
Everything You Should Know About Sound (2016) - simonebrunozzi
https://waitbutwhy.com/2016/03/sound.html
======
hprotagonist
_We often measure loudness using a unit called the decibel (named after
Alexander Graham Bell)._

Not really. Decibels themselves are unitless: we just have to use them because
the magnitude of sound pressure levels varies across a real large range so
it’s very useful to have a compressive scale.

Pedantry ahoy!

1\. The unit of loudness is the _phon_. Loudness is not independent of
frequency:
[https://en.wikipedia.org/wiki/Phon](https://en.wikipedia.org/wiki/Phon)

2\. sound _intensity_ is measured as a sound pressure level (SPL
[https://en.wikipedia.org/wiki/Sound_pressure](https://en.wikipedia.org/wiki/Sound_pressure))
in dB: re 20muPa in air, 1 muPa in water. It is a frequency independent
measure, though there are perceptual weighting scales (a- and b-weightings are
common) to normalize things for occupational exposure limits.

Sound intensity and sound loudness are related but not at all the same.
Loudness is a _percept_ , intensity is a physical measure of a system.

I can play a 100 kHz tone at extremely high sound pressure levels and you will
never hear it. It’s not loud, to a human. It’s probably quite loud if you’re a
bat.

~~~
LeonM
Thanks for the clarification. Learned something new today =)

What I've always wondered though: can loud sounds outside the frequency range
that is audible to us cause hearing damage?

So, say we have a 120dB 60kHz audio source, would that be dangerous for our
hearing? Would we be able to sense it (although not hear it)

~~~
mncharity
"It is known that exposure to noise of a certain frequency does not induce a
higher TS at that frequency, but rather at 0.5 to 2 octaves above that
frequency, depending on the species"[1] ("auditory threshold shift (TS)").

[1]
[https://www.frontiersin.org/articles/10.3389/fnagi.2015.0000...](https://www.frontiersin.org/articles/10.3389/fnagi.2015.00007/full)

A meta observation: When the internet was new, and then the web, there was a
puzzle of how society might adapt to information becoming more easily
accessible than it had been. Bomb making instructions was a commonly used
example. Society responded narrowly, by for instance outlawing such
instructions. There's an entwined puzzle around _understanding_ information.
If science education someday fails less pervasively, how might society adapt?
It's perhaps not a near-term challenge, but maybe something to think about.

~~~
whenchamenia
Bomb making intructions are not illegal however.

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ansgri
Maybe somebody has a recommendation on a similar article, but on _programming_
sound? Somehow there's much less material on working with sound (especially
low-level details about low-latency realtime sound effects) than on working
with images.

~~~
jamesb93
Programming sound is hard. Real-time stuff is almost always coded in C or C++
because it needs to be performant and latency is always a consideration. One
dropped sample can be heard and you can't afford to delay because things need
to feel responsive if they are real-time. There are lots of interesting
projects out there but there is also a wealth of creative coding in the area,
especially in environments like Max/MSP, PureData who are creating algorithms
from scratch inside and possibly porting them out to other code bases. One
good project is [1] which is aimed at making C++ coding for audio easier and
more accessible. The focus is quite wide and aims at webaudio, VST, faust code
etc.

[1] [https://github.com/iPlug2/iPlug2](https://github.com/iPlug2/iPlug2)

~~~
ansgri
I've figured as much about performance, somehow while working with video I
rarely reach for C++ lately, the firm realtime nature of sound makes
performance harder than video even though the bitrate is orders of magnitude
lower. Thanks for the link, will be looking at that and PortAudio.

------
codeulike
_The next time you’re talking to someone, I want you to stop and think about
what’s happening. Your brain has a thought. It translates that thought into a
pattern of pressure waves. Then your lungs send air out of your body, but as
you do that, you vibrate your vocal chords in just the right way and you move
your mouth and tongue into just the right shapes that by the time the air
leaves you, it’s embedded with a pattern of high- and low-pressure areas. The
code in that air then spreads out to all the air in the vicinity, a little bit
of which ends up in your friend’s ear, where it passes by their eardrum. When
it does, it vibrates their eardrum in such a way as to pass on not only the
code, but exactly where in the room it came from and the particular tone of
voice it came with. The eardrum’s vibrations are transmitted through three
tiny bones and into a little sac of fluid, which then transmits the
information into electrical impulses and sends them up the auditory nerve and
into the brain, where the information is decoded. And all of that happens in
an eighth of a second, without any effort from either of you._

Something quite intruiging - to me - about sound (and in particular music and
speech) is that it is spread across time. At any one instant of frozen time,
nothing in particular is happening with a sound - this is different to vision,
where if you imagine freezing time, you get a still image. So the way sound is
processed before it reaches the level of consciousness must need some kind of
time buffer. So when, say, you are appreciating a drum pattern in a piece of
music, your brain is processing the last few seconds of sound, noticing and
highlighting the patterns and then presenting it to your consciousness as a
kindof rhythmic artifact that you can appreciate and anticipate while it is
still happening.

~~~
neohaven
If you freeze video, you get an image. If you freeze sound, you get an "image"
on the speaker. It "stays" where it is. (At the voltage potential it is.) We
put HPFs on subwoofers so they don't stay offset like that.

The same way vision in human sight makes very little sense without things
being able to move (integration over time), hearing in the human sense seems
to be the same. Recording the change in pressure over time of the surrounding
air, instead of photons hitting you. But I am pretty certain that "vision"
that works in "single blips" with no refresh is just as pointless as a paused
audio recording for survival of an animal.

~~~
ses1984
Why would you put a high pass filter on a subwoofer?

~~~
michaelgrafl
To eliminate DC Offset (i.e. 0Hz).

------
ShroudedNight
> each increase of 10 dB doubles the loudness.

I understand a 10 db increase to represent an increase by a factor of 10, and
that an increase of log(2) = (~3) db was a doubling.

~~~
creeble
That's power, not necessarily loudness.

Loudness has to do with perception, which also has a lot to do with content.
For most people (a fairly wide margin here) _music_ sounds about twice as loud
when it's +10dB.

~~~
marvy
yep, he vaguely mentions this in footnote 6

------
0898
I enjoyed this part: "Researching for this post and learning about what sound
is gave me a new perspective on the tree falling in the forest with nobody
there to hear it question. I now think that no, it doesn’t make a sound. It
makes an air pressure wave and that’s it."

------
Foivos
Do I get this correct? In the case of a sound so loud that is leaving vacuum
behind, there should be another wave on the opposite direction that fils up
the vacuum, when the original wave gets very weak. Does this second wave make
a sound and/or destroy things?

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amingilani
It's uncanny how similar sound and radio waves are in terms of propagation,
despite being entirely different types of energy.

That said, there isn't a lot of visual diagrams on radio wave propagation. If
you happen to know of some great diagrams, please share.

~~~
wrycoder
[https://www.youtube.com/user/ElectromagnetismAnim/videos](https://www.youtube.com/user/ElectromagnetismAnim/videos)

