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Underloop
Advanced Member
United Kingdom
3,895 posts
Joined: Mar, 2002
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 Posted - 2006/03/13 : 14:07:56
OK guys, I've had a few people asking me recently about the whole dB thing when looking at volumes - and why so many different scales etc so I thought I'd do a quick tutorial on it, definitions and general useful things to know.
Scales
To put it simply, all dB scales are a measure of the volume relative to another volume. However, you will see several different scales. The 2 main ones are:
-120dB (its actually minus infinity, but -120 is a nice round figure) up to 0dB
0dB up to 120dB (infinity again, but 130dB off the top of my head is the level at which you start to feel pain).
The reason that these 2 scales are different is due to the reference value that the sound is measured against.
With the negative scale the sound is usually measured against another reference sound source, or limits within the equipment or sound file before distortion occurs. For instance 0dB would be the point in a wav file where the signal is at its maximum peak.
The positive scale is taken against a standard value. For acoustic noise this is usually 20uPa (20 micro pascals - a measure of pressure). As a guide a typical nightclub would be at 110dB, whereas a jet aircraft taking off would be about 130dB if you were stood at the edge of the runway (not advisable!!). This is often referred to as the dB SPL (Sound Pressure Level) scale.
Peak or rms
As anyone involved with music knows, a sound wave does not just sit happily at a single level, it oscillates between positive and negative, so what point do you take as your level? 2 popular measures are the peak amplitude (the maximum value) or the rms (root mean square) which effectively gives the average level of th signal. The rms is the most common value used as this is a good indication of what the ear interprets as volume. A signal may have one or 2 peaks in its time history, so will have a high peak, but the rms may be significantly lower, so you would hear a lower level signal.
Amplitude or Power?
*sigh* yet more options! OK, this is where stuff starts to get a bit more confusing. It really only relates to when playing about with amps and electrical power going into a speaker. There are 2 equations used to calculate the dB level. One is used for power (watts) and the other for amplitude (everything else!). These are below for any other geeks who are interested lol:
Power: dB = 10*log10(power/reference power)
Amplitude: dB = 20*log10(amplitude/reference amplitude)
What does this mean? Well firstly, dB is a log scale! Also, the only differece between the equations is one is multiplied by 10 and the other by 20. Whoopy do I can hear you thinking so I'll leave the geeky section and get to what it actually means. Basically, when measuring power, a 3dB increase will double the level, whereas on the amplitude scale, a 6dB increase doubles the level! 99% of the time you will only need to be bothered with the amplitude scale, so your looking for 6dB increases and decreases!
Cutting the Crap
OK, now you can probably see why there is so much confusion over the whole dB thing. Here are the main things you need to know if your producing/engineering:
- If the dBs are being seen are negative and you have no indication of a reference then 0dB is your top limit - do not exceed it!!!
- If the dBs are positive and you have no indication of a reference then you'll probably end up hovering round 100-110dB, but it all depends on what your actually measuring.
- If you are given a reference then you are comparing a signal to another signal or limit. Use your own judgement
- Generaly a 6dB increase or decrease is either doubling or halfing your signal respectively. For instance, if you have a wave form reading -6dB that is half the maximum level it could be. If it is at -12dB then it is at 1/4 of the level it could be (1/2 x 1/2 as you have gone down 2 steps of 6dB). This is a confusing aspect of log scales
OK, thats pretty much all you need to remember for now.
At some stage, if theres any need for it (ie let me know!) then I will put together a frequency tutorial covering amongst other things the aspects of the spectrum analysis tool found in packages such as Soundforge, and how frequencies relate to musical notes.
I hope this was useful. Any questions then post them below along with any feedback both good AND bad! I can only write better tutorials that help you guys better if I know areas I need to improve on!
Cheers
'Loop
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"We don't stop playing because we grow old;
we grow old because we stop playing."
- George Bernard Shaw
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bulby_g
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United Kingdom
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Joined: Apr, 2004
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 Posted - 2006/03/13 : 14:33:36
This is great well worth the read thank you. I for one would much appreciate a frequency tutorial if you get the time :). I find myself EQ'ing stuff by ear without a great deal of knowledge or understanding of what i'm doing.
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The Deviant
Senior Member
United Kingdom
329 posts
Joined: May, 2004
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Posted - 2006/03/14 : 12:16:34
Thanks, but I have a question, I have read (in computer music mag) That a 3db gain is equal to a perceived double in volume, so is 6db acually a double in volume and 3db only perceived to be. Or is that 3db thing just bollocks.
Also do you know anything about Line levels, My soundcard has 3 different input and output level settings, and they are, +4dBu, consumer and -10dBV. I can put +4dBu singnal into my monitors and its and alot louder than -10dBV, but I can only put a -10dBV signal into my receiver. IF you have anymore info on that I be happy if you share it. (I hate having to know how and why stuff works  )
A frequency tutorial would be good, that is something Im really interested in and I hope I can learn more.
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Underloop
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Posted - 2006/03/14 : 22:54:51
OK, this is where it all gets a little bit confusing! OK, lets take a real life situation. You have a 100Watt amp, and you switch it for a 200Watt amp you increase the power output by just 3dB - this is the power scale. However, on a traditional mixing desk/vu meter it will be measured in volts, so a 6dB increase is required to double the power! You can try it for yourself by plugging in "2" into the equations for power and amplitude dB above (remember that the bit in teh brackets in the equations are simply ratios, so 2 represents a double).
Basically, wether it is 6dB or 3dB depends entirely on what you are measuring, whether it is amplitude or power! I deal with dBs every day of my working life and I still find it pretty confusing. Half of the reason for me writing this was to gain a better understanding myself! heh One thing which may be worth considering - and not something I have the chance to check right now as I'm on my laptop not studio PC, is wether the scales in popular DAWs has been altered to represent the power scale rather than amplitude. One thing I will say though is that music is all to do with hearing, not whats on the level meters, no matter how pretty it is! lol Trust your ears! This tutorial is there as a guide more than a rule. Check the scale in whatever dAW you use though by adjust ing a signal by 3dB and then by 6dB and see what you think - I'm off to do this later, but I hardly ever use the meters as anything more than a rough guide for offline processing - when processing/mixing online I always go off my ears.
Regarding the levels your card will output (you got an MAudio by any chance?) firstly these are different scales, or more simply they use different reference levels. Heres a rundown of the different references and their names. It probably better explains some of the stuff I was mentioning before:
dBV represents the level compared to 1 Volt RMS ie 0dBV = 1V
dBu represents the level compared to 0.775 Volts RMS with an unloaded, open circuit, source (u = unloaded).
dbFS - relative to digital full-scale. This is the negative scale I mentioned earlier
dB SPL - A measure of sound pressure level - used for measuring acoustic levels more usually
When we talk about reference, a good way to remember it is that the reference is the value that will give 0dB
You probably already know, but in case anybody reading doesn't, the output from your soundcard is in volts so the output in volts is a good indication of the levels you see out of your speaker.
re-arranging the dB equation above for amplitude you get:
amplitude = reference*10^(dB/20)
Plugging the above values into that equation will give you the voltage outputs (I think its at maximum output, or 0dBFS but I'm not 100% sure)
+4dBu ref = 0.775V Therefore output is 1.23V
-10dBV ref = 1V Therefore output is 0.316V
This means that -10dBV is approx 11.8dB down on +4dBu (or 1/4 of the level)
I may be wrong but I always thought that consumer grade WAS -10dBV, but I think my soundcard has both consumer and -10dBV so I shall investigate later lol. I always go for +4dBu when I can as I find it gives a hotter signal as you nnoticed, and with a better dynamic range, but check your equipment first for its maximum rms voltage input!
I hope you followed all that, and that it makes things a bit clearer!
I've just stumbled upon a very good article on SOS when I was checking my facts for this which can be found here
http://www.soundonsound.com/sos/1994_articles/feb94/decibels.html
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"We don't stop playing because we grow old;
we grow old because we stop playing."
- George Bernard Shaw
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Underloop
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United Kingdom
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Joined: Mar, 2002
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Posted - 2006/03/14 : 23:31:11
OK, just come upstairs to have a play around in the studio PC, and in Logic a -6dB drop on the master fader does appear to halve the volume, but I would say it was subjective. -3dB I wouldn't have said was half though.
I then went into Soundforge and similar things were heard. Looking up in the Soundforge online help section is the following info:
quote:
Decibel (dB)
A unit used to represent a ratio between two numbers using a logarithmic scale. For example, when comparing the numbers 14 and 7, you could say 14 is two times greater than the number 7; or you could say 14 is 6 dB greater than the number 7. Where did we pull that 6 dB from? Engineers use the equation dB = 20 x log (V1/V2) when comparing two instantaneous values. Decibels are commonly used when dealing with sound because the ear perceives loudness in a logarithmic scale.
In Sound Forge software, most measurements are given in decibels. For example, if you want to double the amplitude of a sound, you apply a 6 dB gain. A sample value of 32,767 (maximum positive sample value for 16-bit sound) can be referred to as having a value of 0 dB. Likewise, a sample value of 16,384 can be referred to having a value of -6 dB.
Which would tend to imply that Computer Music can be misleading 
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"We don't stop playing because we grow old;
we grow old because we stop playing."
- George Bernard Shaw
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The Deviant
Senior Member
United Kingdom
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Joined: May, 2004
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Posted - 2006/03/15 : 18:21:27
Thank you for taking the time to write that  , Much appriciated mate, It cleared alot up for me. Im slowly starting to understand it better.
Yeah I have and M-Audio soundcard. Never used the consumer output though, so don't know what the level difference is compared to the other two.
Thanks.
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skid
New Member
Australia
68 posts
Joined: May, 2003
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Posted - 2006/03/16 : 13:48:15
wow nice post...i'm just learning this stuff in my Digital Musics class at uni and it was pretty confusing...this guide helped me understand it better...
hope to see u post more tutorials and guides like this in the near future
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Underloop
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United Kingdom
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Joined: Mar, 2002
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Posted - 2006/03/16 : 19:06:59
Glad someones finding it useful at least!
OK, I *will* write a tutorial on frequency and all that shiznit, *but* on one condition!
Currently the donations for the Progress Event in aid of Save The Children are sitting at just over £200. When the donations hit £250 I will post a tutorial up!!! 
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"We don't stop playing because we grow old;
we grow old because we stop playing."
- George Bernard Shaw
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