ELI5: How are synthesised voices (Alexa, Siri etc) made?

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There are several ways this is done. I worked in a commercial system in the early 2000s but not a whole lot has changed since then. Your idea is one of the more popular ways of producing natural sounding speech. A large amount of audio is recorded by a single speaker, and then chopped up into tiny bits and then put back together in different combinations. Older systems like this would use full phonemes (the basic sounds of speech) like the “s” sound combined with the “e” sound to make the word “see”. This was unnatural because there’s always some mismatch at the start and end of the sounds. By cutting the recordings into smaller pieces the speech can be made more natural sounding when it is put back together. This process of cutting up the recordings takes a lot of time although it can be sped up by using machine learning and speech recognition techniques with some human intervention. We needed about 6 hours of quality recordings including all possible sound combinations multiple times to make a new voice. Less than that would work but 6-12 hours would give the best results.

There are other ways of producing speech that involve mathematical models of the human speech system. This is how some of the earliest computer speech was produced in the 70s and 80s, but it is less natural sounding (like the speech synthesizer that Stephen Hawking used)

Finally more recently there are AI/machine learning generated voices that use neural networks or other “deep learning” techniques to generate the speech sounds.

All of these techniques can be combined to make very good sounding voices these days. When I was doing work in this area some of the biggest challenges were around making the tone and cadence of the voices more human-like. We are still not all the way there but I am really impressed by the progress that has been made even in the past few years.


My mum has got ALS and as she’s losing her speech we recently digitised her voice so she can talk through that in the future.

All she had to do was record 60 sentences with a budget microphone, send it off and a few days later they sent back her digital voice file and now when she typed anything it talks like her.


A friend of mine is a voice over artist that has done work for GPS systems and automated banking hotlines. She told me they recorded her saying every letter of the alphabet, many variations of vowel sounds and common key words and phrases. Skynet does the rest…


I saw an article some years ago who interviewed the woman who provided the voice for Siri, and she said that at the time she didn’t even know it was going to be for an AI, and that they basically just had her make a lot of sort of “nonsense” noises that were pretty much broken down words/syllables.

Because there was so much she did it for a while I’m pretty sure.


I worked at Google. Actually helped with Google assistant’s voice (provided some voice clips for fun).

What they do is hire a voice actress, and basically get them to read the dictionary several times with differnt inflections for each word.

My meager addition was just an hour of reading random phrases so that the AI can help mix in non professional inflections into the mix so she doesn’t sound so computerized. Ultimately that project was scrapped because the research shows that people like computerized voices.

Anyway, now that you have a huge database with all words and different inflections, you feed a few billion hours of captioned audio to a machine learning algorithm, which processes each phrase and learns the right inflections on how to pronounce each word given its position and context in the sentence.

Then, all you have to do is give this AI a sentence and it’ll spit out the right inflectioned words 99% of the time.

That’s the majority of it, there are algorithms for constructing words that aren’t in the database but the accuracy is something like 90-95% rather than 99.5% so they don’t use that.


ELI5: Why does a full stomach lessen the effects of alcohol?

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I once read that it was because when your stomach is empty, the alcohol proceeds straight to the intestines to be absorbed into your blood stream. But if there was solid food in your stomach, you stomach’s sphincter would be closed, forcing the alcohol to wait there before being absorbed. This spreads out the absorption time, reducing the intoxicating effects, like others said, but also gives time for some of alcohol to be broken down by enzymes in the stomach and be rendered non-intoxicating.


It slows down the intake of alcohol in your small intestine, because alcohol is “watered down” by food, so its effects are lessened and prolonged in time. Otherwise, if you take a shot on an empty stomach, it gets sucked in almost immediately, and all at once, assuring you get a hard hit.


Nope. First you need to know alcohol is absorbed in the large intestine rather than the stomach. When you’re belly is full it takes longer for the alcohol to get where it needs go go and there’s more stuff to deal with for your body.

Jn other words your tolerance is the same, stay up long enough and all the extra booze you drank will keep you entertained. 🙂


So basically with drugs the quicker something enters the blood stream the bigger the high. I had a professor explain it to me like imagine getting to 100 mph it feels way more exciting going from 0-100 in 6 seconds than slowly going through the gears.

Like others have said the food slows the process of ingestion of the alcohol so you don’t a big hit of it at once. You will still eventually get all the alcohol just in a more measured manner so you don’t feel the big rush of getting loads dumped into your blood stream


Alcohol absorbs into your body all through your GI tract. Your intestines absorb it the quickest, while the stomach absorbs at a much slower rate. When you eat before, your intestines are more full so the alcohol spends more time in your stomach, slowly being absorbed into your body. When you drink on an empty stomach, the alcohol makes its way to the intestines where it’s absorbed much quicker.


ELI5: why do digital thermostats have both heat and cool settings, as opposed to a singular temperature control

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Depends on the thermostat, but generally cool is connected to an AC unit while heat is connected to a furnace. When heat is on the temperature might go above the selected temperature and when cool is on it might go below the selected temperature. It would be extremely inefficient to make the temperature be exact since a house has a pretty large volume. Thus the heating/cooling system is more of a greater than/less than system.


Instructing the system to repeatedly cycle between heating and cooling modes to maintain a narrow temperature range would consume A LOT of electricity/fuel as the system would potentially be running nearly constantly.

While on a hot summer day, you really don’t want to run the heater just because the AC ran a little longer than necessary.


There are two interesting parts to this answer. The first is covered by most of the other comments, in that it would be impractical to maintain the temperature at exactly one degree (e.g. 70). Instead it makes more sense to have a cool setting – “keep it at least as cool as 70 degrees”, and hot setting – “keep it at least as warm as 70 degrees”, to allow a little wiggle room.

The other portion not covered in the other comments (at least so far) is that how cold or hot it feels is based on more than just the absolute temperature. Other interesting factors include things like direct sunlight, air flow, humidity, and air pressure. Besides the seasonal differences you may have for these that make you feel like 70 in the winter feels different than 70 in the summer, AC also serves as a dehumidifier, contributing to an AC induced 70 feeling different than a heat induced 70.


The same reason why analogue thermostats have them. “Heat” turns on the furnace and “cool” turns on the AC unit. Which is the same reason why one feels colder than the other. On heat the thermostat is trying to reach its sets temperature so the vents are blowing hot air. On cool the opposite it happens. In order to reguajte the temperature of the whole house the thermostat is place away from vents. So a room may be cooler or warmer than the set temperature when the thermostat turns off.


70 is the goal temp, but HVAC systems use much cooler or much hotter air to efficiently get the temp to that level. An AC might blow 40 degree air, while a furnace blows 100 degree air. Each does do until the hot/cold air mixes with the ambient air to hit the desired temp.

Also, an AC turns on and cold until temp drops to 70, while a furnace turns on to warm until it heats up to 70.


ELI5: Why does our anus have receptors for spicy stuff?

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The TRPV1 is the receptor that capsaicin (spicy) binds to. It exists all over your digestive tract and nervous system. Your rectum and anus are included in this. These receptors are designed to detect “heat” to keep you safe. But they can get confused with foods that taste “hot.” They cause a reaction to cool you down, and you have a response that says, “No, this is dangerous.”

So you sweat, and blood flow increases, causing redness. These receptors exist in all of your digestive tract, which is why you may experience stomach pains or some cramps after eating this. Not all of it digested and so the response is activated again at the anus.


Its just heat receptors but the skin around the anus is not skin, its membrane, it absorbs chemicals easily, the remaining spice is absorbed and the capsaicin does its trick, fooling you into thinking its hot when its not.

If anything i can feel spice in my intestines as its very uncomfortable and i get diarrhea from it.


There is no specific receptor for “spicy stuff”. At least that is not the intended purpose of it. The nerve endings that get activated by the capsaicin molecule are the same that transmit signals about heat/pain. So as long as there are those nerve endings, and they can be hit by the molecule, which works best on mucosa, then you can feel the spicy stuff. As for the reason why those nerve endings in the anus exist in the first place, this is more difficult to answer. I do not agree with the other answer that it does not serve any purpose. But I must admit that I can not see an obvious one either. It may just be for protection. This is the main reason why pain receptors exist. So damage is avoided.


It’s not just your anus – it’s your entire skin too. And this receptor isn’t really for detecting spicy stuff – it’s actually to detect heat (i.e. high temperature). Your skin has heat receptors to prevent damage, so that you can quickly move away from something hot (e.g. pull away your hand) before it burns you (too badly). For some reason, these heat receptors (at least some of them) are also chemically sensitive to a molecule that’s present in spicy foods like chili peppers (specifically the *capsaicin* molecule).

So the reason the anus has these receptors is because your entire skin has them. Only, your anus isn’t protected by an outer, tougher layer, like say the skin on your arms, and so your anus is generally more sensitive. But if you rub a hot chili pepper on your arm you’ll feel it burning too.

In addition to being more sensitive, the reason the anus may stand out as “uniquely” sensitive to spicy food (or the remains thereof) is that these receptors don’t exist in your intestines. So you don’t feel that extra spicy Thai Curry going through your guts, but only when you poop it out as it suddenly hits those pesky receptors again, making it seem like your anus is the culprit, when arguably it’s more the absence of these receptors in your gut that is causing this surprising sensation (basically, just be happy it doesn’t burn the whole time).


Cool answers. Now, is there anything that can be done to mitigate this fiery nightmare, once the food is down the hatch?


ELI5: How are status pages for major CDNs and major backbone providers designed to be up even though the provider is down?

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The computational requirements for a status page are extremely low compared to a full website such as e.g. reddit or facebook. It’s extremely cacheable, lightweight, and read-only. There’s no user authentication or policy enforcement logic. It’s also, for obvious reasons, hosted on separate infrastructure from the main site.

Furthermore, even during an outage event, a status site is going to receive less traffic than the actual site, since only a more savvy subset of the users will bother checking the status page.


The most obvious way to implement this is to host the status page with a competitor using completely independent infrastructure. Another trick which can be implemented is that if any part of the status page fails in some way it will just fail by showing the service as failed. So for example with todays fastly issue when the front end servers worked but the backend network had issues the frontends can be configured to show a status page with everything failing if they can not find the back end servers. This obviously does not work if there is any errors very early in the pipeline but at that point it is likely a problem with the clients network provieder anyway and the status page would be of no help.


A park you visit could be closed, but the signs at the perimeter and leading to it will still be there. It’s much like this. The status pages are kept specifically in heavily redundant places, sometimes reside in multiple places, so that they can always be reached, and not at the same physical location as the site itself.


Ahn noticed the fastly outage didnt ya.
But to the point they host their main infrastructure separately from the status page.
For example you host the status page in a Azure (Microsoft) datacenter in the region eu-west1, while the main infrastructure is hosted at AWS in us-east1…

Simplified example but you get the gist.
It could still be with the same provider but different regions too.


They aren’t always. There was a major S3 outage in us-East-1 a few years ago, and AWS’ status page couldn’t update the status icon because all the icons were themselves stored in S3.


ELI5: why do fish or other underwater creatures yawn in a similar way to humans? If they breathe through gills shouldn’t yawning be done more by flairing their gills wider instead of opening their mouth?

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They aren’t really yawning, they’re just breathing. Gills are where the water goes out, not in. Fish breathe in water, then force it through their gills to take in oxygen


Fish do not yawn, at least not the way we do. They do open their mouths sometimes, but that is typically to attract mates or deter aggressors.

Stickleback fish open their mouths sometimes very wide and move their bodies in a way that looks like stretching. Meanwhile Siamese fighting fish seems to be yawning upon seeing another fish, sometimes even of its own species. However, then it jumps into an aggressive attack, which kind of shows that it is not really yawning, even if it looks like that for us.

Siamese fighting fish may look like they are yawning, but they are actually just warning their competitors about an incoming attack.


I’ve seen a few different cases where fish open their mouths wider than usual:

* After eating, to stretch out their jaws and maybe clear food fragments from their teeth?
* When they smell something, I guess cause they want a bigger breath of it to figure out whether it’s food
* Social signaling to other fish, depending on species…but I’ve only seen this from bettas and cichlids


ELI5: how does alcohol dehydrate a person? Is it through the actual act of drinking or that it makes you urinate more and you don’t replenish with water?

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Alcohol is a diuretic which causes your kidneys to pull more water from your body and produce more urine. If all you drink is alcoholic beverages you are not replacing the water you are losing, which can lead to dehydration and hangovers. Best advice, for every drink you have, drink a glass of water.


The other comments about alcohol being an antidiuretic hormone inhibitor are correct but a study I can’t find atm found that upto 4% alcohol still hydrates you as the ~96% water you’re drinking outweighs the keeping less water. And really you’re only going to the loo more because you’re often drinking a lot of liquid when out drinking.


There is a hormone called ADH that tells your kidneys to just filter water but keep some and send some to the bladder. Alcohol won’t let it be produced by your brain so your kidneys just assume that there is too much water in your blood stream and send it all to your bladder.

So you pee like you are well hydrated, while actually becoming dehydrated. The common term “breaking the seal” is really just the start of this cycle of your bladder not holding anything back.


Your kidneys think you are being poisoned by the toxic alcohol and therefore trying to wash it all away, by using whatever water in your body it can find. To that end your dehydrated.


Am I wrong? I always chug a lot of water during the day before I start drinking to avoid the problem of being dehydrated. I believe it works for me, unless it’s a Placebo


ELI5 Why did thatched roofs not rot and collapse from the rain and damp? Or if they did rot, how frequently were they replaced?

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They did rot, but because of how they are layered on the roof, it sheds water pretty efficiently (that’s how they didn’t rot quickly, water shedding ability and tight layering protecting lower layers) and a well thatched roof in modern times can last 50 years before it’s beyond use. Historically, at least in parts of England, a new layer of thatch was added to the aging/weathered layer and some houses have been found to have seven feet of thatch on them with the oldest layers being 500 years old.


Present tense, not past. Some of the thatched buildings in my village are 400+ years old and it’s not a particularly old village. Here most people get their thatch replaced every 25 years or so, but you can just get it spruced up a bit rather than replacing the whole lot at once.


The thatch was sometimes treated with linseed or flax oil. It dries to a waterproof hard coat.
I was a roofer and my co-worker was from Poland told me this.


Thatched roofs use things like straw because they have an incredibly large C:N ratio. So the high carbon and low nitrogen make it slow decaying. Hence straw lasting so long


there are some interesting videos about this, especially repair or build videos where they explain as they go along


ELI5: How do keys in music work? Do they just pick random notes to flat or sharp? And what’s the difference between a minor key and a major key?

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In music a Scale is a set of rules that the notes follow. It tells you what the relationship between the notes are.

Jumping from one note to the one right next to it is called a half-step. It means that two notes are right next to eachother.

A whole-step means you are jumping over one note, so if youre on a note, and going a wholestep up, that means that there is a note inbetween the two you played.

So a scale tells us how far apart from eachother all the notes in that scale are. Theyre the rules. How many Half-Steps, and Whole-Steps there are.

I will mark Whole-Step as “W” and Half-Step as “H”.

So the rules in a Major Scale are: W, W, H, W, W, W, H

And the rules in our Minor Scale are: W, H, W, W, H, W, W

So now we know how the notes in our two scales relate to eachother. But we still gotta know which notes we should play!.

So we pick one note, that we want to be our “Root”. This is the very first note in our order, and if we know what the first note is, we know what all the other notes are since we know how far apart they need to be from eachother.

So picking a root note is called choosing a Key, the Key is what the first note played is, and the Scale is how far from the first note all the other notes are.

So G-Major and C-Major are both Major scales, but their first note is different, and so the following notes are different from eachother. But the distance between them is the same. Going from G to A is the same as going from C to D. Its a Whole-Step. They sound the same, one is just brighter than the other. What key you use depends on which one is the most fun to play, a piano will use more or less black keys depending. Other instruments will be tuned differently, and require different techniques. And a singer will have a specific range of notes they can comfortably sing, which will influence the key that the instruments play in.

The difference between Major and Minor is how far the notes are from the root, and generally Major is seen as “Happy” and Minor as “Sad”.

So the notes being played in music dont matter, its the relationship between the notes. As long as the distance between all the notes are the same, the song sound the same. Minor and Major are different **Scales**, not **Keys**, so the distance between the notes are different, and so they sound different.

As to which keys get sharpened and flattened it doesnt matter, but there is a system. When reading out the notes you want to keep the alphabetic order as good as possible. The notes we play with are A, B, C, D, E, F and G.

Ill represent a sharp as “#” and flat as “b”

So if im counting my notes in C-Minor, i can do it like:

“C, D, D#, F, G, G#, Bb”. But it gets messy, i have two D’s and two G’s, and im missing an E and an A. Hard to memorise.

If i choose to flatten those notes instead though i get:

“C, D, Eb, F, G Ab, Bb”. And now its all alphabetical, and no repeat letters. Ab and G# sound the same, which one is used just depends on the alphabet. Makes it much easier to memorize.


Think of the notes on a piano. The shortest distance between two notes, for example from a white note to the black note immediately above or below it, is called a semitone. So C – C# or B – Bb is a semitone, and so is E – F and B – C, because there are no black notes in between these white notes. A tone is made up of two semitones e.g. C – D (two white notes with a black note in between) F# – G# (two black notes with a white note in between) or even B – C# (white, white, black).

Every major scale has the same pattern of tones and semitones between the notes of the scale, regardless of the starting note. That pattern is tone, tone, semitone, tone, tone, tone, semitone.

The easiest scale to visualise this is C major – it’s played using only the white notes of the piano, because the pattern of white notes on the piano from C up to the next C follows that same pattern of T T S T T T S (T=tone, S=semitone). This scale has no sharps or flats because no notes need to be raised or lowered to follow the tone/semitone pattern.

The next most common scales are G major and F major. If we play all white notes starting on G or F the tone/semitone pattern doesn’t work so G major needs the 7th note to be F# to follow the pattern and in F major the 4th note is lowered to Bb. From then on subsequent major scales will add more sharps or flats (but not both) depending on the starting note to continue following the major scale pattern.

For minor scales, the pattern is T S T T S A S (where A=augmented 2nd which is an interval of 3 semitones). This is for a harmonic minor scale, which is usually what people refer to when they talk about minor scales. Again the patterns of tones and semitones in each harmonic minor scale will remain the same regardless of which note you start on.

Because major and minor keys have different patterns of tones and semitones for the scales they use, this gives them a different sound, or colour, to the listener even if both scales begin and end on the same note. It’s why you hear people refer to major keys as ‘happy’ and minor keys as ‘sad’.


This is also western music only. I figured that was assumed in major and minor scales. If anyone is familiar with other tradition’s music, I’d love to read about it in a comment.

(If you look at a picture of a piano keyboard, this will make a lot more sense

Notes repeat themselves in divisions called octaves which go from a note to the same note at either twice or half the frequency. This is what going up or down an octave means. It’s also what you’re doing in the “Do a deer…” song where you start at Do and end at a higher Do.

Inside each octave there are twelve notes. If you look at a piano keyboard, you’ll see the keys are arranged in groups of twelve–7 white and 5 black keys. On a guitar, that’s what the frets are, and the 12th fret is one octave higher than the natural string note. The two different keys are how we pick out which of the 12 notes are part of our progression through the octave.

It does this by starting at the root note. This is what the C in Cmaj means. It’s telling you to start at a C note. (On a piano, this is the white key to the left of the group of two black keys). From there we either go a “whole step” and skip a note to get to the next one in the scale, or we go a “half step” and use the next one. Conveniently, the piano keyboard is set up for the white keys to be the C major scale. We can now see that to get from one C to the next higher C takes a whole step-whole step-half step-whole step-whole step-whole step-half step, and takes us through CDEFGABC.

Let’s say we want to work out the E major key instead. We start at E (white key to the right of the two black keys) and follow the whole-whole-half-whole-whole-whole-half pattern. This covers an absolute TON of black keys. These black keys are what the flats and sharps are and we only call it a flat or sharp depending on which letter we call it. If it’s a half step up, it’s a sharp, and it’s a flat when it’s a half step down. The other rule is that we don’t reuse or skip letters.

Our E scale started as E??AB??E since we didn’t know what the black keys were. Knowing what we know now, the next note is F. Except it’s the black key a half step past the “regular” F so it’s an F# (or F sharp). The same is true for all the other ones and the full E major scale is EF#G#ABC#D#E.

For a minor scale, all the same rules apply other than the different pattern. Minor scales go whole-half-whole-whole-half-whole-whole. It’s the same number of steps. It just has different “stopping points” for the notes.

To see if you get it try the following:

* What notes are in the C minor scale?
* (Hard mode question) What minor scale has the same notes as the C major scale?


thank you for all your explanations… it all makes sense now


So different approach here to understand the physics behind it. Notes are just different frequencies. There are certain frequencies that sound good together. For example if you take a base (root) frequency and multiply it by 2 you get a nice sound (the same note just an “octave” higher). If you take a frequency and multiply it by 1.5 it sounds nice (perfect fifth). In western music we usually stick to eight of these ratios that we’ve decided sound good and call this group of ratios a scale. What’s with those annoying sharps and flats? Well it turns out that our brains are annoying and the ratios we like don’t follow a perfect pattern, so we had to add some sharps, flats, and nudge some other frequencies around a little bit so we could make instruments that are actually playable. Luckily for us our brains don’t really mind the slight deviations.


ELI5: How do bees get back to the hive when it starts pouring rain if they can’t fly in the rain, especially when it keeps raining for days and days?

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The truth is that not all bees get back to the hive and when conditions are extremely unfavorable they may even die while not able to return. However, bees are exceedingly good at reading and even predicting the weather. They are so sensitive to weather conditions (air pressure, humidity, etc) that humans can predict very short term weather by observing their behavior.

For example, you’re out tending your apiary when a sudden squall hits and you’re wondering if you should pack it up for the day and head home. All you have to do is look at the hive entrance and watch the activity. If the bees are going out as much as they are coming in then the weather wont last that long. If you see no activity or only traffic heading into the hive that storm will most likely last a while.

As for bees that get caught in the rain, they’ll find shelter that they can hang out in until the weather subsides and will die if they cannot fly back to the hive in time.


It’s currently raining lightly…. My bees are doing orientation flights and a short hop over the fence to a flowering hedge behind the house. Don’t think they are doing long range to the acres of blackberries about 1/2 mile away


I’m sorry, I live in Southern California…what is rain?


African bees can handle the rain actually, as well as their hybrids (which is why the hybrids were made in the first place. They’re hard working bees!)

Bees predict the weather better than meteorologists!


They evolved on a planet where this is sometimes the case, and so have ways to detect when it’s going to rain so they don’t get caught out. Bee’s forage within 2 miles of the hive usually preferring closer than that and so need a way to know when it will rain based on how quickly they can fly 2 miles in a bee-line back.

Hives that forage at distances where the hive doesn’t gain weight, forage in rain, forage at distances where they can’t get back, too many die in other ways, etc, don’t pass on their genes