to a certain setting and expect it to always be 83Db. it depends on what you're pumping into them. So to mix at 83db you have to calibrate your monitors so that a -18dbfs signal comes out to your ears in teh room as 83db, but then you have to make sure that everything you monitor after that is coming out as -18dbfs in your daw or recording desk.

in short its just a unit of reference, that can be used as an absolute only when there is one to refer to.
-dan
Old 2nd February 2009 | Show parent
  #14
Gear Guru
 
theblue1's Avatar
 
🎧 15 years
Quote:
Originally Posted by jujumies ➑️
theblue1, it actually did help. I have studied the whole day this issue and I think I am getting the picture now. I somehow simplified these different scales way too much. And previously I haven't been in a situation where I have needed the knowledge...

Simo Orpana
Studio Bad Mama
It took me more than a little while to get my head wrapped around the whole business, too.

When I was in a recording program at a 2 year community college (I was actually in two at different schools, but the other one was very, very loose and the original teacher knew almost nothing, technically, and actually taught people some really cockeyed stuff) the teacher made us all buy scientific calculators and work through all the dB math. While I could do the math, it still took me a while before I could really get a feel in my mind for what was actually going on.

Don't feel bad about it taking a while to sink in...
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