Please explain negative decibels in layman’s terms

skydog

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If 0db is the threshold of human hearing, what constitutes a negative decibel?
 

DunedinDragon

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db are just a measure of sound pressure generally, but it depends on what it's measuring. For example when measuring a signal coming into a preamp you might see a measurement like -4db/+5db meaning the signal is minus 4db or plus 5db under/over unity (0db) depending on the peaks and valleys of the incoming signal. It's just a measurement value like inches.
 

mycroftxxx

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The decibel is a logarithmic unit of power or energy (it’s used both ways). The reference level depends on what physical process you’re measuring. For audio recording, which is what I expect you’re referring to, 0 dB would be set to some specific magnetic intensity for analog audio tape, or some number (maybe 0xFFFF) for 16-bit digital, etc. Negative values just mean lower than the reference.

For acoustic power, if you set 0 dB as the quietest sound a typical human can hear, you can still have acoustic power lower than that -it just won’t be audible to that typical human.

And just for completeness: the bel, named after Alexander Graham Bell, is the base unit; but the bel is rather large, so we typically use a tenth of a bel - the decibel - instead.
 

BlueWolf

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Decibels are typically used to measure Sound Pressure Level (dB SPL), but you have to keep in mind that a decibel is a relationship between two values of SPL. In particular, decibels are designed to allow us to comfortably talk about numbers of largely different magnitudes. A decibel expresses the ratio of one value to another on a logarithmic scale. The number of decibels in a given case is ten times the logarithm base 10 of the ratio of two SPL quantities. In simple words, decibels tell you that 10 to the power of this number divided by 10 gives you the ratio of these two quantities. For example, where x(n) means x to the power n, something that is 100 times as powerful as a reference value is 10(2) times more powerful, so that becomes 20 decibels. Decibels are logarithmic to make it easier to deal with numbers that have very large or sensitive ranges. For example, SPL, which is measured in Pascals, can range from 0.00002 Pa for the normal threshold of hearing to 200 Pa for the sound of a military jet taking off at 30 m. Representing numbers on a dial in .00001 Pa increments from 0 to 200, i.e. 20,000,001 values, is just not practical, so we instead use dB which collapses the range to 0 to 140 in 1 db increments, i.e. 141 values.

So how do we get to negative dBs? Well, while 0 decibels typically means you’re at the threshold of human hearing, this is only true because we are using the threshold of human hearing as the reference value. This not always the case. Decibels can be used when the reference value is set elsewhere. For example, when dealing with professional sound equipment, 0 dB refers to the loudest level before distortion begins. In this case, negative dBs can be a good thing. I know that my A/V receiver uses dBs in this way, so that when I am watching my TV, the volume adjustment is almost always in negative dBs. If I crank it into positive dBs, the neighbors will call the police.

I hope that’s not too complex and helps.
 
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MikeMcK

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The biggest thing that's missing from most explanations is the fact that a decibel always starts from a ratio of two values in the same units (Volts, Watts, etc.). IOW, the measured value is compared to some reference value... if the measured value is equal to the reference, that is 0 dB. If it's less, it's a negative amount in dB, and a positive dB value means it's greater than the reference.

Example: "dBm" means "dB referenced to 1 milliwatt of power". So a measured value of 1 mW can be called, "0 dBm".

The confusing part in using "dB" to describe audio volume is that no one ever tells you what the reference level is. That's kind of OK once you get an idea of "how loud 80 dB" is. You'll know that more than 80 is louder, less than 80 is quieter.

But the "dB" used in audio does have a reference value. It's called "audiometric zero" and is a rough idea of the quietest sound a human should hear (averaged, because in reality that's different for everyone).
 
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andrekp

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We think of decibels traveling forward in time, since that’s what we hear now. But negative decibels are created by anti-matter objects and travel BACK in time, making you lose hearing in your childhood.
 

poppunk

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1,467
Actual attempt to explain in layman's terms (requires a high school education):

For Audio Applications:

0 dB - Sound is the same as the reference level. In OP's question that's a standarized value for "threshold of human hearing". This value is the same (1x).

+20 dB - 10x larger than 0 dB

-20 dB - 1/10th the value of 0 dB (0.1x)

You can have negative dB SPL values becuase sound still exists even though the "standard human" can't hear it under "standard conditions".

Note that as mentioned upthread, you have to know what the "dBs" you are talking about are refernced to. In audio applications common references are dB SPL (measurement of actual sound in atmosphere), dBu (referenced to 0.775 volts), and dBV (referenced to 1 volt). It's a enormous misnomer used by the general public to say how loud things are in "dB".
 

MikeMcK

Gold Supporting Member
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6,790
Actual attempt to explain in layman's terms (requires a high school education):

For Audio Applications:

0 dB - Sound is the same as the reference level. In OP's question that's a standarized value for "threshold of human hearing". This value is the same (1x).

+20 dB - 10x larger than 0 dB

-20 dB - 1/10th the value of 0 dB (0.1x)

You can have negative dB SPL values becuase sound still exists even though the "standard human" can't hear it under "standard conditions".

Note that as mentioned upthread, you have to know what the "dBs" you are talking about are refernced to. In audio applications common references are dB SPL (measurement of actual sound in atmosphere), dBu (referenced to 0.775 volts), and dBV (referenced to 1 volt). It's a enormous misnomer used by the general public to say how loud things are in "dB".
Yeah, this might be a little off-topic, but because dBu seems mysterious to a lot of people, even engineers...

When an engineer sees 'u' at the end of a unit, they automatically think, "OK, that's how we write 'micro', because our keyboards don't have the Greek letter 'micron' (µ) on them. And since 'µ' refers to one millionth, this is some unit divided by a million, just like 'uF' or 'µF' is one millionth of a Farad."

But in this one rare case, the 'u' is just a 'u', said to stand for "unity". Like a lot of audio standards (phone plugs, 48V phantom power) the dBu standard started in the telephone industry. Bell Labs engineers needed an RMS voltage unit in a certain range. So they defined '0 dBu' as "whatever RMS voltage dissipates 1 mW of power in a telephone ring circuit".

Since the ring circuit has a nominal impedance of 600 Ohms, that arbitrary-sounding reference became the square root of 600/1000, so 0 dBu = 0.775 V RMS.
 
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dB is just a way of expressing a ratio in a logarithmic sense.

The formula is:

20 * log(signal of interest/reference signal)

Because of the way in which logarithms work, the log of 1 is zero. This means your signal of interest is equal to the reference. If the ratio is less than 1 then you get negative numbers. If the ratio is greater than 1 you get positive numbers.

But dB by itself it doesn’t mean anything until you specify what the reference signal is. This is usually done by adding another term after the dB, such as dB SPL, or dB FS.

Exploring these in a bit more detail. The reference level for SPL is the the nominal threshold of human hearing (and is typically measured in “phons” IIRC). Sounds below 0dB SPL will need to be amplified for most humans to hear them. Sounds above this will be audible without amplification for most humans.

Looking at dB FS (Full Scale), the reference is right there in the name it’s the maximum level you can put in a fixed width digital word. It’s where your converters saturate. So most of the levels that we express in this form will be in negative dB numbers since we want to keep them below the reference.

There’s a whole bunch of other dB references too including some that refer to voltage and current (dBV and dBu), but hopefully the examples above have helped a bit.
 

ylo

Silver Supporting Member
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926
"dB is just a way of expressing a ratio in a logarithmic sense..." Here is another take on this: Negative decibels make sense because 10 raised to 0 power is unity (one), and 10 raised to the -1 power is one tenth. The decibel level is based on the exponent.
 

MKB

Silver Supporting Member
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9,315
The biggest thing that's missing from most explanations is the fact that a decibel always starts from a ratio of two values in the same units (Volts, Watts, etc.). IOW, the measured value is compared to some reference value... if the measured value is equal to the reference, that is 0 dB. If it's less, it's a negative amount in dB, and a positive dB value means it's greater than the reference.

Example: "dBm" means "dB referenced to 1 milliwatt of power". So a measured value of 1 mW can be called, "0 dBm".

The confusing part in using "dB" to describe audio volume is that no one ever tells you what the reference level is. That's kind of OK once you get an idea of "how loud 80 dB" is. You'll know that more than 80 is louder, less than 80 is quieter.

But the "dB" used in audio does have a reference value. It's called "audiometric zero" and is a rough idea of the quietest sound a human should hear (averaged, because in reality that's different for everyone).
There is no negative dB unless there is a reference to go negative from. Maybe dB doesn't even exist without a reference...

I make my living in fiber optics and we deal in negative dB all the time. The normal value is dBm, which has 1mW of optical power as the reference (which is a lot of power in our world). I don't know how many times I have to scold techs that think you "take down the laser power" by going from -10dBm to -5dBm. Or -80dBm is "higher" than -50dBm. NO. These are usually the techs that call optical fiber "wires". That'll get you laughed at for sure.
 

Franktone

Gold Supporting Member
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3,237
use the Intensity formula I=Io x 10^(B/10)
where Io= 1x10^-12 W/m2 which is the lowest limit of hearing, and B is in dB
Try out a value of B=-10dB for example

I=1 x 10^(-12) x 10^(-10/10)
=1 x 10^(-12-1)
=1 x 10^(-13) W/m^2
=0.0000000000001 Watts/square meter, which is still a positive quantity, but not loud enough for us to hear.
 

MikeMcK

Gold Supporting Member
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6,790
There is no negative dB unless there is a reference to go negative from. Maybe dB doesn't even exist without a reference...

I make my living in fiber optics and we deal in negative dB all the time. The normal value is dBm, which has 1mW of optical power as the reference (which is a lot of power in our world). I don't know how many times I have to scold techs that think you "take down the laser power" by going from -10dBm to -5dBm. Or -80dBm is "higher" than -50dBm. NO. These are usually the techs that call optical fiber "wires". That'll get you laughed at for sure.
There's no maybe about it... dB definitely does not exist without a reference. But most people's first exposure to dB is on a stereo's volume knob. If you don't know that it's referenced to audiometric zero, you think of it as an absolute, like Watts or Volts.

Or sometimes it's a measure of amplification, where the reference is the input value. It all boils down to two equations:
  • For power (like a power amp measured in Watts): x dB = 10 x log10(p_measured/p_reference)
  • For potential difference (like a signal measured in Volts): x dB = 20 x log10(v_measured/v_reference)
In either case, if the measured value is smaller than the reference, it's negative.
 

HugoLC

Member
Messages
1,039
I don't know but I did pass a ear test last year and they told me I could hear -5db in the test
I was very surprised since I play a plexi into a 4x12 of EV each week without ear protection since years , anyway good thing it is
 

skydog

Silver Supporting Member
Messages
12,862
I don't know but I did pass a ear test last year and they told me I could hear -5db in the test
I was very surprised since I play a plexi into a 4x12 of EV each week without ear protection since years , anyway good thing it is
The last hearing test I took was in one of those isolation booths. The nurse, upon leaving, told me to press the controller button whenever I heard a tone. A minute or so later she reentered the booth and asked me why I wasn’t clicking it. I told her I didn’t realize the test had started. She told me it had started when she initially left the booth. I hadn’t heard a thing!
 




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