Luyện nói tiếng Anh bằng Shadowing qua video: Intensity, Loudness, and Timbre

Hi everybody and welcome to another video in the audio processing for machine learning series.

Last time we talked about basic features of sounds, so what sound is, waveforms and introduce the concept of frequency.

This time we continue delving into the features of sound, talking about intensity, power, loudness and timbre.

So first of all I want to talk about a couple of concepts, so intensity and power.

And these are like connected together.

So let's start from the power of sound.

Well, this is, we all know the power of sound, right?

But we don't want to talk about the power of sound here.

We actually want to talk about the reverse of that, the sound power.

So what's the sound power? well this is a physical measure

that we can express like this it's basically the rate at

which the energy is transferred this is basically the idea of the concept of power in general but

if we talk about sound power specifically this is the energy

per unit of time emitted by a sound source in all directions

across I mean like the air across like the the medium

that the sound is traveling in and we measure power in watts okay

and we indicate that with a capital W okay

so this is like the idea of sound power now connected

with sound power we have another thing that's called sound intensity And sound intensity is simply sound power per unit area, right?

And we can measure that as watts divided by squared meters, okay?

So the higher, like, the intensity and obviously the higher, the kind of, like, perception,

like, of loudness of that sound that we're going to have.

Okay, so now I want to ask you, like, a question.

So how much power do you think a thunder, like a super heavy thunder has?

Or how much power do you think a concert orchestra has?

Well, you may think like quite a lot, right?

Because like we perceive it as very loud.

Definitely like the heavy thunder, right?

And it gave us like a few scares, like I guess in the past when we were a child.

Okay, but that's not really the case.

So both a concert orchestra and a heavy thunder usually have a sound power of one watt.

Now if you are not familiar with like any reference like with water I'm gonna give you one.

So your usual like a light bulb used to be a hundred watts.

OK, so you like a whole orchestra is a hundredth, basically, of a light bulb in terms of power.

Same thing for this heavy thunder.

So yeah, this is quite mesmerizing, isn't it?

OK, but basically this tells us another very interesting thing, which is connected with a concept called the threshold of hearing.

So humans are capable of perceiving sounds which have extremely small intensities.

And the threshold of hearing is at 10 to the minus 12 watt over squared meters, right?

So this is like an extremely small number, but still we're capable of hearing that.

So obviously like the threshold of hearing like is the minimum intensity of sound that we can appreciate.

Okay, so now another kind of threshold which is like very important for us is called the threshold of pain.

So which is like the threshold beyond which we start to have like hearing pain, right?

Because like the sound like is like too intense.

okay and that's it at 10 watts over squared meters so

if you just like calculate the difference between 10 to the minus 12

and 10 you appreciate that like the amount the range of intensity

that we can perceive is enormous it's basically like 13 orders of magnitudes which is incredible

And that's why we use the so-called intensity level,

like for describing the intensity of a sound.

And that's why we put the intensity level on a logarithmic scale, because like the range is really, really enormous.

And the intensity level is connected with this measure,

which we call this unit of measure, which we call decibels.

I'm sure like you may be familiar with this but you may be wondering, you may have wondered, okay but what's a decibel?

Well it's a measure of intensity level and it's a ratio between two intensity values.

I just like found that I have like a small like typo here,

it's not ration because I don't know what that is but it's a ratio between two intensity values, right?

and so we use an intensity of reference

which sometimes is the threshold of hearing and then we compare

that against a current like intensity and we applied

that ratio a logarithm and so

if we want to take a look at the function

that describe decibels as a function of intensity is this one right

so we have 10 by log of an intensity divided by

the intensity of the threshold of hearing okay cool okay

so now you may be wondering but what's zero decibels and

so for that we need to pass in uh the intensity of the threshold of hearing

and so if we do

that this ratio uh basically like goes down to uh to one

and logarithm of one is equal to zeros

and we multiply 10 by 0 which is 0 okay

so a 0 at 0 decibel we are basically appreciating the

uh intensity of the threshold of hearing now this is not a universal because it really depends on the

kind of like reference intensity that you use in the intensity level

but usually like you'll find a lot of people like using

the threshold of hearing like as a as an intensity um a level of reference okay

so another thing uh to notice about the intensity level is

that every time we go up by three decibels the intensity tends to double

so if we are at zero decibel we are at the um

intensity of like the threshold of hearing but if we go up by three we are just like doubling

uh that uh intensity cool okay

so now i want to show you a cool table

which by the way is but i like i took it from a book

which is an amazing book like on audio music processing that's called fundamentals of music pro assessing it

and it's by mueller so many of the like uh pictures like

graphs and tables i'm going to use here are just like taken from

that book and i highly suggest you to go check

that out because it's great and it will give you like way more context

that one i'm covering here okay

but here you have a table where we have a lot of like different sound sources

and we have like the intensity measured in watts over a squared meters the

and the relative intensity level and that's like compared in terms of like the threshold of hearing.

Okay so like when we whisper we have an intensity which is at 10 to the minus 10

which is basically 20 decibels so a normal conversation happens at an intensity of 10 to to the minus six,

which is 60 decibels.

And the threshold of pain, which is we know is already intensity 10,

it's 130 decibels and the jet.

So when you have like a jet engine at takeoff,

this is at intensity 10 to two, and it's 140 decibels.

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Okay, so this gives you like more

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or less like an idea of like the different sounds

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and the relative intensity but now let's move on to another aspect

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that i think like it's extremely fascinating which is loudness now while intensity

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and power are two objective measures loudness is a subjective measure

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of intensity it is really like how loud we perceive a sound

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and as we've seen for um a frequency pitch which is like the relative like um

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kind of like subjective measure of frequency obviously it correlates with frequency

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but like there's some level of like uh subjectivity

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and some level of like mapping that's not like linear not the direct

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and the same thing happens here between intensity and loudness okay so um

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loudness so first of all depends on duration and the frequency of the sound in terms of duration

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So we usually hear as at equal intensity,

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we hear shorter sounds as less loud than longer sounds.

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So for example, if I have a sound which is at,

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say, 3 decibels and it only lasts for 100 milliseconds,

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I'll hear that as less loud than a similar sound with the same 3 decibel intensity level,

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but which lasts for 600 milliseconds.

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So this is like kind of like very fascinating because at the end of the day the intensity is the same

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but the perception of loudness is different.

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And a similar thing happens also with frequency.

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So frequency, depending on which frequency the sound is, we're going to hear that sound at a different loudness level.

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And obviously loudness is also correlated with age.

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So people of different age are going to hear like sound with the same intensity, but like with different loudness, right?

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And for loudness, so what researchers have done is basically they went out there

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and they did a lot of experiments to understand how we perceive sound and measured that with psychological experiments.

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And they came out with this measure, unit of measure called phones.

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So now let's take a look at phones.

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So here you have a very interesting chart that's called like equal loudness contours.

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So now on the y-axis you have the sound pressure level measured in decibels.

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So that's the intensity level.

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Here on the x-axis you have

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frequency in a logarithmic scale and it's measured obviously in Hertz you should know

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that by now and here like this curves

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that you see like in red so these are called equal loudness contours

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so along these curves we hear a sound to at a same intensity so for example on this curve here

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we hear sound at 80 phones, which basically is like we hear it at the same perceived loudness.

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However, what changes is the intensity level across the frequencies.

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So for example, when we are like at a very low frequency, say 20 hertz, we need a lot of intensity,

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so 120 decibels to perceive that sound at 80 phones.

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But then when we go up, say, to a thousand hertz,

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then to perceive that sound at 80 hertz, we need way less intense of a sound, which is around 80 decibels.

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And so, which basically means that, like, at lower frequencies, we perceive sounds,

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like, as less loud if they're equal in intensity.

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And this is like interesting.

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And if you look at like where we are the most efficient with loudness here,

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it's around like from a few hundred Hertz up until like 5,000, I would say 7,000 Hertz,

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which is like more or less like the range of like human speaking and human like singing, which may not be like a case at all.

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Right.

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So we are kind of like tailored to be the most efficient at that frequency range.

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And then like when the frequency goes up again,

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so we still, yeah, we are less efficient like at hearing sound, at perceiving like the loudness.

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And so we need more intensity to perceive like the same level of loudness.

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Okay, so this is like extremely, extremely interesting.

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And I'm always like fascinated about like the correlation between like physical measures and kind of like perceptual measures.

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Like and the way like these two things, yeah, are connected together.

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And now we enter like yet another aspect of sound, which is kind of like highly subjective and difficult to grasp.

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And that's timbre.

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okay so what's timbre? well I wish we knew

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because like researchers interested in sound music

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and all of these kind of things like I've studied timbre for a long time the problem is

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that we don't have like a real and comprehensive definition of timbre

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but we have like a few hints so

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if you ask a musician what timbre is like they would probably tell you like is the color of sound

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but that's a weird definition like for some something like which we hope like could be like a measurable thing right and but

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if we talk about this like from a kind of like

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more like programmatic like logical perspective we could think of like uh timbre as the diff between two sounds

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which have like all aspects equal like in terms of like intensity frequency

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and duration so you do a diff

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and what remains like is uh sound there uh timbra sorry

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it remains like timbra well uh the point is like

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if you have like the the same uh like notes like c5 for example uh played on a trumpet trumpet

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and on a violin with the same intensity you still hear

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that there's a difference there uh

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but it's not intensity it's not uh frequency it's not duration it's timbre okay so uh

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and timbre is usually described with words like bright dark dull

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harsh worm all of this like sounds a little bit fuzzy right

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and that's because it is fuzzy

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but now we don't know what our timbre is perfectly

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but we have a clear idea that timbre is somewhat multi-dimensional

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so many features come into place to define timbre this is different from other aspects of sound like frequency

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or intensity where we just have like one uh like one measurable one observable there okay

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so what are like some of these features

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that come into place with timbre so here i've listed three which which are like the most important ones.

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So one is called the sound envelope, then we have the harmonic contents, and then amplitude and frequency modulation.

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Now, I'm quite sure that most of you are completely oblivious about like what these things are,

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but don't worry, because I'm gonna cover like all of these things in depth, and we're gonna have like quite a lot of fun going through all of these aspects of sound.

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Okay, so let's start with sound envelope, which is called auto amplitude envelope.

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So what's that?

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Well, when we think of a sound, definitely like this is true for like musical sounds,

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like notes and things like that on different instruments.

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So the sound usually has like an envelope, right?

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And so this envelope can be divided with a model that's called the ADSR model, which stands for attack, decay, sustain, release model.

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So basically the idea is that the amplitude of these sounds has an initial spike, and that's like the attack.

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So for example, it's when you strike a key on a piano, you have like a spike in amplitude,

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which also has some kind of like noisy sounds due to the hummers like of the piano.

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And that's like a type of like transient sound.

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Then you have like a decay, which is where the sound stabilizes,

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a sustain period, which is where the the sound like remains like more

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or less like constant in amplitude and then you have a release

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which is like just like the fading out phase of a sound now the interesting thing is

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that different types of like sounds have different envelopes

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and this is like definitely true for musical instruments

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so let's take a look at this once again I'm using

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like this uh figure from the mueller's like book um fundamentals of music processing okay

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so here we can uh like um compare two different envelopes for a piano sound

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so here like you just like press a key

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and you wait for it like to to just like decay and here you have a violin sound okay

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and you see that the adsr

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and the envelope of this two um sounds it's like very very different

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so with the piano you have like a short attack a

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transient here then you have like a little bit of a period of decay

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and then you have a sustain and finally you have a release down here right

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and this is different for for violin as you can appreciate

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here the attack is way longer in terms of time and that's

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because like the attack is less sharp okay then you really

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don't have like a period of decay here you just have like some kind of like sustain

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and then you have the release when uh yeah just like end the sound okay

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so sound envelope is an important feature

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that determines the um the sound uh okay so one thing

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that i that i want to tell you here is

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that for example

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if you remove the attack part from a key sound from a from a piano sound what happens is

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that it's difficult to recognize that

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that is like a piano that's like playing right that's

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because like we we associate like a piano sound with its like sharp transient quite a lot okay

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so now let's move on

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to the second aspect which is harmonic content

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but in order to understand how what harmonic content is we

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need to understand what a complex sound is made up of

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so a complex sound is made up of a superposition of uh many sinus

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or like fundamental uh like simple uh like sounds right so So we can think of like a partial as a sinusoid,

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which is used to describe a sound.

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So the many different sinusoid that we have,

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which are superimposed together to create a complex sound are called partials or harmonic partials.

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So the lowest partial is called the fundamental frequency.

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And this is usually like the one that gives the pitch name to a note,

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if we are talking about like a note, for example, musical note.

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And now the harmonic partial,

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so the harmonic partials are frequencies that are like an integer multiple of the fundamental frequency.

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So let's say we have a fundamental frequency, which is at 440 Hertz, which is a four four now

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if we if we uh take a look at the second partial there

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so a harmonic partial what happens is that we have to multiply

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that 440 by 2 and this gives us like 880 hertz now the third partial

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will be 3 multiplied by 440

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which is um 1320 hertz now you can move on

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and basically the idea is that's like the complex sound is

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going to made up of all of these like partials

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and the harmonic content tells us how much energy we have in each of these uh partials

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and that determines somehow the timbre of the sound.

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Now not all sounds are perfectly harmonic

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so there's a lot of inharmonicity in some sounds and

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that would determine inharmonicity we indicate a deviation from a harmonic partial

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so that's for example if we had like some frequencies that are not perfect harmonic partials of the fundamental frequency.

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Now if we take a look at music instruments for example we know

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that um pitch pitched instruments tend to be like harmonic whereas

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like percussive instruments tend to have a lot of inharmonicity right

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so and usually obviously like noise are are highly completely like inharmonic right cool

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so this gives you like a an idea

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but now let's try to like listen to some of these things

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so for showing uh the harmonic content we should go

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and check out like a jupiter notebook that i just like uh broached okay

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so here i'm not gonna go through like the code

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because like the the point here is not like to show you like some of these things

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because we're gonna cover them like in future videos anyways but i want to to show you

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like some sounds and then the relative spectrograms okay so yeah

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so here just like i load some sounds

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and here like i have like a nice function

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that i can use to plot a spectrogram now what's a spectrogram

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so you'll hear about this like a lot moving forward

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but basically the idea is that with a spectrogram we get a snapshot of like the different energy in different frequencies

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of a sound across the duration of that sound

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and we can visualize that with a nice um plot

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which is this one okay so now let's try to um listen to a sound

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and so here what i want to show you like is two sounds

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and we'll start with a violin sound uh which is at i believe at c4 okay

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so that's middle c and so let's listen to this sound

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okay yeah let's okay good so that's c4 on a um violin okay

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so now let's plot the spectrogram for this

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and here you have it okay in all of its glory

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so basically the idea here is that on the x-axis you have time

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measured usually in seconds on the y-axis you have hertz

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and this is like a logarithmic scale

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and here like each point here has a color

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and the color tells you uh like the uh intensity of a um yeah of

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that frequency at that like specific time and

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so here like you have this color bar

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that tells you like how to reach like the intensity based on color like the redder

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and the more intense like the that frequency is okay

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so as you can see here

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so we we have uh a lot of like energy here in these like frequency band

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which is around 256 precisely this is a 261

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which i believe like should be like the hertz for c4 for the frequency for c4

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and yes and here we expect a lot of energy

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but then we have like the the partial the harmonic partial here

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which is double that frequency which is a 512

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and as you can see here we have like uh that coming in and then we multiply 256

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61 by 3 and we have the third partial

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and you can see it here the fourth the fifth cool it's super cool

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and you can go up and up and up and up

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and you still get like all of these partials okay and

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so as you can see there's not much like in harmonicity here

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so the partials like are very well uh define here okay

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so now let's compare this with a piano sound okay

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so now i have i believe like it's a c5 piano sound

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so let's listen that okay so you you heard that

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so this is c5 so it's an octave above the violin sound

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that we just heard and so let's take a look at the

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at the spectrum here for this piano sound and so as you can see here we have a lot of activity

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around 512 and this is like a correct right because this is like the fundamental frequency that we expect at

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and then over here you have like the first like harmonic

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partial you go up the second the third the fourth

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but as you can see the the higher you go and the less present like the partials are

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and if you compare this like against this right you see the difference okay

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so this is like way more sustained across

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so the energy is more spread out across the whole like

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partials whereas here like with the piano like is it seems to be like a little bit less

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so right and so by looking at this you can say okay

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so one of the reasons why like this two sounds are different is because of the

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distribution of the energy across the different partials being different okay cool okay now don't worry like

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if you don't have an idea about like what spectrograms are

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or how we we got to them or anything like that

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because this is gonna be a key topic

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and we're gonna cover this like in a lot of detail

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because like it's all about spectrograms

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and believe me spectrograms are key to analyze sound okay well

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i didn't want to go here i just like spoiled what's coming next

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but let's move on okay

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so now we have an idea about harmonic content now we have like one final aspect of timbre

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so we said envelope harmonic content and now frequency and amplitude modulation

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so basically modulation in the uh the sound like itself so what's frequency modulation so frequency modulation

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is also called in music circles as vibrato right and basically here you have um periodic variations

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in frequency and in music you have this for expressive purposes

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but uh like we can create an effect a vibrato effect by using like frequency modulation

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and this can be formalized and but basically the idea behind this is

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that you start like with a signal

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that you want to use to modulate the frequency of a a carrier signal

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which is this one like in blue down in the middle

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so you apply like this message signal signal on the carrier signal

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and you get these results down below

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and as you can see here that's a frequency modulation

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because the frequency starts uh like this

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but then the frequency kind of like um goes down okay

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and then it goes up and then it goes down right

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and so here we have a frequency modulation

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so now you may be wondering but how does like

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that sound like now if you're a musician obviously you know what a vibrato is

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but if you're not i'm gonna show you and this is where

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that amazing video comes in and

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so here we have a violinist who explains how to play vibrato on the violin

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so the first thing you'll hear is like the uh kind of like alternating like two notes right

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and then after that you'll hear like how that can become a vibrato

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yeah that's very dull right it's very very dull but

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if you change the speed there you'll hear like the typical violin sound the vibrato sound let's hear

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and until you reach normal vibrato speed

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and you need to right that's super cool now every time like you listen to violins

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or orchestras like the strings usually use like this vibrato sound quite uh intensively okay

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so uh one thing

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that i wanted to say is like as a keyboard player

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unfortunately we don't have this luxury of having like a vibrato

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but sometimes like you'll see pianists who like kind of like strike a key

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and then after that they kind of try to do a vibrato obviously

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that doesn't have like any effect on the sound

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but still like you have that

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so it's a it's like a nice like quirk okay but

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if you have a midi keyboard uh midi keyboards have this thing

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which is like after touch like

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and after like you strike a key then you can like vibrate the sound

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and it's gonna like vibrate but not on a piano i can assure you

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that i've tried it doesn't work okay uh

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so now the other type of like modulation

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that we have is called amplitude modulation

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and in musical terms this is called tremolo

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and this is still like a periodic variation but it's not in frequency

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but in amplitude and once again music we can use this like for expressive purposes okay

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so how do we uh get to amplitude modulation basically it's the same idea

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so we have a signal message signal we have a carrier signal

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and we apply the signal on the carrier signal

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but this time not on the frequency but on the amplitude

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and so you you're gonna have like this effect that does like ah so you like

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like bursts and release lack of amplitude, like at regular intervals.

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Now, I want to show you this once again.

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So let me just like go here.

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So I have a sound down here that I want to show you with a tremolo, right?

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Amplitude modulation.

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So let's hear that.

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But yeah, let's go back to zero.

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Okay, so yeah, you get the idea.

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So there like you have like this tremolo effect

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and probably you are familiar with that because like you've heard it like in many different like musical pieces and whatnot.

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Right, but basically the idea here is that when we modulate the sound like this

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both like on the amplitude and frequency uh dimensions what happens is

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that we we change the timbre of sound now we don't really change

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that much like the perception like of the of the frequency

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itself like of the amplitude is more like a quirk

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that changes the time breath it's as if like these things were like localized

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and had an effect like on time breath okay

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so here you have it like all the different things like

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that determine like timbre that we kind of like know of

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or have like reconstructed

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so timbre for sure is a multifactorial sound uh like dimension dimension of sound

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and it has like three main things

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so we already reviewed them say amplitude envelope the harmonic content

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or distribution of energy across different partials

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and then the signal modulation both like in frequency and amplitude okay

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so by now you should have like a good idea of sound like

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and all the different aspects of sound so we last time like we saw

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that sound like is a mechanical wave

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that propagates through air it is characterized by frequency like the intensity we can describe it with frequency intensity timbre

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and obviously like some of these things are also like quite subjective like pitch loudness

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and of course uh time breath okay

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so now you have like this very nice introduction

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and we are now ready to go to the next level

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which is introducing audio signals

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and specifically next time we'll are gonna tackle two very interesting

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like topics well for for sure we are gonna introduce like all your signals

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and then talk about audio to digital conversion called ADC the acronym

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and digital to audio conversion or DAC which is kind of like the inverse

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process okay so i hope like you've enjoyed this video

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if that's the case remember to leave like

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if you haven't subscribed yet please do so if you have any questions or doubts or anything

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please like just like leave a comment in the section below

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and talking about questions i just want to remind you

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if you haven't or well if you if you are already here that's great

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but if you are not please uh join the sound of ai slack community

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so there you'll find a lot of people interested in like

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audio processing machine learning ai music all this kind of stuff you can get feedback

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and talk with other people

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so i'll leave you a link to sign up to the slack community in the description below

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so i hope like you really enjoyed this video it's all for today until the next time cheers

Phương Pháp Shadowing Là Gì?

Shadowing là kỹ thuật học ngôn ngữ có cơ sở khoa học, ban đầu được phát triển cho chương trình đào tạo phiên dịch viên chuyên nghiệp và được phổ biến rộng rãi bởi nhà đa ngôn ngữ học Dr. Alexander Arguelles. Nguyên lý cốt lõi đơn giản nhưng cực kỳ hiệu quả: bạn nghe tiếng Anh của người bản xứ và lặp lại to ngay lập tức — như một "cái bóng" (shadow) đuổi theo người nói với độ trễ chỉ 1–2 giây. Khác với luyện ngữ pháp hay học từ vựng bị động, Shadowing buộc não bộ và cơ miệng phải đồng thời xử lý và tái tạo ngôn ngữ thực tế. Các nghiên cứu khoa học xác nhận phương pháp này cải thiện đáng kể phát âm, ngữ điệu, nhịp điệu, nối âm, kỹ năng nghe và độ lưu loát khi nói — đặc biệt hiệu quả cho người luyện IELTS Speaking và muốn giao tiếp tiếng Anh tự nhiên như người bản ngữ.

Luyện nói tiếng Anh bằng Shadowing qua video: Intensity, Loudness, and Timbre

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Phương Pháp Shadowing Là Gì?