The Little Vulgar Book of Mechanics (v0.6.0) - Sound I
Last updated: February 13th 2022
Just added a new section to the book: Sound I.
Sound I #
"In Space, no one can hear you scream." – ALIEN (1979), movie tagline.
This is one of those "facts" that people repeat all the time: There is no sound in space. Which is fine. It's true enough. The problem is that people will often "explain" it further, by saying it's because "sound needs an atmosphere in which to travel," which is wrong.
So let's start there: "There's no sound in Space." Yes, correct. "There is sound here on Earth." Also correct, obviously. And, using your powers of deduction, you can conclude: Therefore, sound requires... something, in order to exist.
So far so good.
Before we go on, though, let me set a rule: We do not say that sound "travels." Death metal screamers: I don't want you to think of your screams "traveling" from your mouth to the mic. There is no such traveling entity, or agent. There is no "scream" that is "traveling" from your mouth to the mic.
If you think sound "travels through the air," you are gonna be a shitty singer, engineer, and spouse. And you will never understand anything. And you will live a life of misery and despair. And your wife will divorce you. Do not think of sounds "traveling through the air."
Why? I'll explain below. Let's start with some basics first.
Sound I - Medium I #
OK, so I opened this section with the Alien tagline, but let's switch to Star Wars for a second, because I have an anecdote for context.
The other day I came across some comment on an Internet sci-fi forum thing, where someone was saying that Star Wars is "soft sci-fi." That it is "more fantasy than sci-fi."
Impressive. Noticing that SW is not scientifically accurate. How did Mr. big brains arrive at such insight? Was it the people vanishing out of their clothes, when getting killed by a 900-year-old little green monkey with a sword made of light?
No. He says the reason Star Wars movies are "more fantasy than sci-fi" is that they have "sounds in Space" during the battles. "In real life, you woudn't hear those explosions," he says. Because there is no sound in space. Because "sound needs an atmosphere in which to travel."
So that's how whales and dolphins communicate sonically, is it? By shooting their whistles up into the air, so they can "travel" across the atmosphere?
No. Dolphins and whales rely on sound, but it's all underwater. It's not that sound "needs an atmosphere," it's that sound needs a medium.
Back to Alien's tagline, let's play out the hypothetical situation:
The two of us are in Space. Well, three of us, counting the xenomorph. The xenomorph bites your leg or something. You scream at me, for some reason. Always blaming me for everything. Your vocal cords and shit vibrate normally, as usual. I.e. all the mechanics of your screaming are working as expected. But those vibrations in your body are not disturbing anything else, outside of you.
No water molecules, no air molecules, nothing. "In Space, no one can hear you scream," because the vacuum of space is not a proper medium for propagating the vibrations we perceive as sound.
Think of a sound-friendly medium as a collection of particles. A bucket where everything happens. It could be the ocean. Or some atmosphere. Or air in a helmet. The particles have some mass, so you can disturb them, and they can disturb each other. And by "disturb them" I mean make them vibrate. And by "vibrate" I mean displace them, back and forth, with respect to an equilibrium position.
The short slogan is: Vibration makes sound. More complete slogan: Vibration makes sound if you're in a medium that propagates vibrations interpretable as sound.
Why do I say "interpretable as sound"?
Do I mean that not all vibrations will be sound to our ears?
Sound I - Frequency I #
Grab a piece of metal. Like a metallic bar. And hit it. I assume you're not in Space, so you're in a vibrating system. Ideally, the metal bar you're hitting is a tuning fork. A metal tine that goes "diiiing!" when you hit it, generating a very basic type of vibration, which we perceive as a neat, basic sound. The rate at which it vibrates, we call frequency. The unit is hertz (Hz). E.g. If it vibrates 440 vibrations per second, we say it's a frequency of 440 Hz.
The metal tine, or tuning fork, is called a "diapasón" in Spanish, by the way. I will refer to it in all three ways, to keep you on your toes. In the past, musicians used this device as a referece to tune their instruments to. Nowadays they use better technologies. But tuning forks are still used for musical purposes. I'll give an example soon in this section.
Back to frequencies: Can our ears sense all frequencies?
No. Unfortunately (or not?), the range of frequencies our ears perceive as sound is limited. We can't hear some frequencies that other animals can. Also, unfortunately (definitely), this range shrinks as we grow older, not so much cos of age, but more cos we live in noisy cities, listen to loud shit on headphones, go to war, try to date militant feminists, etc. I.e. We put our ears through constant torture.
But, injuries and mutations aside, humans can hear in the frequency range from 17 Hz to about 17,000 Hz.
To guitar players: I'm not gonna talk about strings yet. That's a higher level of complexity. Elasticity, tension, etc. Various physical properties I'd prefer to actively include into the conversation, but would make this introductory section too long. Metal tines are simple objects that make simple oscillations, so I'm sticking with them here.
Stop hitting the diapasón for a second, though. I have a history question:
How are these two things related?
- World War II aircraft.
- A certain classic, mellow 1970s piano sound.
Sound I - Pitch I #
Vibration is what happens mechanically. What emerges acoustically we call a pitch. The pitch is what the singer, or musician in general, cares about. E.g. A basic wave of 440Hz from a vibrating tuning fork, we will perceive acoustically as a pitch, which we call "A."
Back to the question: How are World War II aircraft historically related to a classic, beloved mellow 1970s piano sound?
I'm sure you've heard that mellow 1970s sound. It's a peculiar type of piano. A piano that was designed by an American piano teacher and inventor, who would make miniature pianos from scrapped airplanes during WW2, and would craft his piano lessons as a form of therapy to soldiers. His name was Harold Rhodes.
Rhodes pianos sound super smooth, and don't go out of tune. This is because, unless you're a real son of a bitch, and really try hard to deform it, a metal tine will never go out of tune. And, unlike traditional pianos, whose keys are hammers that hit strings, the Rhodes' keys are hammers that hit metal tines. Inside a Rhodes piano you have basically a bunch of tuning forks, of the right shape and mass, to vibrate at specific frequencies, producing the right pitches for making music.
Sound I - Noise I #
Go back to the Rhodes piano, and press one key. As I said, each key is a hammer that hits a specific metal tine, and each tine vibrates at a specific frequency to get a specific musical pitch. So you've pressed a single key. We're now hearing a pitch. There is regularity to the vibration generated by one note, which translates to regularity, or periodicity, of the vibrations of air around your ears.
(The musician may say "note" instead of just "pitch" when he talks about more aspects of the sound, such as duration, timbre, etc. That's for a separate conversation. See Music I.)
Now, what happens if you press several keys at once?
E.g. Press the first, third, and fifth white keys, left to right. What are we hearing now? It's an addition of vibrations, obviously. (We've actually been hearing an "addition of vibrations" all along, but I won't get into that yet.) This is more complex sonic information to our brains. We can call it a "chord." But what happens if you press all the piano keys at once?
Use both arms or something. And a leg, maybe. But be careful. Press all the keys at once. What are we hearing now? This is much more information. In fact, it's so much "information," that it's not information at all. We're gonna call this noise. Musical tones have an order to them, whereas noise is basically the addition of so many irregular vibrations that our brain treats them as non-information.
Now get off my Rhodes, you fucking animal.
Sound I - Checkpoint #
So far we've gone over:
- Medium and vibrations.
- Frequency and pitch.
What's another fundamental property of sound, that I haven't talked about yet?
Sound engineers, look at your console...
Sound I - Level I #
Vibrations can be subtle, strong, and every intensity in between. That's what makes sound have different levels, which we measure in decibels (dB). The dB scale is logarithmic, because it makes the mathematics of sound better express human perception, which, in turn, makes engineering easier. Scratch that: It makes sound engineering possible. So we measure sound intensity in decibels (dB) so we can engineer cool stuff.
I mentioned in Problem Solving I that logarithms are simpler numbers that better serve how we perceive and use some things. The dB scale is a perfect example of this.
If your mixing console used faders made with a plain linear resistance, it would work like this: The top 4/5 of the fader's range would change little of the sound loudness. All the loudness variation would be in the bottom 1/5. In other words, it'd be stupid. A non-logarithmic fader on your mixing console would be wasteful and impractical.
Consider this: The pressure of the loudest known sound is more than one billion times the pressure of the faintest sound. Now ask an engineer to design you a usable measurement tool for that range. Is he gonna build you a 1 kilometer long fader or something? Or a normal size fader that's super oversensitive? A logarithmic range of 0 dB to 200 dB is more practical.
How about a mathematical function? Imagine the graph. Time as the horizontal axis. Sound level as the vertical axis. A curve where a value could be zero, but also a billion? So many pointless values. Wasted ink (or pixels.) Anyway, you get the point: Logarithmic scale = Good. Logarithmic scale = Practical.
There are variations of the dB scale, too. E.g. The dBA scale is like the dB, except adapted to account for the different reactions our ears have to different frequencies. A 100 Hz tone at 100 dB has a certain loudness to our brain, equal to the loudness of a 1000 Hz tone... at 80 dB. Our ears hear some frequencies more than others. The dbA scale is "A" weighted, using some curves that approximate human hearing.
But let's leave further talk of the dB to more advanced sections.
Like with frequency range, we also have a minimum and maximum of intensity that we can handle. Very low intensity vibrations are just silent to us, obviously. Too strong vibrations can harm, and even permanently damage, our ear drums. 40dB is about the lowest we can hear, and 120dB is very loud, and also the point at which our ears start to hurt (which is why 120 dB is called the "threshold of pain.")
By the way, it goes without saying: Silence is, therefore, what happens when the pressure around your ears isn't being changed by anything intensely enough to disturb your ear drums.
Speaking of silence, here's an old track of mine! Zander Noriega – Enjoy the Silence
Gotta plug my shit at all times, you know!
Sound I - Medium II #
Important point: The medium itself does not flow. There is no matter moving to "carry the sound." The medium is not being restructured. The "waves" transmit energy from one place to the next, without the medium moving any matter around. The vibrating tuning fork is not making pieces of atmospheric mass travel to your ear.
Have you ever been to a sports game, or watched it on TV? You know the audience "wave" thing they do? Well, if you take the audience as the medium, notice how we can talk about the wave "moving." The wave is "traveling," right? But notice that nobody in the audience is literally traveling with the wave. The particles (each audience member) are barely moving in that axis. They're only vibrating (sitting up and down). The wave is not moving matter along with it.
Now, think of each person's energy. Loosely speaking, there's some stored energy in each person. When they're sitting, i.e. doing "nothing," their energy output is zero. When they get up, they are emanating some
y amount of energy. So the wave, is an organized pattern of energy, which if you think about it, lets us say something like this: The wave moves energy from place A to place B, without moving matter from place A to place B.
Yet another way to put it: The motion you used when exciting the particles in the tuning fork, has been communicated to your ear drum, which is elsewhere in the medium.
Sound I - Waves I #
Here's another rule: In this conversation, whenever I say "wave," do not think of those rings that form when you throw a stone into calm water.
Think sphere. That's what we're dealing with, in sound, in practice, always: Spheres. Sound waves = Spheres.
Imagine a sphere made of many sub-spheres. I.e. Layers, or levels, like an onion. Think of each layer as either condensed, or rarefied air. Those are, respectively, the crests and troughs of a sound wave. Because when you disturb a particle in the air, it will disturb its neighbors above, below, etc. A spherical propagation of vibrations.
Say you're a metal singer, and you're thinking "I'm sending my powerful vox from my mouth to the mic!" while recording. No, no, no. This is what you must think instead:
I'm vibrating every bone in my skull and torso. Shaping my mouth to direct some of the energy. The vibrations are propagating spherically. This energy sphere is expanding and colliding with the walls, floor, and ceiling, etc. Now they are vibrating. Creating additional energy spheres, which overlap with my original sphere (which I'm still generating, as I sustain my scream). A sum of all this is being caught by the mic.
Your "voice" is the sum of the whole atmosphere vibrating, because of your whole body and the room's acoustic properties. This is why body posture, mic position, and room acoustics, all matter.
A powerful, monster, beast scream, is not something that sounds like it's right next to your ear. Any child can get up close and disturb your ears greatly. A fucking mosquito can make itself heard when near enough. (Clarification, though: The mosquito's sound is from its little wings flapping super fast. That's the buzzing sound.) A beast is a thing whose roars shake the whole fucking room. So room sound in metal vocals is often key.
However, too much reverberation makes things sound "distant," and a "beast in the distance" is usually less of a threat! Keep that in mind too.
So how do you make your metal (or video game, or movie) monster scream to sound threatening? Gotta find that sweet spot: Clear and dry enough for the brain to go "Oh shit, the beast is near me!" yet with a room sound (either real or manufactured in the mix) to make the brain go "Oh shit and this beast is powerful cos it's making the room vibrate." Case in point, of course: Corey Taylor from Slipknot. Corpsegrinder from Cannibal Corpse.
Recording studio acoustics is not just about slapping some shit on the walls to "kill reflections" or "isolate." Proper room acoustics must take into account the artistic necessities of the types of instruments (including vocals) meant to be recorded in it.
Try setting up a mic on a snare. Place it near the snare, pointing at it. Record it. How does it sound? Like shit. Go back and tinker with the placement. It doesn't fucking matter: The close snare mic always sounds like shit, no matter what.
This is why room mics, overhead mics, "mic leak," and/or artificial reverberation, are always added to the close snare sound in the mix. To recreate the complex natural signal. So don't panic, recording engineer: Close snare mic sounds like shit. You always have to "fix it in the mix."
But I digress. Acoustics and sound engineering should be separate sections.
Enough about screaming monsters. Let's go back to the tuning fork.
Strike the tuning fork again. It's vibrating now, and you're in a sound-friendly medium, so the disturbed tine disturbs the particles in the medium (atmosphere in this case), and the medium lets particles disturb their neighbors. This "chain of disturbances" has a certain order, or pattern, that we call a "wave." Sound waves are spherically propagating compressional waves. They are made of compressions and decompressions of the medium.
Actually, let's go back to this: Remember how I mentioned room sound and "reverberation"? That brings up the point I wanna talk in the next section.
Walk into a cathedral. Snap your fingers. Or say "hello." You'll notice that the sound doesn't just cut off instantly. Even seconds after you stopped causing vibrations, you're still hearing the "tail" of the sound so to speak. That's what we call reveberation. But that's not what I wanna talk about. Back to physics, let's ask this question:
When does sound stop? Or, why do the spherically propagating vibrations ever stop?
Sound I - Energy I #
The push-pull disturbances create the wave. Its characteristics are the frequency, intensity, etc. The properties we've gone over. As each particle disturbs its neighbors, the vibrations preserve (or are based on) these characteristics. The chain of push-pull disturbances, i.e. the wave, extends all the way to the atmospheric pressure around your ear drums. Energy dissipates and vibrations stop, according to the medium, and this dictates how you perceive the way sounds die off.
Remember how I've been talking about a medium being "sound-friendly," and "having properties that define how things vibrate in it." That's been too vague so far, so let's ask the question: What exactly determines how things vibrate in a medium?
For this, let's make the guitarists happy (and I happen to be a guitarist, too. Lucky me!) and use their instrument for this example:
Imagine an idle guitar string. Undisturbed, doing nothing. In its equilibrium state. Then you strike it with your pick. With a strong death metal palm-mute. This supplies energy to it. It starts to vibrate. As it vibrates, it dissipates its energy, radiating it away as heat and/or sound. You know this. Jumping to audio for a second: This is the "tail" of the amplitude of your waveform. That's the string dissipating energy. Eventually the energy dissipates completely and the string returns to its equilibrium state.
I mentioned in Sound I - Level I that intensity of sound corresponds to intensity of vibration. And I told you here that vibration stops when all energy dissipates (as sound and/or heat.) Well, then: Sound level corresponds to energy level.
Obvious, perhaps, but it can't hurt to say it explicitly.
Now you know what I mean by "properties of the medium." I mean properties such as its particular combo of "vibratory" and "dissipative" forces.
Sound I - Space I #
"In Space, no one can hea–"
Oh God, not that shit again!
Don't worry, I mean normal space, this time. Vulgar, lowercase "space."
I didn't wanna finish this introduction without at least mentioning this important aspect of sound: Its spatial content.
If you're an engineer mixing music, you are likely often trying to make something sound "wide." As in "stereo" vs. "mono." Usually, the reason your "double-tracked" metal guitars "don't sound as wide" as those in someone else's mix that you admire, is that the left and right sound content is too similar. But I digress! The point is: The perception of spatial content in sound emerges from, among other things, the differences in time and intensity of the disturbances, caused by anything with consider a single agent, on each of our ears.
- (Text/HTML) Interaural time difference @ Wikipedia
- (Text/HTML) Audio Spatial Representation Around the Body
See current full book's WIP here.