
What is a decibel? - mirceasoaica
https://tomhazledine.com/what-is-a-decibel-anyway/
======
korethr
A small nit-pick: >The highest possible level of sound produced by audio
equipment is 0 dBFS.

...is only applicable to audio equipment operating solely on digital signals.
Humans don't hear digital signals, we hear pressure changes, so at some point,
those digital signals need to be transformed into analog voltage signals to
drive pressure transducers (loudspeakers). Those analog voltages are measured
by a different reference, a standardized voltage, as opposed to the absolute
max of a digital scale. Analog voltages are commonly specified in dBV or dBu
(0 references of 1 volt and ~0.775 volts, respectively).

The majority of the pro audio gear I've seen interacts with analog signals. So
at some point in a piece of gear that handles both analog and digital signals,
you're going to have to define a reference point between your digital and
analog levels. This can vary from one piece of equipment to the next. The
professional audio gear worth it's salt that I've seen can handle analog
signal levels of +18dBu or more. Consumer and pro-sumer audio gear have lower
limits, and can vary wildly. So 0dBFS signal going into your USB audio
interface might come out as a +16dBu analog signal, while the same digital
signal going into a professional mixing console might come out as a +24dBu
analog signal.

Edit: Grammar and spelling

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tomhazledine
That's a great point. The piece was written from a very web-centric point of
view. Even though I used analogue mixing desks as my jumping off point, I only
really looked at decibels in a digital context.

In fact, all the digital VU meters I've looked at do account for positive
values, so I definitely shouldn't have breezed over that aspect so quickly.

~~~
wrigby
This varies from product to product; for example, meters in Pro Tools are
labeled directly in DBFS[1]. So are Yamaha digital mixers (at least the LS9
and M7 - not sure about the newer ones).

On the other hand, the Allen & Heath GLD has its meters labeled differently,
and I'm not quite sure what they're referenced to (they go up to +12). This
can be a bit frustrating, since I never know how close I am to digital
clipping, but in practice I aim to get everything around 0 dB on the meters
and I have at least 12 dB of headroom.

Wikipedia has a neat list[2] of different analog reference levels for 0 dBFS.

1: Pro Tools metering -
[http://i.imgur.com/YcUBR6H.png](http://i.imgur.com/YcUBR6H.png)

2:
[https://en.wikipedia.org/wiki/DBFS#Analog_levels](https://en.wikipedia.org/wiki/DBFS#Analog_levels)

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hprotagonist
"a dimensionless way to compare values that vary dramatically on a linear
scale" ?

So many people totally omit the reference measure, which drives me abolutely
batty, particularly when we're talking about sound.

In air, dB SPL is referenced to 20 micropascals. In water, it's 1 micropascal,
so the medium matters.

Since loudness is a psychophysical percept, it's also dirty pool to speak of
loudness in this way. Intensity can be quantified, loudness varies by
listener.

~~~
jpmattia
> _Since loudness is a psychophysical percept, it 's also dirty pool to speak
> of loudness in this way._

It's worth noting that there are standards for loudness as well.

For example: The FCC broadcast spec for commercials uses ITU 1770, which
involves a filter. The resulting measurement is then labeled in units of dB
LKFS. (LK are filter params, which are fixed by the spec, FS is full-scale, ie
this is done digitally. (As an aside, video streaming could sure use some more
uniformity for its inline commercials too.))

~~~
matt4077
I've noticed very recently that Youtube started to play commercials louder
than whatever I was watching. I don't remember google stooping to such tricks
ever before,

~~~
jpmattia
> I don't remember google stooping to such tricks ever before

I kinda doubt that anyone has consciously tried to make the commercials louder
than the content. It's a tough problem to get right: The television industry
has been at it a lot longer, and they still barely make it work. The digital
outlets still seem to be finding their footing on this issue.

FD: My biz delivers commercial content to TV, digital and cable outlets (and
we normalize the audio). Normalization used to be a bit of a black art, but
failure to get the audio levels right results in FCC fines to the stations, so
you have to work hard to make it happen.

~~~
contras1970
> _I kinda doubt that anyone has consciously tried to make the commercials
> louder than the content. It 's a tough problem to get right: The television
> industry has been at it a lot longer, and they still barely make it work._

right, that's why commercials are _consistently_ louder than the "content".

~~~
hprotagonist
[https://arstechnica.com/tech-policy/2010/09/super-loud-tv-
co...](https://arstechnica.com/tech-policy/2010/09/super-loud-tv-commercials-
one-step-closer-to-extinction/)

7 years later..

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Matthias247
That text mentions that dB is relative, but misses how the values are defined.
A simple definition could be:

\- If we are talking about dB in context of amplitude 6dB more means twice as
much amplitude, 6dB less (or -6dB) means half the amplitude.

\- If we are talking about dB in context of power 3dB more means twice as much
power, 3dB less (or -3dB) means half the power.

dB is often also used to measure attenuation instead of gain. Here a positive
dB value would actually mean less output compared to reference value.

And: For dB values often a suffix is used, to denote to what the value is
relative to. E.g. dBV is relative to 1V, dbm is relative to 1 milliwatt.

~~~
pitaj
What you said is kinda confusing so I'm gonna clarify

3dB = √2 which is the amplitude ratio which is equivalent to doubling the
power, since power is square proportional to amplitude

6dB = 2, so a 6dB increase in power would be doubling the power, and a 6dB
increase in amplitude would be 4x the power

a negative dB is just the reciprocal of the radio, so -6dB is equal to 1/2

~~~
ramshorns
> _6dB = 2, so a 6dB increase in power would be doubling the power_

Pretty sure this part isn't right. Decibels are about power ratios, not
amplitude ratios. 10 dB is a power ratio of 10, so 20 dB is an amplitude ratio
of 10. 6 dB is a power ratio of 10^(6/10) ≈ 3.981 or an amplitude ratio of
10^(3/10) ≈ 1.995.), while 3 dB is a power ratio of 1.995 or an amplitude
ratio of 1.413.

(It's probably sometimes defined so that 3 dB is a power ratio of exactly 2,
making 30 dB 1024 instead of 1000.)

~~~
pitaj
Yes I was wrong about the 6dB. That's a quadrupling of the power, not a
doubling. It's twice the amplitude.

Unfortunately I can't edit my comment.

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AndrewKemendo
Great write up and balancing the relativity of decibels to frequency and
environmental conditions is basically what audio engineers do all day.

In high school long ago I used to compete in Car Audio SPL competitions.
Whenever we would compare notes about who has the highest readings, we would
always reference spl at what frequency and where it was measured (windshield,
kick panel or dash).

This is largely because in a pure SPL "Drag race" competitors typically
compete on a single frequency rather than a song for example. It was
instructive to compare frequency and spl because some cars resonant
frequencies (fs) were better suited to different fs values of drivers
(speakers) and then further down the line certain amplifiers optimized certain
frequencies, down to understanding which MOSFETs they used.

So it was all about finding and matching the fs of your car, the drivers and
the amps and then how much power you could push at once to make the whole
system resonate the most optimally at a certain focal point in the car. Really
mind boggling maths and physics when you really get down to it.

If anyone is interested in going deeper, John Hilliard did some amazing
research into decibel understanding and managing SPL as part of the Apollo
project. Basically he's the guy that made sure that the sound from the Saturn
5 engines didn't bounce off the deck and destroy the rocket on lift off.

~~~
mietek
Could you supply a reference for John Hilliard’s research?

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EvanAnderson
Google cache:
[http://webcache.googleusercontent.com/search?q=cache%3Ahttps...](http://webcache.googleusercontent.com/search?q=cache%3Ahttps%3A%2F%2Ftomhazledine.com%2Fwhat-
is-a-decibel-anyway%2F&oq=cache%3Ahttps%3A%2F%2Ftomhazledine.com%2Fwhat-is-a-
decibel-anyway%2F&aqs=chrome..69i57j69i58.824j0j4&sourceid=chrome&ie=UTF-8)

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shawkinaw
All of that (which was quite interesting), yet he doesn't actually say what a
decibel is. It is:

• 0.1 bel

• +/-1 bel = +/-10 dB = factor of 10 increase/decrease in power

~~~
GuB-42
The bel is never used in practice.

Although I am probably wrong, I always thought that "decibel" actually meant
"decimal bel" because it represents a decimal logarithm and that the "bel"
unit was devised as an afterthought to fit the SI prefix.

Still, it is a good way to remember that 10 dB = multiply by 10, because
"deci" means 10.

~~~
jpmattia
> and that the "bel" unit was devised as an afterthought to fit the SI prefix.

It's named after Alexander Graham Bell, in 1928 at Bell Labs.
[https://en.wikipedia.org/wiki/Decibel#History](https://en.wikipedia.org/wiki/Decibel#History)
(Yes I worked there, it was part of indoctrination training... :)

> because "deci" means 10.

Really, it means 1/10\. ("Deca" is 10.)

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platz
Ossmann's 'Software Defined Radio with HackRF' lesson 3 - Decibel:

[https://www.youtube.com/watch?v=J1FbhBqBYLM](https://www.youtube.com/watch?v=J1FbhBqBYLM)

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donpdonp
The wikipedia article is also very informative. I recently had to learn about
dBV (decibel volts) when trying to understand the spec sheet for a MEMS
microphone.

[https://en.wikipedia.org/wiki/Decibel](https://en.wikipedia.org/wiki/Decibel)

~~~
lfowles
I got intimately familiar with that page and sources while trying to extend
Boost.Units with Decibel support. Bit of a nightmare, in the end I just
hardcoded in support for dBm?[VW]?

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nthcolumn
The sound of mysql crashing is about 10 decibels.

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linsomniac
It's a tenth of a bel, right...? :-)

