Helpful ReplydBFS vs dB

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SH
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2011/05/02 23:24:29 (permalink)

dBFS vs dB

My daughter is using Sonar for a science experiment to measure the relative volume of different sounds.  The levels are -35dB, -25dB, -19dB (dBFS) etc.  When the question of the negative numbers came up, we Googled 'Decibels' and got pages of "+60 dB conversation level, +110 dB chain saw" etc.  That scale/method of measurement is probably more appropriate for her science experiment.  Question:  What (if any) is the relationship between the two dB scales?  I'm sure her teacher is more familiar with the positive dB scale.
#1
Karyn
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Re:dBFS vs dB 2011/05/03 06:38:04 (permalink)
dBFS = how much quieter a sound is compaired to a sound that would make your equipment read full scale.

dB (used in audio measurement) = How much louder a sound is when compaired to the threshold of human hearing. ( defined as a sound pressure level of .0002 microbar)

The important part to note (to impress teacher) is that the decibel unit is a comparative measure, NOT an absolute.  The decibel will tell you how much louder or quieter one sound is when compaired to another, it will NOT tell you the precise volume of a sound (although you could work that out)




For the purpose of the school work, dBFS would give perfectly usefull results, is readily available in Sonar and can be calibrated easily.   Just start with the loudest sound you're expecting to test and set the sound card input gain, (or portable recorder mic gain) to show Full Scale.  Now leave the mic gain alone, (put gaffer tape over it) and all your recordings/test will be relative to your loudest example.

Remember that for any of your results to be valid the sounds must be recorded with the same mic at the same distance to the sounds.      EDIT : or note the distance to the sound in the results. ex: Jet plane @ 100m
post edited by Karyn - 2011/05/03 07:32:55

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Karyn
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Re:dBFS vs dB 2011/05/03 07:31:22 (permalink) ☄ Helpful
I should have said that the advantage of decibels is that you can simply add/subtract them to convert between different scales.

So taking a few examples from Wikipedia

Vuvuzela  +120
Jack hammer +100
Passenger car @ 10m  +70
Washing machine +50
Whispering  +25
Leaves rustling +10

Now record your own Vuvuzela (you do have one don't you?) and set gain to read Full Scale.  0bDFS
The other items would show as
Jack Hammer -20
Passenger car @ 10m  -50
Washing machine -70
Whispering  -95
Leaves rustling -110


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Re:dBFS vs dB 2011/05/03 08:29:53 (permalink)
Sounds like a fun experiment

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Re:dBFS vs dB 2011/05/03 10:31:22 (permalink)
dB (used in audio measurement) = How much louder a sound is when compaired to the threshold of human hearing. ( defined as a sound pressure level of .0002 microbar)

Technically, this would be dB SPL.  (or see below about dB by itself...)

"dB" all by itself doesn't really have a meaning. dB scales are always in reference to some standardized level. And when the measurement equals that standardized level you get 0 dB-in-whatever-scale. 

In the case of dB SPL, when the measurement equals .0002 microbar you get 0 dB SPL, and it usually goes up from there.

In the case of dBFS, the reference is 'full scale' which, in digital, is the clipping point of the converter. So when you're at the clipping point you get 0 dBFS and you go down from there.

That's why you sometimes get positive and negative values for dB--it all depends on the reference level.

When you interface digital gear with analog, you want to calibrate levels. Since analog usually has 'headroom' you calibrate so that -20 dBFS equals 0 dBu on your analog meter--depending on the headroom of course.

With the K-20 Metering system you set your levels so that -20 dBFS RMS equals 83 dB SPL.  With dB, it's all relative. You have to know the reference level for dB to mean anything.

dB by itself is only used when talking about a change in level. "I turned it down 6 dB" or "that pre gives 60 dB of gain". In that case, you're talking about the change in an already assumed dB scale of some sort.
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Re:dBFS vs dB 2011/05/03 11:23:30 (permalink)
And it's not a linear scale.

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Re:dBFS vs dB 2011/05/03 11:39:02 (permalink)
Karyn
Now record your own Vuvuzela (you do have one don't you?)
Who here doesn't? *

*IIRC they handed them out at the door.
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Re:dBFS vs dB 2011/05/03 12:22:21 (permalink)

 
Time for all of you to head over to Beyond My DAW!
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Re:dBFS vs dB 2011/05/03 12:27:39 (permalink)
tarsier



dB (used in audio measurement) = How much louder a sound is when compaired to the threshold of human hearing. ( defined as a sound pressure level of .0002 microbar)

Technically, this would be dB SPL.  (or see below about dB by itself...)

"dB" all by itself doesn't really have a meaning. dB scales are always in reference to some standardized level. And when the measurement equals that standardized level you get 0 dB-in-whatever-scale. 

In the case of dB SPL, when the measurement equals .0002 microbar you get 0 dB SPL, and it usually goes up from there.

In the case of dBFS, the reference is 'full scale' which, in digital, is the clipping point of the converter. So when you're at the clipping point you get 0 dBFS and you go down from there.

That's why you sometimes get positive and negative values for dB--it all depends on the reference level.

When you interface digital gear with analog, you want to calibrate levels. Since analog usually has 'headroom' you calibrate so that -20 dBFS equals 0 dBu on your analog meter--depending on the headroom of course.

With the K-20 Metering system you set your levels so that -20 dBFS RMS equals 83 dB SPL.  With dB, it's all relative. You have to know the reference level for dB to mean anything.

dB by itself is only used when talking about a change in level. "I turned it down 6 dB" or "that pre gives 60 dB of gain". In that case, you're talking about the change in an already assumed dB scale of some sort.

+1 to this (and what Karyn said).

I will add this: dB's are a logarithmic way of expressing a ratio. A ratio of 1:1 = 0dB; ratios greater than 1 = +X dB; ratios less than 1 = -X dB.

When you specify a reference standard (like dBFS, dB SPL, dBu, dBV), you are comparing a single value to a predefined reference point ("X is -20dBFS" means X is -20dB compared to the predefined reference of 0dB = full scale digital). 

If you are comparing 2 values, you just use "dB" (X is +20dB compared to Y).

The most common audio reference points are defined like this:
0dBFS = Full Scale digital, the largest number that can be represented digitally
0dB SPL = 20 micropascals (Sound Pressure Level)
0dBu = .775Volts RMS
0dBV = 1Volts RMS, so 0dBV means X=1V RMS, +X dBV means greater than 1V RMS, and -X dBV means less than 1V RMS.

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MNorman
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Re:dBFS vs dB 2011/05/03 14:59:54 (permalink)
Just to complete this, 20 micropascals = .0002 micro bars = about 2.9 billionths of a psi
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Re:dBFS vs dB 2011/05/03 15:02:22 (permalink)
MNorman


Just to complete this, 20 micropascals = .0002 micro bars = about 2.9 billionths of a psi

A programming language used to describe tiny a pie bar?
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Re:dBFS vs dB 2011/05/03 15:41:26 (permalink)
Karyn, go pick up a cheap SPL meter for around $30. Your daughter will have fun going around measuring sound levels and afterward you can use it to calibrate your studio monitors.


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Re:dBFS vs dB 2011/05/03 17:47:28 (permalink)
This one's pretty good, if a little pricey:

http://www.radioshack.com/product/index.jsp?productId=2103667
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Re:dBFS vs dB 2011/05/03 17:48:26 (permalink)
MNorman


This one's pretty good, if a little pricey:

http://www.radioshack.com/product/index.jsp?productId=2103667

I have that one.
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Re:dBFS vs dB 2011/05/03 18:08:37 (permalink)
Yeah, me too.  Gets the job done.  And it has a hold for a tripod.
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SH
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Re:dBFS vs dB 2011/05/03 18:44:56 (permalink)
Karyn


I should have said that the advantage of decibels is that you can simply add/subtract them to convert between different scales.

So taking a few examples from Wikipedia

Vuvuzela  +120
Jack hammer +100
Passenger car @ 10m  +70
Washing machine +50
Whispering  +25
Leaves rustling +10

Now record your own Vuvuzela (you do have one don't you?) and set gain to read Full Scale.  0bDFS
The other items would show as
Jack Hammer -20
Passenger car @ 10m  -50
Washing machine -70
Whispering  -95
Leaves rustling -110
This really puts it into perspective and helps alot.  Thanks.
 
<< dBFS = how much quieter a sound is compaired to a sound that would make your equipment read full scale. >>
 
Does this mean "Jack Hammer -20dB" in the example above could be different perceived volume if recorded on or played back on different equipment?  Does this scale (dBFS) only refer to recorded sound? (ie. "... your equipment")
 
 
post edited by SH - 2011/05/03 18:46:05
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Karyn
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Re:dBFS vs dB 2011/05/03 21:01:48 (permalink)
SH

 
<< dBFS = how much quieter a sound is compaired to a sound that would make your equipment read full scale. >>
 
Does this mean "Jack Hammer -20dB" in the example above could be different perceived volume if recorded on or played back on different equipment?  Does this scale (dBFS) only refer to recorded sound? (ie. "... your equipment")
 
 
The perceived volume on playback would depend on the size of amplifier you play it through...   The recording level will be determind by the gain of your input amp.  This is where calibration is important.
 
If you buy an expensive (or cheap) sound level meter then you're paying for the calibration of its mic amp to be accurate.
 
 
dBFS (Full Scale) is simply a level that makes your meters hit Full Scale.  It is a good choice to use because it is very easy to calibrate. You simply choose the loudest sound you want to record and adjust the input gain untill the meters read Full Scale.

If you have two or three or more recorders, perform the same calibration on them with the same sound source at the same distance and all your equipment will give you the same readings.
 
Use a table of known levels from Wiki to convert your dBFS readings to dBspl if you wish as I showed you before.

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Re:dBFS vs dB 2011/05/03 21:42:55 (permalink)
What is the source of the sound? If you're trying use a meter to calibrate  or compare you need a steady state sine wave. 1 Khz sine is a typical reference for normal hearing.

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Re:dBFS vs dB 2011/05/03 23:00:50 (permalink)
Just when I think there's nothing to learn in the Coffeehouse.... :D

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Re:dBFS vs dB 2011/05/04 06:01:03 (permalink)
Rbh


What is the source of the sound? If you're trying use a meter to calibrate  or compare you need a steady state sine wave. 1 Khz sine is a typical reference for normal hearing.

Read the OP,  "My daughter is using Sonar for a science experiment to measure the relative volume of different sounds."
 
All she needs do is set the input gain so that the loudest sound she's testing reads Full Scale (0dBFS) then all the other sounds will be x y z dB quieter than that.   As long as the input gain is not changed (I sujested gaffer tape on the knob) then there is no further calibration to do.
 
 
Example:
 
She records the household washing machine from 1m distance. Sonar shows a level of -60dBFS.   She goes to Wikipedia and finds the SPL for the average washing machine is around +50dBspl  so she has a reference.
 
Now she records a guitar amp with her kid sister/brother/family pet bashing away on the strings.  Sonar gives a level of -30dBFS.
Straight away she has fullfilled the aim of the school project.  The guitar is 30dB louder than the washing machine. 
 
Further, she can take her reference sound from Wiki (Washing machine +50dBspl) add 30dB and say the guitar was playing at +80dBspl.
 
As long as the input gain is not touched that will work all day long with no further calibration.

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Re:dBFS vs dB 2011/05/04 08:17:58 (permalink)
+1 - that's exactly how to carry out that experiment, well...except for the gaffer tape.  Karyn's kind of gaff-"happy."

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Re:dBFS vs dB 2011/05/04 08:26:59 (permalink)
You can never have enough gaffer tape.

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Re:dBFS vs dB 2011/05/04 09:36:46 (permalink)

Karyn makes me proud.

I dunno why, she just does.

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Re:dBFS vs dB 2011/05/04 09:43:20 (permalink)
All I'm going to say on the subject is that gaffer tape really pulls out the hair.

Julibee, it's not so much that there's nothing to learn in the Coffee House as much as the fact that sometimes, we learn a little too much.
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Re:dBFS vs dB 2011/05/04 10:39:02 (permalink)
MNorman


Just to complete this, 20 micropascals = .0002 micro bars = about 2.9 billionths of a psi

This reminds me of Queen and Bowie.
 
(Oh yeah, and I also have that Radio Shaft SPL meter - works great!)

 
Time for all of you to head over to Beyond My DAW!
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Re: dBFS vs dB 2016/12/27 07:44:55 (permalink)



post edited by Tuqa.K.A - 2016/12/27 11:41:13
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Re:dBFS vs dB 2016/12/27 08:05:36 (permalink)
Karyn
...
The important part to note (to impress teacher) is that the decibel unit is a comparative measure, NOT an absolute.  The decibel will tell you how much louder or quieter one sound is when compaired to another, it will NOT tell you the precise volume of a sound (although you could work that out)
...



Still, one must remember that the particular measurement is "relative" to its environment, and from that one could say/conceive that the particular sound is around this or that level. 

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Re:dBFS vs dB 2016/12/27 08:23:09 (permalink) ☄ Helpfulby craigb 2016/12/27 09:33:08
A horse went in a bar, the bartender asked; "Why the long face?"...while others were concerned about the opposite end.
...I think that was around 6 years ago...for what it's worth.
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Re:dBFS vs dB 2016/12/27 09:34:01 (permalink)


 
Time for all of you to head over to Beyond My DAW!
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Re:dBFS vs dB 2016/12/27 10:54:05 (permalink)
               oooooohhhhhhh I love to hear a good horse joke as much as the next Pony ...
The thing is when it comes to horse jokes ,  why did the horse walk in the Saloon in the first place ?
Bars and Saloons an be very dangerous places for a horse unless they are carrying a Colt ( hey I made a funny )
Also when it comes to these horse walking into a Bar jokes, Why does the horse when he finally makes it inside the Saloon always get a total a$$hole for a bartender asking him the same old tired cliche of a question ?
                                                       "why the  long face "? 
 
             
 
When was the last time you had to spend weeks at a time on a hot and dusty trail with a cowboy sitting on your back ?
Most of the time these cowboys don't even know what a bar of soap looks or smells like when they are out on the trail .
Oh boy , did I happen to mention that after a few days of trail life these cowboy guys really start to smell ....
They all eat beans , drink coffee and sleep next to the campfire with all their cloths on ..
Then when they wake up in the morning they go to take a poop on the side of the trail and they don't even use toilet paper ...
Once we saddle up and get back on the trail , my day turns into an eternity of having to chug along all day long with out so much as a complaint out of me "Your faithful horse "
I sometimes have days where there is a lot of action going on ..I'm being chased by Indians , Stage coach robbers and getting shot at ..those day go by fast and if nobody gets hurt ...they are a lot of fun....
On the boring days where there is not much going on , the hours seem to last forever while I have to listen to the cowboy fart all day long while chewing tobacco and spiting it right past my head  ...shhhheesssh ..
 
I guarantee that all the A hole Bartenders in all these horse jokes would stop asking the stupid question of "why the long face " if he had the ability to walk a mile in my horseshoes 
 
When a horse walks into a bar all he want's is one thing ..to get so drunk he can forget his troubles and end up like this guy for a few hours 
            
             Drunk and soundly asleep in a field with out a care in the world .
 
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