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.

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

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

Sounds like a fun experiment

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.

And it's not a linear scale.

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.

Here you go:

http://www.nwdreamer.com/misc/vuvuzela.mp3

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.

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