so, ..what exactly what is a deciBel?

[sabianq]

so, ..exactly what is a deciBel?

hey all, im new to these parts.

i was just looking and saw a couple of threads inquiring about decibels,

a little history about me.

i currently work for the government as an audio/video engineer, i have been in the field for 15 years, and have come across many different, strange and cool set ups.

i have done work for IBM, AOL, Intel, the President of the United States and many other high profile entities.

my home system includes
Ashly parametric EQ's
Mackie 16 X 8
Raine active crossovers
QSC and ashly amps
various speakers Genelec, Tannoy, Optimus (yes i know, radio shack), Bose Meridan and Paradigm.

Shure and AKG microphones


I have been recording for just under 4 years
i am currently working on my studio
http://www.audioforums.com/forums/showthread.php?t=13097

i frequent a forum called

Audioforums.com

the reason i wanted to post here and describe the decibel was it seemed to me that some of you were struggling on the meaning of decibel

I hope this helps.


one common misconception is that 0 dB means no noise level,
I had the opprotunity to go to a demonstration where a company was showing off a compact new line array system called the Cobra System, (a very impressive system, capable of super clear and crisp distortion free 127 dB spl at well over 1 meter) that system rocked.

Either way, the "engineer" after the demonstration proceeded to tell the room that 0 decibels ment "as quiet as it can get and that there are no places on earth where there is 0 decibels" i promptly raised my hand and voiced my dissent but was shrugged off. I will not name the company for ethical reasons. however, that incident prompted me to start this thread.



Decibels are always expressed in ratios and is always a relative quanity, so a decibel is a ratio compared to a reference.
here is an example:

lets look at lengths, what if all lengths or distances were compared to a meter.
you would say, "this rope is 10 time longer than one meter"

it is the same with a decibel although the reference is sometimes applied.
say +10 dB is ten times more than the reference, which is defined as 0 dB.

decibels are logarithmic ratios
so if i want twich as much, i actually only want "6 dB more" [20*log (2)=6]

dBu, dBm, dB SPL, dBFS are expressions of decibels with defined references.
in 1960 the term dBu was introduced by the Neve Corporation, and it means decibels compared to a voltage reference of 0.775 volts.(which is why some of your recivers have a 0 dB rating on the dial) dBm is a decibel compared to a power reference of 1 milliwatt. and dBFS is a decibel compared to full scale PCM: that is dBFS represents the highest digital level PCM can encode.

so one can see how it is possible to have negative decibels.


now if you look at dB SPL

it is a little different.
it is still a reference but a reference to the threshold of human hearing

here is a brief history.

In the early 1900's Bell Laboratories did the most comprehensive study of human hearing ever. They decided to come up with a standard measurement of how loud we hear things. The initial idea was that they would increment the measurements by twice as loud as the previous. Each increment would be called a "Bel" out of respect for Alexander Graham Bell. The plan was to divide the Bel into 10 equal parts (the decimal system) and call each part a "deciBel". So it was established that 1 deciBel was 1/10th of twice as loud.

it goes on to explain,

A base reference was needed so that the measurement system could be used accurately. Methodic and extensive testing yielded information leading the scientists to conclude that black women, age 18-22, had the best hearing of all people. Bringing in extremely sensitive barometric pressure sensors they measured the changes in the air pressure of the room the subjects were tested in. Using a 1kHz tone they found that the quietest sound their subject could hear was at .0002 Micro-Bars pressure. This is the point that Bell Labs referenced to 0 dB-SPL (Sound Pressure Level).

one of the more interesting findings during the study was that human hearing is not linear, meaning that twice the barometric pressure does not create a sound that we perceive being twice as loud. The following chart represents the results of their findings.


0 dB-SPL @ 1kHz = .0002 Micro-Bars
10 dB-SPL @ 1kHz = .000635 Micro-Bars -OR- .0002 * 3.157 = Twice as loud

the whole article can be read here at:


http://www.ptme.com/et/audio/reference/sound/dB-SPL.htm


so in essence,
0 dB SPL represents the minimum pressure of a sound wave that can be precieved by a human ear.

10 dB SPL is twice as loud as 0 dB SPL
and 20 dB SPL is twice as loud as 10 dB SPL.

30 dB SPL is twice as loud as 20 dB SPL and so on

so if you do the math, you find out that the loudness increases logarithmically as you go up in dB SPL.

50 dB SPL is 32 times louder than the threshold of human hearing.
60 dB SPL is 64 times louder than the threshold of human hearing.

and 120 dB SPL is 4096 times louder than the threshold of human hearing.

and 130 dB SPL is 8192 times louder than the threshold of human hearing.

you can see why it would take alot of power to get any louder.

i hope this helps.
anyway,
have a good day

 

[Sheep]

At first, I thought you were asking about it, and then I read the first part of your post. If you didn't know what a decible was, and you did all that, you must have some dumb luck on your hands

By the by, welcome to the forums.


Sheepstar

 

[Mr. Lamb Fries]

At first, I thought you were asking about it, and then I read the first part of your post. If you didn't know what a decible was, and you did all that, you must have some dumb luck on your hands


Sheepstar

I thought the same thing, Sheep

That was very interesting! thanks for the DETAILED explaination.
BTW, Welcome to the forum (echo, echo, echo)...Keep the posts comming sabianq!

 

[MDS]

Excellent overview of the dB, sabianq. However, I think it would be beneficial for you to clarify this point:

in 1960 the term dBu was introduced by the Neve Corporation, and it means decibels compared to a voltage reference of 0.775 volts.(which is why some of your recivers have a 0 dB rating on the dial)... dBFS is a decibel compared to full scale PCM: that is dBFS represents the highest digital level PCM can encode.

People tend to get confused by 0 dB when we talk about the 0 dB point on the receiver's volume display vs 0 dB meaning 0 dBFS for PCM. I tried to explain it in another thread called 'What is 0 dB on a Yamaha ...'.

The wording you chose implies that 0 dB on the receiver's volume display is .775 volts - it is not. [I don't think that is what you meant, just that it could appear that way to those who are still confused] 0 dB is an arbitrary setting somewhere below the receiver's max rail voltage and as you adjust the volume control from that point, the voltage swings up or down by .775 volts (changing the output SPL by 1 dB). [Actually from any point, but the relative volume display of -infinity to +something is used mainly so that 0 dB can be chosen as the 'reference level' from which anything below is a negative dB reading and anything above is positive].

 

[Mudcat]

Sort of like something I posted last year

See post #4 in this thread.


http://forums.audioholics.com/forums/showthread.php?t=5285

 

[sabianq]

Excellent overview of the dB, sabianq. However, I think it would be beneficial for you to clarify this point:

in 1960 the term dBu was introduced by the Neve Corporation, and it means decibels compared to a voltage reference of 0.775 volts.(which is why some of your recivers have a 0 dB rating on the dial)... dBFS is a decibel compared to full scale PCM: that is dBFS represents the highest digital level PCM can encode.

People tend to get confused by 0 dB when we talk about the 0 dB point on the receiver's volume display vs 0 dB meaning 0 dBFS for PCM. I tried to explain it in another thread called 'What is 0 dB on a Yamaha ...'.

The wording you chose implies that 0 dB on the receiver's volume display is .775 volts - it is not. [I don't think that is what you meant, just that it could appear that way to those who are still confused] 0 dB is an arbitrary setting somewhere below the receiver's max rail voltage and as you adjust the volume control from that point, the voltage swings up or down by .775 volts (changing the output SPL by 1 dB). [Actually from any point, but the relative volume display of -infinity to +something is used mainly so that 0 dB can be chosen as the 'reference level' from which anything below is a negative dB reading and anything above is positive].

sorry about that
maybe this is a little better.

0 dBu A reference voltage point equal to 0.775 Vrms. This reference was originally labeled dBv (lower case) but was too often confused with dBV (upper case), so apparently, dBv was changed to dBu.

0 dBm A reference power point equal to 1 milliwatt. To convert into an equivalent voltage level, (the impedance must be specified). For example, 0 dBm into 600 ohms gives an equivalent voltage level of 0.775 V, or 0 dBu (see above); however, 0 dBm into 50 ohms, for instance, yields an equivalent voltage of 0.224 V.

so your pre amp voltage and impedance determines where the 0 dB is in reference to your amp voltage.


i think thats right MDS.
feedback?

 

[mulester7]

my home system includes
Ashly parametric EQ's
Mackie 16 X 8
Raine active crossovers
QSC and ashly amps
various speakers Genelec, Tannoy, Optimus (yes i know, radio shack), Bose Meridan and Paradigm.

.....SabianQ, I'll say WELCOME, and let's hear your opinion of those QSC amps for home usage, and which ones do you have?.....outstanding post on them there decibulls....I only know lots of 'em sound real nice with the right stuff....mercy, the people who are just showin' up....(makes me wonder if Clint made a phone call and slipped in a Ringer on us)......

 

[BMXTRIX]

It seems that we are discussing dB-SPL and not unamplified signals.

Question: How does this apply to unamplified signals. Like what is coming off the CD player runs through a preamp at line level at a specific voltage.

Is that voltage, when your receiver shows 0dB - .775 volts? Is it arbitrary as indicated below? How is it on pro-sound mixing gear?

I would think (and am happy to be wrong) that receivers would pass .775 volts to their amplification section at 0dB at which point the power of the amplifier combined with the efficeincy of the speakers creates dB-SPL.

I get stuck using some commercial audio mixing equipment and have levels to set which, unfortunately, has to have been self-taught. Any thoughts appreciated.

I did read what MDS said below... but why would 0 dB on a receiver be arbitrary? I don't get it.

 

[sabianq]

i have the rmx850 and it is a super quiet amp over a s/n ratio of over 100 db, I love it for public address applications.
the QSC amps are very robust amps and run very quiet (low fan noise)

i want to get some qsc PLX 2402 amps. <drool> the s/n ratio is a breathtaking, and the dynamic range and frequency response is absolutly amazing.


in my studio i am using ashly amps with active crossovers.

super quiet and very robust.

i always use active crossovers (before the amp in the signal path so the only thing connecting the amplifier to the driver is the speaker cable) this presents the maximum damping factor at all times, regardless of frequency. an added benefit is that my whole system is balanced.

for work we use QSC powerlight series and crown macro-tech with eaw drivers in linear spiral line arrays.

the auditorium i do most of my work in has meyers drivers and crown amps rane crossovers and parametric EQs with an Electro Voice 48 channel mixing board. Shure wireless and feedback elimination processors (i actually dont like the feedback processors, (IMO i think they are noisy) i would rather use gate/ compressors/limiters. but i dont make the rules (yet anyway).using an Electro Voice 48 channel mixing board.





hahahh ho heheh.
ahhemm <cough> <hack>

anyways, you all seem to be pretty cool people.
im glad i found this forum.

 

[MDS]

I did read what MDS said below... but why would 0 dB on a receiver be arbitrary? I don't get it.

This is my favorite topic because it is so damn confusing.

The key point to get from sabianq's posts is that a decibel is a ratio of two quantities. He gaves us the rundown of all the different types of measurements and their reference values.

Let's take the 3 that are most applicable to the discussion of receivers:
1. 0 dB SPL
0 dB SPL is the threshold of hearing; ie the lowest perceptible sound. 120 dB is the threshold of pain. 0 dB has a defined value (in microbars or pascals) and being a ratio, a dBSPL is a ratio of the value assigned to 0 dB and the other value.

2. 0 dBFS ('full scale digital')
0 dBFS is the maximum value that can be encoded digitally and depends on the bit depth. Remember that the A/D process samples an analog signal so many times per second and assigns a number (the 'sample value') to represent the amplitude of the signal at each point in time. Using CD audio, the bit depth is 16 and the sampling frequency is 44,100 Hz. Therefore there are 44,100 16 bit numbers for every second of audio. Those values can range from -32,767 to +32,768. [2^16 = 65,536 (64K) but the values are signed because the waveform has both a positive and negative component, leaving only 15 bits for the actual value.]

So 0 dBFS for 16 bit audio is a value of -32,767 or +32,768. Any sample values lower than that are expressed in negative decibels - decibels below full scale. The test tones of a receiver are at -30 dB, a cd mastered in the 80s has an average power level of around -18 dB, and a modern compressed cd has an average power level of around -10 dB to -12 dB.

0dB in this case is the top of the range, while in contrast 0 dB SPL is the bottom of the range. 0 dBFS doesn't tell you what the output SPL will be in your room because that depends on how much the signal is amplified, the sensitivity of the speakers, the room, etc. One thing that is certain though is that a CD with a level of -10 dB is going to be louder than a CD with a level of -20 dB when the volume setting on the amp is the same.

3. 0 dB reading on the receiver's volume display
I said it was 'arbitrary' because it is not directly related to 0 dBSPL or 0 dBFS and you can't directly compare volume settings from one receiver to another. Think of each number as a step between min and max. The absolute scale might be from 0 to 100 while a relative scale might be from -80 to +20. They are the same - there are 100 'steps' between min and max. [As a side note, Onkyo manuals say that 82 on the absolute scale is equivalent to 0 on the relative scale].

An amp/receiver has a specific rail voltage (say 40 volts). The voltage always swings between -40 and +40 volts. The volume control is an attenuator. When you turn it below max, it attenuates the voltage. If the volume control is dB accurate, then moving it up or down one detent will change the voltage by an amount that causes the output SPL to go up or down by 1 dB.

0 dB on the display is chosen as the 'reference' point simply so that it mimics 0 dBFS of digital audio. You've calibrated to a known SPL when the setting is at 0 dB on the display so that anything below that is negative and anything above that is positive. While you can't exceed 0 dB in a digital sample (any analog signal that would equate to a value greater than the max gets truncated to the max), you can exceed 0 dB on the receiver because 0 is not the max (in my example above, the max was +20).

When sabianq said 0 dB is .775 volts he meant that 0 dBSPL (barely or not audible) is the setting where the receiver is putting out only .775 volts - not that .775 volts is output when the volume display reads 0 dB. If 0 dB on the display is roughly 80% of max, then the voltage would be roughly 80% of the rail voltage.

Maybe that helps to clear things up...but then again maybe it makes it worse.

 

[mulester7]

I always use active crossovers (before the amp in the signal path so the only thing connecting the amplifier to the driver is the speaker cable) this presents the maximum damping factor at all times, regardless of frequency. an added benefit is that my whole system is balanced

....outstanding.....brought a tear to my eye......

 

[sabianq]

aww..

you guys are so cool..

i have been very busy at work latly and havent had time to follow up.

i hope my situation will change.

 

[BMXTRIX]

MDS - I kind of get what you are saying. I understand the concept of it being arbitrary.

Then again - I don't.

Line level audio comes in carrying a certain power behind it. If that power were to hit an un-attenuated amp. Full power... it would be outputting a strong wattage to some speakers (depending on amp, etc.)

If you were to take that same line level audio, and send it through a audio mixer and set that audio mixer to 0db, it would pass that audio through and out at the exact same level it came in. Or it should. (Polycom Vortex and Gentner AD800 do this).

On the other hand, if you bump up, or down the line level audio coming in, you are changing the voltage output on the backside sending a lower, or higher level signal out. Or is it still line level out with peaks and valleys altered?

Most of all - in a receiver, why isn't the same true? Is it in the CD digital realm that the change is happenning? I don't know. I don't think that the bits have anything to do with it. I would think that there could be positive gain or negative gain pushed onto the incoming line level audio before that signal is passed onto the amplificaiton system. At 0db I would think that the output would be identical to the input. All preamp level.

Or is the amp being attenuated?

I don't think that is the case though - I think it is the signal to the amp that is being adjusted.

I am fine being wrong - this is not something I care that much about really. But, it does affect me when I play with digital mixers and have used them in the past. Brining levels up and down to help balance a room out for video teleconferencing and voice reinforcement. 0db is the magic number on the mixers. It isn't arbitrary for sure within them.

 

[MDS]

Bmxtrix,

Think of all these voltage manipulations as scaling operations. For a different perspective, consider that you want to write code to generate random numbers between 0 and 10. Your scale is 0 - 10. You use a function like rand() that generates random numbers between 0 and 1 and then multiply by 10 (and drop the fraction). You've scaled the range of 0 - 1 to the range 0 - 10.

Increasing the SPL by 1 dB requires 'scaling' the voltage by 1.26 [10*Log(1.26) = 1 dB].

0 dB is just a mark on a given scale but its meaning varies:
1. 0 dBFS for digital audio is the top of the range as explained before (a sample value +32767 or -32768 for 16 bit audio). -infinity is the bottom of the range (a sample value of zero). If you have a wave file and its average power is - 10 dB and you use the volume process in an audio editor to increase the level by 1 dB, it will multiply every sample by 1.26. The signal is now 'hotter' and when it gets scaled to the preamp voltage level between 0 and 2 volts, it will be higher (closer to 2 volts) than if the signal were at a lower level, like -12 dB.

2. 0 dB on your mixer is the point where your incoming voltage is neither increased nor decreased. For simplicity, assume the mixer's scale is only -1 to 1. At -1, an incoming voltage of 2 volts is changed to 1.48 volts. At 0, the voltage remains 2 volts. At +1, the voltage is changed to 2.52 volts. If the device to which that signal is sent cannot handle more than 2 volts then the signal is 'clipped'.

3. The receiver is going to scale the incoming voltage in a range of 0 - 2 volts from the preamp to a range that is 0 to its max rail voltage. The number on the volume display indicates by how much the voltage is going to be increased. So 0 dB on that scale is not related to the others - it is one point in the range from min to max. If instead of a relative volume display of -80 to +20 you use the absolute display of 0 - 100, it becomes clearer. [Again Onkyo says that 82 on the absolute scale is equal to 0 dB on the relative scale for their receivers]. Given that 82 (or 0) is near the top of the range, it can't possibly be the point where the voltage is not changed as in the mixer case.

Scaling, scaling, scaling...

 

[BMXTRIX]

...it can't possibly be the point where the voltage is not changed...

I know I sound like a pain...

But, why? This sounds EXACTLY like what they mean. They know clipping is likely to occur at a level above the voltage that is incoming and they have tested it to be an 18db increase over incoming line levels. This makes total sense to me.

It doesnt make sense to me that 0db in a receiver would be a positive or negative manipulation of incoming voltage. It seems just the opposite would make much more sense.

NOT that I expect companies to actually make sense.

And I notice you are the only one answering these questions. Which I greatly appreciate, but wish there were half a dozen people putting in their two cents as well. With more info, or the same.

*sigh* - I get it, I'm just having trouble believing it. I guess that's what I get for being an control system designer. I question every little thing I look at and believe manufacturers for about half a nano second.

Thanks for your patience.

 

[MDS]

Let's say the receiver's volume scale was labelled A-Z with A being the minimum setting and Z being the max. Would it make sense that the volume increases as you turn the knob from A to T, then remains constant at U, then continues to increase from V to Z?

The markings on the volume display are just a range between min and max.

 

[rschleicher]

Not to add to the confusion, but it is also worth pointing out that the formulas for computing the dB difference between two voltage levels, and between two power levels, are a bit different (but consistent with each other).

dB = 10log(P2/P1) (power ratio, e.g. watts)
dB = 20log(V2/V1) (voltage ratio)

If you consider a fixed impedance Z (like a speaker impedance), you can see that the two definitions above are consistent. P (watts) = V (volts) ^2 (squared) / Z (ohms).

Thus the ration P2/P1 is equal to the ratio (V2^2/Z)/(V1^2/Z) = (V2/V1)^2

Therefore dB = 10log(P2/P1) = 10log((V2/V1)^2) = 20log(V2/V1)

The "rules of thumb" are that 10x power = +10dB. But 10x voltage is actually +20 dB, since it is 100x the power.

Similarly, double the power is +3 dB, but double the voltage (and hence four times the power) is +6 dB.

Yet another reference-based dB version in common use is the dBm, which is dB relative to a power of 1 mW (milliWatt). dBW is dB relative to 1 Watt. Thus 0 dBW = +30 dBm.

In testing amplifiers, it is common to assume an 8 ohm reference impedance. 0 dBW = 1 Watt = 2.83 Vrms into an 8 ohm load.

Sorry about rambling on!

 

[sabianq]

and dont forget, the volume control on your amplifier is actually a sensitivity control. when you turn up the "volume" you are actually increasing the sensitivity of the amplifier to the input signal.